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The origin of reference intervals: A College of American Pathologists Q-Probes study of "normal ranges" used in 163 clinical laboratories.

* Context.--Standards have been developed for establishing reference intervals, but little is known about how intervals are determined in practice, interlaboratory variation in intervals, or errors that occur while setting reference intervals.

Objectives.--To determine (1) methods used by clinical laboratories to establish reference intervals for 7 common analytes, (2) variation in intervals, and (3) factors that contribute to establishment of "outlier" intervals.

Design.--One hundred sixty-three clinical laboratories provided information about their reference intervals for potassium, calcium, magnesium, thyroid-stimulating hormone, hemoglobin, platelet count, and activated partial thromboplastin time.

Results.--Approximately half the laboratories reported conducting an internal study of healthy individuals to validate reference intervals for adults. Most laboratories relied on external sources to establish reference intervals for pediatric patients. There was slight variation in intervals used by the central 80% of study laboratories, but some laboratories outside the central 80% had surprisingly low and high limits for their reference intervals. In some cases the intervals used by 2 laboratories had no overlap. For example, one laboratory considered a hemoglobin of 13.8 g/dL in a woman to be "low" while another considered the same value to be "high." Three percent of reference intervals contained a limit that qualified as an "outlier" using standard statistical tests; we could not identify any practice associated with adoption of outlier intervals.

Conclusions.--Many laboratories adopt reference intervals from manufacturers without on-site testing of healthy individuals. Reference intervals used by facilities that forgo on-site testing are not statistically different from intervals validated with on-site studies.

(Arch Pathol Lab Med. 2007;131:348-357)

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Laboratory test results are commonly compared to a reference interval before caregivers make physiological assessments, medical diagnoses, or management decisions.

The importance of reference intervals is underscored by US regulation: the Clinical Laboratory Improvement Amendments of 1988 require that laboratories that introduce an unmodified, US Food and Drug Administration-cleared or approved nonwaived test system "verify that the manufacturer's reference intervals (normal values) are appropriate for the laboratory's patient population." (1) Laboratories that modify US Food and Drug Administration-approved tests or develop their own assays are required to establish their own reference intervals for their assays. Regulations also specify that reference intervals be included in laboratory reports or made available upon request to individuals who order tests.

Reference intervals are of 2 types. (2) The most common type has been termed health-associated and is derived from a reference sample of persons who are in good health. (3) For example, the reference interval normally reported for serum potassium is health-associated. The central 95% of the healthy adult population tested by many laboratories has serum potassium levels between 3.5 and 5.1 mEq/L, and these limits define the serum potassium reference interval. The other type of reference interval has been termed decision-based and defines specific medical decision limits that clinicians use to diagnose or manage patients. For example, a serum total cholesterol of more than 200 mg/dL defines the level at which diet and exercise are first recommended by most authorities to lower cholesterol in otherwise healthy adults. (4) Similarly, the International Normalized Ratio range of 2.0 to 3.0 defines what some authors consider appropriate long-term anticoagulation in patients without atrial fibrillation or dilatation who have St Jude Medical bileaflet prosthetic aortic valves. (5) Reference intervals that incorporate medical decision limits are often defined with clinical trials and adopted by laboratories from the medical literature. In this study we investigated the process clinical laboratories use to establish health-associated reference intervals. These intervals are also popularly known as reference ranges, normal values, normal ranges, biological reference intervals, and expected values.

Health-associated reference intervals vary from laboratory to laboratory. For example, in a survey of 525 clinical laboratories the lower limit of the laboratory's total serum calcium reference interval ranged from 8.3 to 8.8 mg/dL (10th and 90th percentile of laboratories) and the upper limit from 10.2 to 10.7 mg/dL. (6) Some of this variation may have been due to differences among laboratories in clinical service needs, analytic platforms, populations of healthy individuals, or analytic imprecision that was present when reference intervals were determined. Yet some of the variation may have resulted from different approaches laboratories used to establish their reference intervals, as the process for establishing health-associated reference intervals has historically been poorly defined.

No single authoritative source specifies the process that clinical laboratories should use to establish health-associated reference intervals. The document that most closely approaches an authoritative source is the Clinical and Laboratory Standards Institute (CLSI; formerly NCCLS) document C28-A2. (3) The CLSI document is based on the seminal work of Solberg and colleagues, (7-12) who served in the 1980s on the Expert Panel on Theory of Reference Values of the International Federation of Clinical Chemistry and the Standing Committee on Reference Values of the International Council for Standardization in Haematology.

This CLSI standard recommends one approach for establishing reference intervals of a newly developed or modified analytical test system and a second, abbreviated, 20-specimen approach for validating the transfer of reference intervals among comparable analytical platforms. The more involved approach is applicable to instrument manufacturers first determining reference intervals for their test systems or to laboratories that develop their own assays or modify commercially available assays. The less involved approach allows clinical laboratories using unmodified US Food and Drug Administration-approved assays to validate reference intervals supplied by the manufacturers of their analytic instruments.

The current version of the Centers for Medicare and Medicaid Services (CMS) Survey Procedures and Interpretive Guidelines for Laboratories and Laboratory Services indicates a "laboratory must evaluate an appropriate number of specimens to verify the manufacturer's claims for normal values or, as applicable, the published reference ranges." (13) The CMS provides no guidance about what constitutes an "appropriate" number of specimens, when age-specific and sex-specific intervals need to be established, or how data from tested healthy individuals are to be used to verify a manufacturer's claims. The CMS agents make judgmental determinations about each facility's efforts to validate reference intervals on a case-by-case basis during the course of laboratory inspections.

Little is known about how laboratories establish health-associated reference intervals in practice. During 2004, 1.3% of laboratories enrolled in the College of American Pathologists (CAP) Laboratory Accreditation Program were cited by inspectors for failing to validate reference intervals properly, but the specific types of omissions were not documented and it is not clear whether inspectors approached this problem in a consistent manner. A survey of 500 laboratories conducted by the CAP in 2001 found that 390 (78% of laboratories) adopted manufacturers' published values for their reference intervals. (2) Approximately one fifth of laboratories reported receiving help from manufacturers in validating a manufacturer's reference interval, either in the form of statistical consultation, test materials, or procedures. The survey did not collect information about the proportion of laboratories that adopted manufacturers' reference intervals without testing any healthy individuals.

The aims of the present study were to (1) describe methods actually used by clinical laboratories to establish reference intervals for common analytes, (2) document interlaboratory variation in the reference intervals used in practice, and (3) identify institutional factors and practices that influenced the reference intervals a laboratory adopted.

