The detection and measurement of microalbuminuria: a challenge for clinical chemistry.
To earn CEUs, see test on page 18.
1. Describe three forms of urinary albumin.
2. State three quantitative definitions of microalbuminuria (MA).
3. Discuss two benefits of screening for MA in diabetic and nondiabetic patients.
4. Explain the reaction mechanisms for one dye-based dipstick and one immunoassay method of testing for MA.
5. Assess the potential advantages of a high-performance liquid chromatography (HPLC) method for measurement of MA.
Albumin can be detected in the urine in very small amounts in "normal," healthy people. The term microalbuminuria (MA) refers to a range of urinary albumin excretion (UAE) that is above "normal" levels but below amounts referred to as macroalbuminuria or proteinuria, which indicate overt kidney damage. MA is found in 6% of the general population. Of the people with MA, 63% have hypertension, 38% have chronic kidney disease, and 26% are diabetic. (1) MA is an important risk factor for kidney and for cardiovascular disease (CVD) in persons with and without diabetes. Therefore, MA should be detected and measured at high levels of sensitivity and specificity* so that diabetic nephropathy and CVD can be treated as soon as this risk factor appears. Preventing, delaying, or reducing albuminuria is a key therapeutic goal for kidney and cardiovascular protection.
There is no consensus on why albuminuria is an independent risk marker, but there are several likely factors. One is that albumin leakage indicates a general vascular dysfunction, particularly to the blood vessel walls. (2) A second is that vascular inflammation may be caused by the leakage, further damaging the blood vessel walls. (3)
Regardless of the mechanism, there is a growing need to educate the lab community on the demonstrated value of microalbumin detection. Only 21.5% of privately insured individuals who are at risk for kidney disease get tested. (4) In managed care environments, there is only about a 50% screening-compliance rate. (5)
This brief paper speaks to the need for the early identification of MA as a risk factor in diabetic nephropathy and CVD, then it reviews conventional and new albumin tests.
Microalbumin as a risk factor
For diabetic nephropathy
Approximately 13 million individuals in the United States have been diagnosed with diabetes mellitus, (6) and this number could increase to about 14.5 million by 2010 and to about 17.4 million by 2020. (7) Type 1 and type 2 diabetes are the most prevalent forms of diabetes mellitus,** with type 1 affecting 5% to 10% and type 2 affecting 90% to 95% of Americans who are diagnosed with diabetes. (6)
Nephropathy is a major complication of diabetes. The gradual and progressive kidney damage that occurs in diabetic nephropathy is reflected in an increasing UAE, which is detected initially as persistent MA and subsequently as persistent macroalbuminuria (also called proteinuria or clinical albuminuria). (8-10) MA indicates that kidney damage is occurring in spite of UAE being above normal and below pathological levels of kidney functioning. (8,10-14) Consequently, MA is said to indicate "incipient" nephropathy. (14) On the other hand, macroalbuminuria indicates "overt" nephropathy, or kidney failure, which can eventually culminate in end-stage renal disease (ESRD***).
The overall prevalence of MA in persons with a diagnosis of diabetes is 30% to 40%. (8,14-16) MA is uncommon in type 1 diabetes during the first five to 10 years after its diagnosis and is common in type 2 diabetes when it is diagnosed because this form of diabetes often exists undiagnosed for a number of years. (17) Once persistent MA is detected in type 1 or type 2 diabetes, its excretion increases at 4% to 9% per year, with overt nephropathy generally occurring in six to 12 years. (11,14) Overt nephropathy develops in 25% to 50% of individuals with diabetes, especially if hyperglycemia and elevated blood pressure are not controlled, and is aggravated by continued hyperglycemia and development of nephropathy-related hypertension. (10,13,17-19) Not only is diabetic nephropathy the leading cause of ESRD, but ESRD has been increasing in prevalence in the past 10 years due to a rise in the incidence of type 2 diabetes. (14,19)
For CVD in hypertensive patients
In people older than 45 years with stage 2 or higher hypertension, MA seems to be strongly associated with several traditional and nontraditional cardiovascular risk factors and with target-organ damage. (20) Several studies have suggested that MA occurs in about 30% of patients with mild or moderate hypertension, ranging from 7% to 40% depending on age and ethnic group. (21,22)
Several retrospective and cross-sectional studies have reported that the prevalence of CVD is significantly higher among hypertensive patients with MA than hypertensive patients without MA. In a large cross-sectional study (23) of 11,343 nondiabetic hypertensive patients, those with MA had a significantly higher prevalence of:
**** coronary artery disease (31% vs. 22%);
**** left ventricular hypertrophy (24% vs. 14%);
**** previous stroke (6% vs. 4%); and
**** peripheral vascular disease (7% vs. 5%).
In the patients with MA and CVD, the amount of albumin in the urine was also significantly higher than in those who did not present with CVD.
For CVD in the general population
In several studies involving large populations, MA has also been established as a strong, independent risk factor for CVD both in persons who have and do not have a diagnosis of diabetes. For example:
**** A study of individuals in community and academic practices in Europe, North America, and South America found that the presence of MA, defined as a urinary albumin: creatinine ratio (ACR) of [greater than or equal to]2 mg/mmol, was associated with a relative risk (RR) of 1.75 for cardiovascular events (myocardial infarction, stroke, cardiovascular death), 1.92 for all-cause mortality, and 2.42 for hospitalization for congestive heart failure, after adjusting for other risk factors. Moreover, RRs were similar in groups with and without diabetes. (24) For every 0.4-mg/mmol increase in the ACR level, the adjusted hazard for a cardiovascular event increased by 5.9%. Notably, this increasing risk started at urine albumin concentrations as low as 0.5 mg/mmol, well below the currently accepted detection thresholds for a diagnosis of MA.
**** A study involving a Danish population that did not have a diagnosis of diabetes found that the presence of MA, defined as an ACR in the upper 10% range, or >0.65 mg/mmol, was associated with an RR of 2.3 for ischemic heart disease, independent of other risk factors for atherosclerosis. (25) Also, MA presence more than doubled the predictive effect of these other risk factors.
**** A study of a general population in the Netherlands found that the greater the UAC (urinary albumin concentration), the greater the risk of both cardiovascular and noncardiovascular mortality after adjustment for other well-recognized risk factors. (26) A two-fold increase in UAC (e.g., from 5 to 10 mg/L, or from 20 to 40 mg/L) was associated with an RR of 1.29 for cardiovascular mortality and an RR of 1.12 for noncardiovascular mortality. Again, the relationship between UAC and cardiovascular and noncardiovascular mortality was already apparent at UAC levels that are currently considered normal.
**** A study that appeared in the American Journal of Kidney Diseases (15) generated national estimates of the prevalence of MA in the U.S. population. The prevalence of MA (urinary ACR, 30 to 299 mg/g) was 6.1% in males and 9.7% in females. MA was common even among persons without diabetes or hypertension: 5.1% of those without diabetes, hypertension, CVD, or elevated serum creatinine levels had MA. The numbers increased starting at 40 years of age, and were greater in non-Hispanic blacks and Mexican Americans aged 40 to 79 years compared to similar-aged non-Hispanic whites.
MA appears to reflect microvascular disease and macrovascular disease that not only affect the kidneys, but also the cardiovascular system. Consequently, MA is a risk factor for CVD even at levels that are below the normally accepted range for incipient nephropathy. (12,24,26) MA has joined other major, independent risk factors for CVD, including hyperglycemia, hypertension, hypercholesterolemia, and smoking, that may or may not be linked with causal processes. (24,26)
Testing and treatment guidelines
The American Diabetes Association has recommended a range for MA for various modes of urine collection (see Table 1). (27,28) A first void or early-morning spot collection is generally preferred because of its convenience to the patient, a diurnal variation in UAE, and a higher urine concentration that enables easier detection of MA. (8,14,27,28) The ACR corrects for variations in volume of urine output, (8,28) and usually predicts 24-h UAE accurately. (8)
The American Diabetes Association also has recommendations for when screening for MA should be performed on persons with the diagnosis of type 1 or type 2 diabetes. (27, 28) Because MA rarely occurs with type 1 diabetes of short duration, individuals with type 1 diabetes should be tested annually, beginning five years after the time of diagnosis. Because precise dating of onset of type 2 diabetes is usually not possible, persons with type 2 diabetes should be tested annually, beginning at the time of diagnosis.