We considered the aims of this study to be important for two reasons: First, the validation of reference intervals can consume considerable laboratory time and resources. Methods for validating reference intervals that require less effort but that still produce reliable intervals may be attractive to clinical laboratories operating with tightly constrained resources. Second, we were concerned that some laboratories may have adopted unusual reference intervals that posed patient safety risks, because of some sort of oversight or conceptual error. We wished to examine how often "outlier" reference intervals (defined in the "Materials and Methods") were found in laboratories, and whether any particular laboratory practices predisposed to the adoption of outlier intervals.

MATERIALS AND METHODS

Study Design

The study was conducted according to the Q-Probes study format previously described, which relies on a convenience sample of clinical laboratories that subscribe to the CAP Q-Probes benchmarking program. (14) After refinement of a standardized data collection instrument, CAP Q-Probes subscribers were mailed data collection instructions in late 2005.

Participants were asked to provide their laboratories' low and high values for reference intervals for 7 analytes: potassium, total calcium, magnesium, thyroid-stimulating hormone (TSH), hemoglobin, platelet count, and activated partial thromboplastin time (aPTT). Adult values (assuming a 32-year-old male inpatient) and pediatric values (assuming an 8-year-old male inpatient) were collected. In addition, hemoglobin reference intervals were collected for female adult and pediatric inpatients.

For each analyte, the following additional information was collected: unit of measure, primary specimen type, analytic instrument manufacturer, year the reference interval was originally established, year of the most recent revalidation of the reference interval, and year the primary instrument was placed into service. The methods the laboratory used to determine reference intervals for each analyte were also ascertained. Participants were asked whether they tested "healthy individuals" as part of the process they used to determine reference intervals, but no explicit definition of a healthy individual was provided.

Reference intervals for point-of-care instruments and capillary (fingerstick or earlobe) blood collections were excluded. In addition, intervals that were in the process of being reviewed were excluded, as were intervals for secondary laboratory testing sites and analyzers (if more than one testing site or analyzer was in use and reference intervals differed between sites or analyzers).

Participants were also queried about several institutional characteristics: occupied bed size, teaching status, pathology resident training status, government affiliation, institution location, institution type, CAP inspection status, and inspection status by the Joint Commission on Accreditation of Healthcare Organizations.

Laboratory Characteristics

Participants from a total of 163 institutions submitted data. Most of the institutions (97%) were located in the United States with the remaining located in Canada (2), Australia (1), Lebanon (1), and South Korea (1). Approximately 31% of participating institutions were teaching hospitals and 15% had a pathology residency program. Within the past 2 years, the CAP inspected 78% of the study laboratories. Hospital or laboratory inspections were conducted by the Joint Commission on Accreditation of Healthcare Organizations at 66% of participating institutions. Table 1 displays characteristics of participating institutions.

For chemistry assays the majority of institutions reported that they most commonly tested serum, rather than plasma. Ninety-one institutions (56.5%) most commonly tested for potassium using serum; 94 (59.1%) used serum for calcium; 92 (57.9%) used serum for magnesium; and 120 (78.9%) used serum to test for TSH.

Statistical Analysis

Where required, calcium and magnesium reference interval data were standardized to milligrams per deciliter (if they had been reported in millimoles per liter or milliequivalents per liter). Prior to performing statistical tests for association, values were screened for outliers. Several participating institutions did not answer all of the questions on the questionnaire about demographic characteristics, institutional practices, or reference intervals for particular analytes or age groups. These institutions were excluded only from tabulations and analyses that required the missing data elements. All statistical analysis were performed using SAS v9.1 (SAS Institute Inc, Cary, NC).

The low and high values for the reference intervals were tested for associations with the institutions' demographic and practice variable information in Tables 1 and 2, as well as the analytic platform used by the laboratory. Individual associations were first tested using the nonparametric Kruskal-Wallis test. Variables with significant associations (P < .10) were then included in a forward selection regression model. All remaining variables were significantly associated at the .05 significance level.

We used multivariate analysis of variance to perform a joint analysis of the dependent variables: low and high adult reference intervals and low and high pediatric reference intervals. This approach simultaneously tests whether the mean low/high vectors are statistically different for the analyte-specific predictor variables. Since the cell counts for the predictor variables were not equal, the significance level was set at .01.

We investigated outliers/atypical reference intervals using several techniques. First, we screened reference interval limits for outliers using 2 tests, the Tukey procedure (in which an outlier is defined as any value less than the first quartile - 1.5 x Inter-quartile Range or greater than the third quartile + 1.5 x Inter-quartile Range) and a 2-pass/3 SD procedure (in which an outlier is defined as any value that fell more than 3 standard deviations from the mean, either during a first pass with all data included or during a second pass in which outliers identified during the first pass were excluded). We also used a logit regression model to examine whether unusual reference interval limits--those in the upper or lower decile--were associated with any of the demographic and practice variables listed in Tables 1 and 2. For the logit regression, the significance level was set at .05.

RESULTS

When Are Reference Intervals Established?

A number of institutions reported that they did not know the year that their original reference interval had been established or the date of their most recent reference interval revalidation. Nine participants (5.5%) could not provide the year of their most recent revalidation of aPTT reference intervals, while 30 participants (18.4%) did not know the most recent year that potassium reference intervals were revalidated. Among laboratories that reported the year of their most recent revalidation, most had revalidated reference intervals within the past 5 years. Excluding aPTT, approximately two thirds reported that they revalidated their intervals in the same year that a new analyzer was purchased (Table 1). Nevertheless, in some laboratories and for some analytes the most recent revalidation of a reference interval had not occurred for more than 10 years, and one participant indicated the laboratory had last revalidated several reference intervals in 1983, more than 22 years before the study was conducted and more than 8 years before the laboratory's current analytic instrument was placed in service.

Methods Used to Establish Reference Intervals

Approximately half the participants reported that they conducted an internal study of healthy individuals to help establish chemistry and hematology reference intervals for adults, and half relied exclusively upon external sources (manufacturers' inserts, published literature, intervals used at other laboratories, or medical staff recommendations). For aPTT, 130 facilities (82.3%) reported conducting an internal study of healthy individuals to establish reference intervals. These data are shown in Table 2.

Most laboratories relied on external sources to establish reference intervals for pediatric patients (Table 2). Approximately one fourth of participants indicated they performed an internal study on healthy individuals to establish chemistry reference intervals for pediatric patients, and approximately 10% of facilities conducted internal studies to establish hematology and aPTT pediatric reference intervals.

One hundred thirty-six (84%) of 162 laboratories reported that they received some assistance from instrument manufacturers in establishing reference intervals. Of laboratories that received assistance from manufacturers, 111 (82%) reported receiving statistical support, 91 (67%) received consultative support, 55 (40%) followed a procedure for establishing reference intervals that had been provided by the manufacturer, and 27 (20%) received specimens for testing from the manufacturer.

We asked participants several detailed questions about the process they used to establish potassium reference intervals for adults. Of the respondents that indicated they had conducted an internal reference interval study of healthy individuals, half (65 laboratories) indicated that they had tested specimens from between 21 and 50 healthy individuals, and one quarter (32 laboratories) indicated they had tested more than 100 specimens. Of the sites that tested healthy individuals to help establish reference intervals, 56 sites (43%) set their reference intervals using the mean of the tested reference population plus or minus 2 standard deviations, while the remainder of respondents (73 sites; 57%) used test results to "verify" an external reference interval obtained from the manufacturer, a published textbook, or some other source. These data are shown in Table 3. Laboratories that set their reference intervals from the results of internal testing (mean [+ or -] 2 SD) did not tend to test more healthy patients than laboratories that used internal testing to "verify" an external interval (Table 4; chi-square = 2.27; P = .32).

Interlaboratory Variation in Reference Intervals

There was slight variation in reference intervals among the central 80% of study laboratories (Table 5). Upper limits of reference intervals tended to vary slightly more from laboratory to laboratory than lower limits did. There was no dramatic difference between the amount of interlaboratory variation in adult reference intervals and pediatric reference intervals, even though adult reference intervals were more often validated by testing specimens from healthy individuals and pediatric reference intervals were most often obtained from external sources. Reference intervals for the 7 study analytes showed similar levels of interlaboratory variation.

Outside of the central 80% of laboratories there was more substantial variation in reference intervals. A few laboratories had surprisingly low and high limits for their reference intervals. For example, one laboratory reported that its reference interval for adult potassium extended down to 3.0 mmol/L, while another reported that the upper limit of its potassium reference interval extended up to 5.7 mmol/L. In some cases the reference intervals used by 2 laboratories did not overlap, and the upper limit of one laboratory's reference interval was lower than the lower limit of another's. For example, one laboratory considered an aPTT of 30 seconds to be "low" while another considered an aPTT of 30 seconds to be "high." Using the Tukey procedure, 40 (3.1%) of 1271 adult reference intervals contained at least 1 limit that was an outlier. The 2-pass/3 SD procedure identified the same total number of outliers. There were no significant differences in the fraction of outlier reference interval limits among the analytes we studied.

Institutional Factors and Practices Associated With Reference Intervals

We examined all of the institutional factors and practices in Tables 1 and 2 to elucidate variables that were associated with reference intervals. For several analytes, particular instrument manufacturers were associated with higher or lower reference intervals. Tables 6 and 7 illustrate this relationship, showing statistically significant associations between instrument manufacturer and the median value of the upper and lower limits of adult reference intervals, respectively. Tables 8 and 9 show the same relationship for the upper and lower limits of pediatric reference intervals. Interestingly, specimen type did not affect the potassium reference intervals used by participants, even though the potassium concentration in plasma is lower than in serum. (15) The method used to establish a reference interval (adoption from manufacturer versus onsite testing of a healthy population) was not associated with the interval in use.

We examined whether any of the characteristics in Tables 1 and 2 were associated with the use of aberrant adult reference interval limits (in the upper or lower decile). Institutions that established their reference intervals before 2001 were 3.1 times more likely to have aberrant high aPTT limits than were institutions that established their intervals during 2001 to 2005 (P = .02). Institutions that placed test instruments in service before 2002 were 1.3 times more likely to have aberrant low TSH limits than were institutions that placed instruments in service in 2002 to 2005 (P = .01). No other factor was statistically associated with aberrant reference interval limits.

COMMENT

To our knowledge, this is the first large survey describing how reference intervals are actually established by clinical laboratories "in the field." We found that approximately half of all laboratories test healthy adults to establish reference intervals, but few test healthy children. Some of the laboratories that test healthy adults calculate their own reference intervals from their findings, but most use in-laboratory testing to "validate" reference intervals supplied by manufacturers.

Although different approaches were used to establish reference intervals, no particular approach was associated with the reference interval that laboratories ultimately established. In other words, there was no discernable difference between the reference intervals of laboratories that adopted manufacturers' reference intervals without further testing, laboratories that tested healthy subjects to validate manufacturer-supplied intervals, and laboratories that tested healthy subjects to calculate their own reference intervals.

Since the approach laboratories used to determine reference intervals did not appear to influence the interval that was ultimately established, we question the conventional wisdom that it is always necessary for laboratories to validate manufacturers' intervals with on-site testing before adopting the interval locally. If a manufacturer adequately describes the process it used to establish a health-associated reference interval, and the laboratory is using the manufacturer's analytic platform in accordance with the manufacturer's instructions and is testing a similar population, validation of a manufacturer's interval might consist of nothing more than the laboratory medical director making a professional determination that the supplied interval is appropriate for the local laboratory. This approach is contemplated in the CLSI standard, (3) but not in the current version of the CMS survey procedures. (13)

Klein and Junge (16) sound a note of caution about adopting manufacturers' reference intervals without local onsite testing of healthy individuals; the authors have concerns that preanalytic procedures used by manufacturers (such as specimen collection and storage procedures) may not be adequately duplicated by every laboratory, creating the need for local testing of healthy patients to ensure that manufacturers' intervals are locally applicable. While recognizing that the adoption of manufacturers' reference intervals without on-site validation poses some risk, our study suggests these risks are already being assumed by most laboratories that report pediatric reference intervals, since little testing of healthy children is taking place in laboratories even though specimen collection procedures for pediatric patients show a great deal of variability from institution to institution. The establishment of reference intervals for "special fluids" poses problems similar to those seen in pediatrics, as special fluid samples are rarely collected from healthy patients without clinical suspicion of disease, and reference intervals tend to be adopted from external sources without on-site validation. (17) Reference intervals for children and special fluids can be set by multiple cooperating laboratories, (18) but differences between test systems make this approach somewhat difficult to implement. (19)

While we do not believe local testing of healthy patients is required before adopting a well-documented reference interval from a manufacturer, there is nothing wrong with a local laboratory performing on-site testing of healthy individuals using the abbreviated 20-specimen CLSI procedure to validate a manufacturer's reference interval. If no facility performs on-site validation of manufacturers' reference intervals, the entire laboratory community will be relying on manufacturers and their regulators to ensure that initial reference interval studies are performed correctly. We must also point out that a decision by a laboratory director to forgo on-site testing of healthy individuals for the purpose of establishing a reference interval does not relieve the laboratory of its obligation to adequately calibrate a new instrument and test the instrument's analytic accuracy and precision before placing it into service.

We found that for most institutions, interlaboratory variability in the reference interval values for our study analytes was fairly low. The type of test instrument in use was the main source of reference interval variability that we could identify, explaining much of the variation in reference intervals. Variation in reference intervals among analytic platforms is probably appropriate, given that major instrument platforms show analytic bias relative to one another. (20-22)

Were the reference intervals used by most study participants accurate? Without access to the patient populations being tested by each of our study laboratories, we cannot answer this question directly. However, we collected indirect evidence that the potassium reference intervals used by many participants were at least somewhat inaccurate. We found no significant differences in the potassium reference intervals established by sites that primarily tested plasma and those that primarily tested serum, despite the known difference between plasma and serum potassium values (approximately 0.3 mmol/L, depending on the analytic test system). Only 28 sites "converted" results for 1 specimen type to the equivalent level from the other type before reporting, and only 17 sites established different reference intervals for each specimen type. For the remaining sites, potassium reference intervals were likely to have been inaccurate for at least 1 of the 2 specimen types.

What are the consequences of establishing health-associated reference intervals that are slightly inaccurate? Our study was not designed to address this question explicitly. Health-associated reference intervals generally do not define medical decision limits that call for action. Five percent of the healthy population--by definition--falls outside the health-associated normal range, and for most analytes there is no evidence that intervening to bring analyte concentrations into the central 95% of the healthy population will make healthy outliers any healthier. In our personal experiences, health-associated reference intervals are often ignored by physicians, who make management decisions using personal "rules of thumb," using established decision limits relevant to a patient's particular clinical context, or after making reference to a patient's previous values for the same analyte. But small inaccuracies in a laboratory's reference intervals may have a subtle impact on demand for repeat or follow-up testing. Small degrees of analytic bias or imprecision can have significant impact on provider behavior for some analytes, (23) and a similar relationship may exist for reference intervals.

Activated partial thromboplastin time and TSH may represent situations where small aberrations in health-associated reference intervals impact clinical care. Some heparin dosing protocols target a therapeutic aPTT range of 1.5 to 2.5 times the midpoint of the reference interval, which means a laboratory's choice of a reference interval will influence heparin dosing. (24) In the case of TSH there is some controversy about the definition of hypothyroidism, and some authors consider any patient with a TSH above the upper limit of the health-associated reference interval to be hypothyroid. Where this view prevails, the choice of a reference interval will influence the frequency with which hypothyroidism is diagnosed.

We found that a few laboratories in our study had adopted atypical/outlier reference intervals. Overall, 3.1% of reference intervals contained a limit that qualified as an outlier using either of 2 standard statistical tests for identifying outliers. This percentage of outliers is much higher than would be predicted on the basis of chance or from normal variation in the results of on-site testing of healthy individuals. However, we could not identify any institutional practices that were broadly associated with the adoption of aberrant reference intervals. Some outliers may have resulted from participants incorrectly completing the data collection forms or being confused about the units they were reporting.

The clinical implications of using highly atypical/outlier reference intervals is of much more concern than the use of reference intervals that are slightly inaccurate. Test results are "framed" by reference intervals, and the use of aberrant frames can bias decision-making. (25) Our study was not designed to determine whether a laboratory's adoption of aberrant/outlier reference intervals had clinical consequences for patients being served by the facility, but this question deserves further study.

Several limitations of this study should be acknowledged:

* First, data from study participants were self-reported and we could not independently validate all of the data submitted.

* Second, in the outpatient/outreach arena in the United States, approximately 30% of testing is performed by commercial laboratories that were not represented in this study. The approach to establishing reference intervals and the intervals in use by commercial laboratories may be different from those observed in this investigation.

* Third, institutions installing new test platforms commonly perform a comparison study with the platform being replaced, in which a regression slope and intercept are calculated from split specimens tested by each platform. This practice may help laboratories appraise the "reasonableness" of reference intervals supplied by the manufacturer of a new test system, even if no onsite testing of healthy individuals is performed.

* Fourth, the definition of a "healthy" individual may have varied among the study laboratories, accounting for some of the differences we observed in reference intervals.

* Finally, the 163 participants in this study may not be representative of hospital-based laboratories. Participants' willingness to participate in this study might reflect increased concern about their own reference intervals or might alternatively reflect an unusually strong commitment to properly establishing reference intervals. As a result, the findings reported in this study may not accurately represent the methods generally used to establish reference intervals in hospital-based laboratories and may overrepresent or underrepresent the frequency with which outlier/atypical reference intervals are adopted in practice.

Despite these limitations, we believe our data and generally accepted laboratory practices support several recommendations:

* First, laboratories should document how their reference interval for each analyte is established, even when "validating" a local reference interval consists of nothing more than the laboratory director making a professional determination that an instrument manufacturer's interval is applicable to the local population. In conformance with regulatory requirements, this documentation should be saved until 2 years after the reference interval is no longer in use.

* Second, the methods used by manufacturers to determine reference intervals should be adequately described in instrument literature and reagent inserts so that laboratory directors may make informed decisions about whether to adopt a manufacturer's intervals for their own institutions. Conducting local reference interval studies consumes time and money, and manufacturers that provide adequate documentation about their own studies will help conserve their customers' resources.

* Finally, as part of the process of implementing a new test system, laboratories should compare their chosen reference interval to the reference intervals recommended by manufacturers, published in the medical literature, or used by other laboratories. The adoption of outlier (and most likely inaccurate) reference intervals appears to occur with some frequency. Any extreme reference interval--for example, an interval that differs markedly from the manufacturer's or that has limits above the 95th percentile or below the 5th percentile of other laboratories--should prompt a reexamination of the process that was used to establish the local interval and consultation with the instrument manufacturer as required.

References

(1.) Clinical Laboratory Improvement Amendments of 1988 (CLIA), 42 CFR [sections]493.1253(b)(1)(ii) (2003).

(2.) Valenstein P, ed. Quality Management in Clinical Laboratories: Promoting Patient Safety Through Risk Reduction and Continuous Improvement. Chicago, Ill: College of American Pathologists; 2005:99-104.

(3.) How to Define and Determine Reference Intervals in the Clinical Laboratory; Approved Guideline--Second Edition. Wayne, Pa: Clinical and Laboratory Standards Institute; 2000. NCCLS document C28-A2.

(4.) Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). Bethesda, Md: National Institutes of Health, US Dept of Health and Human Services; 2002. NIH publication 02-5215.

(5.) Acar J, Iung B, Boissel JP, et al. AREVA: multicenter randomized comparison of low-dose versus standard dose anticoagulation in patients with mechanical prosthetic heart valves. Circulation. 1996;94:2107-2112.

(6.) Howanitz PJ, Cembrowski GS. Postanalytical quality improvement: a College of American Pathologists Q-Probes study of elevated calcium results in 525 institutions. Arch Pathol Lab Med. 2000;124:504-510.

(7.) Solberg HE. Approved recommendation (1986) on the theory of reference values. Part 1. The concept of reference values. Clin Chem Acta. 1987;167:111-118.

(8.) PetitClerc C, Solberg HE. Approved recommendation (1987) on the theory of reference values. Part 2. Selection of individuals for the production of reference values. J Clin Chem Clin Biochem. 1987;25:639-644.

(9.) Solberg HE, PetitClerc C. Approved recommendation (1988) on the theory of reference values. Part 3. Preparation of individuals and collection of specimens for the production of reference values. Clin Chem Acta. 1988;177:S1-S2.

(10.) Solberg HE, Stamm D. Approved recommendation on the theory of reference values. Part 4. Control of analytical variation in the production, transfer, and application of reference values. Eur J Clin Chem Clin Biochem. 1991;29: 531-535.

(11.) Solberg HE. Approved recommendations (1987) on the theory of reference values. Part 5. Statistical treatment of collected reference values. Determination of reference limits. J Clin Chem Clin Biochem. 1987;25:645-656.

(12.) Dybkaer R, Solberg HE. Approved recommendations (1987) on the theory of reference values. Part 6. Presentation of observed values related to reference values. J Clin Chem Clin Biochem. 1987;25:657-662.

(13.) Clinical Laboratory Improvement Amendments (CLIA), Interpretive Guidelines for Laboratories, Appendix C, Survey Procedures and Interpretive Guidelines for Laboratories and Laboratory Services. Baltimore, Md: Centers for Medicare and Medicaid Services, US Dept of Health and Human Services. Available at: http://www.cms.hhs.gov/CLIA/03_Interpretive_Guidelines_for_Laboratories.asp. Accessed May 2, 2006.

(14.) Howanitz PJ. Quality assurance measurements in department of pathology and laboratory medicine. Arch Pathol Lab Med. 1990;114:1131-1135.

(15.) Solberg HR. Establishment and use of reference values. In: Burtis CA, Ashwood ER, Bruns DE, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. Philadelphia, Pa: Elsevier Saunders; 2006:425-448.

(16.) Klein G, Junge W. Creation of the necessary analytical quality for generating and using reference intervals. Clin Chem Lab Med. 2004;42:851-857.

(17.) Dhondt JL. Difficulties in establishing reference intervals for special fluids: the example of 5-hydroxyindoleacetic acid and homovanillic acid in cerebrospinal fluid. Clin Chem Lab Med. 2004;42:833-841.

(18.) Rustad P, Felding P, Lahti A. Proposal for guidelines to establish common biological reference intervals in large geographical areas for biochemical quantities measured frequently in serum and plasma. Clin Chem Lab Med. 2004;42: 783-91.

(19.) Klee GG. Clinical interpretation of reference intervals and reference limits: a plea for assay harmonization. Clin Chem Lab Med. 2004;42:752-757.

(20.) Klee GG, Killeen AA. College of American Pathologists 2003 fresh frozen serum proficiency testing studies. Arch Pathol Lab Med. 2005;129:292-293.

(21.) Steele BW, Wang E, Palmer-Toy DE, Kelleen AA, Elin RJ, Klee GG. Total long-term within-laboratory precision of cortisol, ferritin, thyroxine, free thyroxine, and thyroid-stimulating hormone assays based on a College of American Pathologists fresh frozen serum study: do available methods meet medical needs for precision? Arch Pathol Lab Med. 2005;129:318-322.

(22.) Ross JW, Miller WG, Myers GL, Praestgaard J. The accuracy of laboratory measurements in clinical chemistry: a study of 11 routine chemistry analytes in the College of American Pathologists chemistry survey with fresh frozen serum, definitive methods, and reference methods. Arch Pathol Lab Med. 1998;122:587-608.

(23.) Gallaher MP, Mobley LR, Klee GG, Schryver P. The Impact of Calibration Error in Medical Decision Making: Final Report. Gaithersburg, Md: National Institute of Standards and Technology; 2004.

(24.) Heparin sodium injection, USP PDR information. Physicians Desk-Reference. Montvale, NJ: Thomson PDR; 2006.

(25.) Tversky A, Kahneman D. The framing of decisions and the psychology of choice. Science. 1981;211:453-458.

Accepted for publication September 8, 2006.

From the Department of Pathology, Baystate Medical Center, Springfield, Mass (Dr Friedberg); the Department of Biostatistics, College of American Pathologists, Northfield, Ill (Ms Souers); the Department of Pathology, The David Geffen School of Medicine at UCLA, Los Angeles, Calif (Dr Wagar); Preanalytical Systems, BD Diagnostics, Franklin Lakes, NJ (Dr Stankovic); and Department of Pathology, St Joseph Mercy Hospital, Ann Arbor, Mich (Dr Valenstein).

The authors have no relevant financial interest in the products or companies described in this article.

Reprints: Paul N. Valenstein, MD, Department of Pathology, St Joseph Mercy Hospital, Ann Arbor, MI 48106-0995 (e-mail: paul@valenstein.org).

Richard C. Friedberg, MD, PhD; Rhona Souers, MS; Elizabeth A. Wagar, MD; Ana K. Stankovic, MD, PhD, MPH; Paul N. Valenstein, MD
Table 1. Characteristics of Participating Laboratories *

 Institutions

Characteristic No. %

Institution type 78 60.0
 Private, nonprofit 18 13.8
 State, county, or city hospital 10 7.7
 Private, profit 6 4.6
 University hospital 5 3.8
 Governmental, federal 5 3.8
 Independent lab 8 6.2
 Other

Occupied beds

 0-150 53 42.1
 151-300 41 32.5
 301-450 19 15.1
 451-600 10 7.9
 >600 3 2.4

Institution location

 City 63 47.4
 Suburban 37 27.8
 Rural 31 23.3
 Federal installation lab 2 1.5

Governmental affiliation
 Nongovernmental 104 80.0
 Nonfederal governmental 21 16.2
 Federal governmental 5 3.8

Written policy for establishing, revising, or updating intervals

 Yes 117 73.6
 No 42 26.4

Unique intervals for distinct populations by patient age

 Yes 140 87.0
 No 21 13.0

Unique intervals for distinct populations by patient location

 Yes 4 2.5
 No 157 97.5

Unique intervals for distinct provider or practice group

 Yes 2 1.2
 No 159 98.8

Unique intervals for distinct populations based on disease type

 Yes 6 3.8
 No 153 96.2

Unique intervals for distinct populations by patient ethnicity

 Yes 7 4.4
 No 153 95.6

Reference intervals revalidated in the same year as new
 instrument acquisition

 Potassium 66 65.3
 Calcium 66 64.7
 Magnesium 68 67.3
 TSH 66 66.0
 Hemoglobin (male) 68 61.3
 Hemoglobin (female) 65 59.6
 Platelet count 66 61.1
 aPTT 33 29.5

* TSH indicates thyroid-stimulating hormone; aPTT, activated partial
thromboplastin time.

Table 2. Source of Reference Intervals *

 Institutions, No. (%)

 Potassium Calcium

Adult
 Internal study of
 healthy individuals 70 (44.3) 70 (43.8)
 Manufacturer's recommendations/
 inserts 55 (34.8) 56 (35.0)
 Published literature/
 textbooks 19 (12.0) 17 (10.6)
 Other laboratories (adopted with
 internal validation) 8 (5.1) 9 (5.6)
 Nonlaboratory medical staff
 recommendations 2 (1.3) 4 (2.5)
 Other laboratories (adopted with-
 out internal validation) 0 (0) 0 (0)
 Other 4 (2.5) 4 (2.5)

Pediatric
 Internal study of
 healthy individuals 35 (25.2) 34 (24.1)
 Manufacturer's recommendations/
 inserts 45 (32.4) 42 (29.8)
 Published literature/
 textbooks 42 (30.2) 44 (31.2)
 Other laboratories (adopted with
 internal validation) 7 (5.0) 8 (5.7)
 Nonlaboratory medical staff
 recommendations 3 (2.2) 5 (3.5)
 Other laboratories (adopted with-
 out internal validation) 1 (0.7) 2 (1.4)
 Other 6 (4.3) 6 (4.3)

 Institutions, No. (%)

 Magnesium TSH

Adult
 Internal study of
 healthy individuals 70 (44.6) 75 (48.4)
 Manufacturer's recommendations/
 inserts 57 (36.3) 46 (29.7)
 Published literature/
 textbooks 16 (10.2) 12 (7.7)
 Other laboratories (adopted with
 internal validation) 8 (5.1) 10 (6.5)
 Nonlaboratory medical staff
 recommendations 2 (1.3) 4 (2.6)
 Other laboratories (adopted with-
 out internal validation) 0 (0) 2 (1.3)
 Other 4 (2.5) 6 (3.9)

Pediatric
 Internal study of
 healthy individuals 36 (25.9) 21 (15.7)
 Manufacturer's recommendations/
 inserts 45 (32.4) 59 (44.0)
 Published literature/
 textbooks 43 (30.9) 33 (24.6)
 Other laboratories (adopted with
 internal validation) 6 (4.3) 7 (5.2)
 Nonlaboratory medical staff
 recommendations 3 (2.2) 5 (3.7)
 Other laboratories (adopted with-
 out internal validation) 1 (0.7) 4 (3.0)
 Other 5 (3.6) 5 (3.7)

 Institutions, No. (%)

 Hgb Hgb
 (Male) (Female)

Adult
 Internal study of
 healthy individuals 85 (53.5) 83 (52.5)
 Manufacturer's recommendations/
 inserts 17 (10.7) 18 (11.4)
 Published literature/
 textbooks 43 (27.0) 43 (27.2)
 Other laboratories (adopted with
 internal validation) 8 (5.0) 8 (5.1)
 Nonlaboratory medical staff
 recommendations 3 (1.9) 3 (1.9)
 Other laboratories (adopted with-
 out internal validation) 1 (0.6) 1 (0.6)
 Other 2 (1.3) 2 (1.3)

Pediatric
 Internal study of
 healthy individuals 35 (23.8) 34 (23.0)
 Manufacturer's recommendations/
 inserts 13 (8.8) 14 (9.5)
 Published literature/
 textbooks 75 (51.0) 75 (50.7)
 Other laboratories (adopted with
 internal validation) 7 (4.8) 8 (5.4)
 Nonlaboratory medical staff
 recommendations 4 (2.7) 4 (2.7)
 Other laboratories (adopted with-
 out internal validation) 10 (6.8) 10 (6.8)
 Other 3 (2.0) 3 (2.0)

 Institutions, No. (%)

 Platelets aPTT

Adult
 Internal study of
 healthy individuals 81 (51.3) 130 (82.3)
 Manufacturer's recommendations/
 inserts 19 (12.0) 12 (7.6)
 Published literature/
 textbooks 44 (27.8) 6 (3.8)
 Other laboratories (adopted with
 internal validation) 8 (5.1) 5 (3.2)
 Nonlaboratory medical staff
 recommendations 3 (1.9) 2 (1.3)
 Other laboratories (adopted with-
 out internal validation) 1 (0.6) 2 (1.3)
 Other 2 (1.3) 1 (0.6)

Pediatric
 Internal study of
 healthy individuals 34 (23.8) 85 (63.0)
 Manufacturer's recommendations/
 inserts 15 (10.5) 11 (8.1)
 Published literature/
 textbooks 68 (47.6) 24 (17.8)
 Other laboratories (adopted with
 internal validation) 8 (5.6) 5 (3.7)
 Nonlaboratory medical staff
 recommendations 4 (2.8) 2 (1.5)
 Other laboratories (adopted with-
 out internal validation) 9 (6.3) 4 (3.0)
 Other 5 (3.5) 4 (3.0)

* TSH indicates thyroid-stimulating hormone; Hgb, hemoglobin; and aPTT,
activated partial thromboplastin time.

Table 3. Methods Used to Establish Potassium Reference Interval

 Institutions

 No. %

Potassium levels run on which specimen type
 Both plasma and serum 100 61.7
 Plasma 25 15.4
 Serum 37 22.8

If potassium levels run on both plasma and serum,
 laboratory:

 "Converts" a result from one specimen type to
 the equivalent level from the other type 28 62.2
 Establishes and reports 2 different reference
 intervals based on specimen type 17 37.8

If an internal reference interval study was performed for potassium
 values, how many healthy individuals were studied?

 20 4 3.1
 21-50 65 50.4
 51-100 28 21.7
 >100 32 24.8

If an internal reference interval study was performed for potassium
 values, how did laboratory set the reference interval?

 Statistical analysis of collected data (eg,
 mean [+ or -] 2 SD) 56 43.4
 Manufacturer's recommendation after comparing
 to collected data from an internal reference
 interval study 55 42.6
 Textbook data after comparing to collected
 data from an internal reference interval
 study 10 7.8
 Another laboratory's reference interval after
 comparing to collected data from an internal
 reference interval study 3 2.3
 Other 5 3.9

Table 4. Number of Healthy Individuals Tested for Potassium and Method
Used to Establish Reference Intervals *

 Laboratories Determining Reference
 Interval Using Particular Method, No. (%)

 Interval Calculated Using Statistical
No. of Healthy Analysis of Tested Results
Individuals Tested (Mean [+ or -] 2 SD)

<50 27
51-100 13
>100 16
Total Laboratories 56 (46)

 Laboratories Determining Reference
 Interval Using Particular Method, No. (%)

 Interval Adopted From Manufacturer,
No. of Healthy After Comparison to
Individuals Tested Internal Tested Results

<50 40
51-100 15
>100 12
Total Laboratories 67 (54)

 Laboratories Determining Reference
 Interval Using Particular Method, No. (%)

No. of Healthy Total
Individuals Tested Laboratories, No. (%)

<50 67 (54)
51-100 28 (23)
>100 28 (23)
Total Laboratories 123 (100)

* No association was detected between the number of tested individuals
and the method used to determine the reference interval; chi-square
= 2.27; P = .32.

Table 5. Variation in Reference Intervals of Selected Analytes Among
163 Laboratories

 All Institution Percentiles

Analyte,
Population, and Limit * n Minimum

Potassium, mmol/L
 Adult, low 162 3.0
 Adult, high 162 4.5
 Pediatric, low 144 3.0
 Pediatric, high 144 4.1

Calcium (total), mg/dL
 Adult, low 162 7.6
 Adult, high 161 9.6
 Pediatric, low 145 7.6
 Pediatric, high 145 9.6

Magnesium, mg/dL
 Adult, low 157 1.3
 Adult, high 157 2.0
 Pediatric, low 141 1.2
 Pediatric, high 141 2.0

Thyroid-stimulating hormone, mIU/L
 Adult, low 154 0.10
 Adult, high 155 3.00
 Pediatric, low 133 0.10
 Pediatric, high 136 4.00

Hemoglobin (male), g/dL
 Adult, low 161 11.5
 Adult, high 161 15.0
 Pediatric, low 148 8.5
 Pediatric, high 148 12.7

Hemoglobin (female), g/dL
 Adult, low 159 10.4
 Adult, high 159 13.5
 Pediatric, low 148 9.0
 Pediatric, high 148 12.7

Platelets, x [10.sup.9]/L
 Adult, low 162 100
 Adult, high 161 316
 Pediatric, low 146 100
 Pediatric, high 143 325

Activated partial thromboplastin time, s
 Adult, low 156 17
 Adult, high 159 28
 Pediatric, low 135 17
 Pediatric, high 138 28

 All Institution Percentiles

Analyte,
Population, and Limit * 10th 25th

Potassium, mmol/L
 Adult, low 3.3 3.5
 Adult, high 5.0 5.0
 Pediatric, low 3.3 3.5
 Pediatric, high 5.0 5.0

Calcium (total), mg/dL
 Adult, low 8.3 8.4
 Adult, high 10.1 10.1
 Pediatric, low 8.4 8.4
 Pediatric, high 10.1 10.2

Magnesium, mg/dL
 Adult, low 1.5 1.6
 Adult, high 2.2 2.3
 Pediatric, low 1.5 1.6
 Pediatric, high 2.1 2.3

Thyroid-stimulating hormone, mIU/L
 Adult, low 0.30 0.34
 Adult, high 4.20 4.68
 Pediatric, low 0.30 0.34
 Pediatric, high 4.40 4.75

Hemoglobin (male), g/dL
 Adult, low 13.0 13.0
 Adult, high 16.5 17.0
 Pediatric, low 10.5 11.0
 Pediatric, high 13.7 14.5

Hemoglobin (female), g/dL
 Adult, low 11.5 11.7
 Adult, high 15.0 15.5
 Pediatric, low 10.5 11.0
 Pediatric, high 13.7 14.5

Platelets, x [10.sup.9]/L
 Adult, low 130 130
 Adult, high 400 400
 Pediatric, low 130 140
 Pediatric, high 392 400

Activated partial thromboplastin time, s
 Adult, low 21 22
 Adult, high 32 33
 Pediatric, low 21 22
 Pediatric, high 32 33

 All Institution Percentiles

Analyte, 50th
Population, and Limit * (Median) 75th

Potassium, mmol/L
 Adult, low 3.5 3.6
 Adult, high 5.1 5.2
 Pediatric, low 3.5 3.6
 Pediatric, high 5.1 5.2

Calcium (total), mg/dL
 Adult, low 8.5 8.5
 Adult, high 10.2 10.5
 Pediatric, low 8.5 8.8
 Pediatric, high 10.4 10.7

Magnesium, mg/dL
 Adult, low 1.6 1.8
 Adult, high 2.4 2.6
 Pediatric, low 1.6 1.8
 Pediatric, high 2.4 2.5

Thyroid-stimulating hormone, mIU/L
 Adult, low 0.35 0.40
 Adult, high 4.94 5.50
 Pediatric, low 0.35 0.46
 Pediatric, high 5.00 5.50

Hemoglobin (male), g/dL
 Adult, low 13.6 14.0
 Adult, high 17.5 18.0
 Pediatric, low 11.5 11.7
 Pediatric, high 15.5 15.5

Hemoglobin (female), g/dL
 Adult, low 12.0 12.0
 Adult, high 16.0 16.0
 Pediatric, low 11.5 11.5
 Pediatric, high 15.0 15.5

Platelets, x [10.sup.9]/L
 Adult, low 142 150
 Adult, high 400 440
 Pediatric, low 150 150
 Pediatric, high 400 450

Activated partial thromboplastin time, s
 Adult, low 23 25
 Adult, high 35 37
 Pediatric, low 23 25
 Pediatric, high 35 37

 All Institution Percentiles

Analyte,
Population, and Limit * 90th Maximum

Potassium, mmol/L
 Adult, low 3.6 4.1
 Adult, high 5.3 5.7
 Pediatric, low 3.6 4.1
 Pediatric, high 5.4 5.6

Calcium (total), mg/dL
 Adult, low 8.8 9.0
 Adult, high 10.5 10.8
 Pediatric, low 9. 9.6
 Pediatric, high 10.8 11.5

Magnesium, mg/dL
 Adult, low 1.8 2.1
 Adult, high 2.7 3.2
 Pediatric, low 1.8 2.2
 Pediatric, high 2.7 3.2

Thyroid-stimulating hormone, mIU/L
 Adult, low 0.47 0.50
 Adult, high 5.60 6.00
 Pediatric, low 0.50 0.70
 Pediatric, high 5.90 8.00

Hemoglobin (male), g/dL
 Adult, low 14.0 14.8
 Adult, high 18.0 18.1
 Pediatric, low 12.5 14.0
 Pediatric, high 16.3 20.0

Hemoglobin (female), g/dL
 Adult, low 12.1 14.0
 Adult, high 16.0 18.0
 Pediatric, low 12.0 13.0
 Pediatric, high 16.0 18.0

Platelets, x [10.sup.9]/L
 Adult, low 150 174
 Adult, high 450 500
 Pediatric, low 184 242
 Pediatric, high 450 539

Activated partial thromboplastin time, s
 Adult, low 26 32
 Adult, high 38 43
 Pediatric, low 26 29
 Pediatric, high 38 43

* Low indicates the lower limit of the reference interval; high, the
upper limit of the reference interval.

Table 6. Relationships Between Instrument
Manufacturer and Adult Reference Interval Upper
Limit *

 Median
 Adult
 Reference
Analyte, Interval
Unit of Measure, No. of Upper
and Manufacturer Institutions Limit

Magnesium, mg/dL (P < .001)
 Vitros 24 2.30
 Dade Behring 53 2.40
 Bayer 8 2.45
 Beckman Synchron 34 2.50
 Roche Hitachi 17 2.56
 Roche 13 2.60

TSH, mIU/L (P < .001)
 Roche 11 4.20
 Roche Hitachi 8 4.45
 Abbott 18 4.67
 Vitros 14 4.68
 Dade Behring 33 4.82
 Bayer 34 5.50
 Beckman Access 27 5.60

aPTT, s (P = .001)
 Dade Behring Actin FS 10 33
 BioMerieux Platelin L 13 33
 Dade Behring Actin FSL 44 35
 HemosIL/Hemoliance SynthASiL 18 36
 Diagnostica Stago STA-PTT A 28 36
 HemosIL/IL Test APTT-SP 23 37

* TSH indicates thyroid-stimulating hormone; aPTT, activated partial
thromboplastin time.

Table 7. Relationships Between Instrument
Manufacturer and Adult Reference Interval Lower
Limit *

 Median
 Adult
 Reference
Analyte, Interval
Unit of Measure, No. of Lower
and Manufacturer Institutions Limit

TSH, mIU/L (P < .001)
 Roche 11 0.27
 Roche/Hitachi 8 0.29
 Beckman Access 27 0.34
 Dade Behring 33 0.34
 Bayer 34 0.35
 Vitros 13 0.47
 Abbott 18 0.48

aPTT, s (P < .001)
 BioMerieux Platelin L 13 21
 Dade Behring Actin FS 10 22
 HemosIL/IL Test APTT-SP 22 23
 Dade Behring Actin FSL 43 24
 Diagnostica Stago STA-PTT 28 24
 HemosIL/Hemoliance SynthASiL 18 25

* TSH indicates thyroid-stimulating hormone; aPTT, activated partial
thromboplastin time.

Table 8. Relationships Between Instrument
Manufacturer and Pediatric Reference Interval Upper
Limit *

 Median
 Pediatric
 Reference
Analyte, Interval
Unit of Measure, No. of Upper
and Manufacturer Institutions Limit

Magnesium, mg/dL (P = .001)
 Vitros 22 2.30
 Dade Behring 50 2.40
 Roche 13 2.40
 Beckman Synchron 30 2.50
 Roche Hitachi 13 2.52

TSH, mIU/L (P < .001)
 Roche Hitachi 7 4.20
 Roche 10 4.64
 Vitros 13 4.68
 Dade Behring 29 4.82
 Abbott 17 4.98
 Bayer 30 5.50
 Beckman Access 23 5.60

Hemoglobin (male), g/dL (P = .02)
 Bayer 13 14.8
 Coulter 81 15.0
 Sysmex 25 15.0
 Abbott 27 15.5

Hemoglobin (female), g/dL (P = .005)
 Bayer 13 14.8
 Coulter 80 15.0
 Sysmex 25 15.0
 Abbott 26 15.5

aPTT, s (P = .003)
 Dade Behring Actin FS 10 33
 BioMerieux Platelin L 12 33
 HemosIL/Hemoliance SynthASiL 14 35
 Dade Behring Actin FSL 41 35
 Diagnostica Stago STA-PTT A 24 36
 HemosIL/IL Test APTT-SP 17 37

* TSH indicates thyroid-stimulating hormone; aPTT, activated partial
thromboplastin time.

Table 9. Relationships Between Instrument
Manufacturer and Pediatric Reference Interval Lower
Limit *

 Median
 Pediatric
 Reference
Analyte, Interval
Unit of Measure, No. of Lower
and Manufacturer Institutions Limit

TSH, mIU/L (P = .02)
 Roche/Hitachi 7 0.27
 Beckman Access 23 0.34
 Dade Behring 29 0.34
 Bayer 29 0.35
 Roche 9 0.40
 Vitros 12 0.47
 Abbott 16 0.49

aPTT, s (P < .001)
 BioMerieux Platelin L 12 21
 Dade Behring Actin FS 10 22
 HemosIL/IL Test APTT-SP 16 23
 Dade Behring Actin FSL 40 24
 Diagnostica Stago STA-PTT A 24 24
 HemosIL/Hemoliance SynthASiL 14 25

* TSH indicates thyroid-stimulating hormone; aPTT, activated partial
thromboplastin time.
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Title Annotation:CAP Laboratory Improvement Programs
Author:Friedberg, Richard C.; Souers, Rhona; Wagar, Elizabeth A.; Stankovic, Ana K.; Valenstein, Paul N.
Publication:Archives of Pathology & Laboratory Medicine
Article Type:Clinical report
Geographic Code:1USA
Date:Mar 1, 2007
Words:8479
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