Several studies have shown that if MA is detected in the initial stages, the onset of kidney disease and CVD can be slowed, halted, and--in some cases--reversed with common blood-pressure drug therapies. (10,29) Effective treatment involves control of blood pressure as well as hyperglycemia; elevated blood pressure can be a contributor to and a consequence of diabetic nephropathy, as well as a risk factor for CVD in persons with and without diabetes. MA usually precedes an elevation of blood pressure in type 1 diabetes and is commonly associated with a moderate blood-pressure elevation even when type 2 diabetes is diagnosed. (9,11) Reviews of the numerous published studies of the effectiveness of drugs in the treatment of diabetic nephropathy have concluded that ACE (angiotensin-converting enzyme) inhibitors have a renoprotective effect in type 1 and type 2 diabetes, that ARBs (angiotensin receptor blockers) have a renoprotective effect in type 2 diabetes, that ARBs may be proven also to have a renoprotective effect in type 1 diabetes, and that the renoprotective effect of ACE inhibitors and ARBs is independent of their blood-pressure-lowering effect. (9,10,19,29) A study conducted by Dr. de Zeeuw and colleagues reports that cardiovascular protection in diabetics can be explained by the therapy's antiproteinuric effect as well, irrespective of changes in all other risk factors. (30) Monitoring lab-test readings after the initial MA diagnosis is, thus, an effective way to evaluate drug efficacy.
The American Diabetes Association has recommended use only of ACE inhibitors or ARBs for MA or macroalbuminuria in type 1 and type 2 diabetes, while recognizing that use of ARBs in type 1 diabetes lacks investigational support. (27,28) It has also recommended that if one of these classes of drugs is not tolerated, the other class should be substituted.
Conventional tests for MA
Various semiquantitative dipstick tests, which involve wetting a chemically impregnated test strip with sample urine, have been used in the office setting for MA detection. If a test is positive for MA, the result can be confirmed and the MA accurately quantified by various laboratory methods. In fact, these laboratory methods have also been used to evaluate the accuracy of the dipstick tests.
One example of a dipstick test for MA is the Clinitek Microalbumin Reagent Strip (Bayer Corp., Elkhart, IN).
In this test, albumin binds to a sulfonephthalein dye, and creatinine forms a copper-creatinine complex with peroxidase-activity that catalyzes the reaction of diisopropyl-benzene dihydroperoxide and 3,3'5,5'-tetramethylbenzidine. (31,32) Both reactions produce colors that are read reflectometrically in a Clinitek 50 portable urine chemistry analyzer and reported as albumin concentrations of 10, 30, 80, or 150 mg/L, creatinine concentrations of 0.9, 4.4, 8.8, 17.7, or 26.5 mmol/L (10, 50, 100, or 200 mg/dL), and as an ACR <30 mg/g, 30 to 300 mg/g, or >300 mg/g. Evaluations comparing this test to reference laboratory testing have shown that:
**** In a total of 144 urine samples from individuals with diabetes and/or renal disease, and with an upper limit of normal of <20 mg/L for albumin concentration, this test gave a sensitivity of 95.4% and a specificity of 78.9%. (31)
**** In a total of 302 urine samples from consecutive patients with diabetes, and with an upper limit of normal of <30 mg/g for ACR, this test gave a sensitivity of 79% and a specificity of 81%. (33)
**** In a total of 200 urine samples from children, adolescents, and young adults with type 1 diabetes, and with an upper limit of normal of <30 mg/L for albumin concentration, this test gave a sensitivity of 89% and a specificity of 73%. (32)
Another example of a dipstick test for MA is the Micral-Test II test strip (Boehringer Mannheim, Indianapolis, IN). In this test, albumin passes via a wick fleece into a conjugate fleece, where it binds specific, gold-labeled antibodies, and then flows to a detection pad. (31,34) A chemical reaction in the detection pad produces a color that is visually compared to color blocks, with colors representing albumin concentrations of 0, 20, 50 and 100 mg/L. Evaluations comparing this test to reference laboratory testing have shown:
**** In a total of 2,228 urine samples from diabetic patients, and with an upper limit of normal of <20 mg/L for albumin concentration, this test gave a sensitivity of 96.7% and a specificity of 71%. (35)
**** In a total of 411 urine samples from consecutive patients with diabetes, and with an upper limit of normal of <20 mg/L for albumin concentration, this test had a sensitivity of 93% and a specificity of 93%. (34)
**** In a total of 96 urine samples from individuals with diabetes and/or renal disease, and with an upper limit of normal of <20 mg/L for albumin concentration, this test gave a sensitivity of 97.1% and a specificity of 33.3%. (31)
Three laboratory methods--immunonephelometry, immunoturbidimetry, and RIA (radioimmunoassay)--have been used for the confirmation and measurement of MA. The performance characteristics of these methods are listed in Table 2.
**** Immunonephelometry: Albumin in the urine sample comes into contact with antibody to human albumin to produce an antigen-antibody reaction. An increase in light scatter from this reaction is analyzed optimetrically to provide MA concentration.
**** Immunoturbidimetry: Albumin in the urine sample and human albumin bound to latex particles compete for a monoclonal antibody, which aggregates the latex particles. Consequently, the amount of aggregation that results is in inverse proportion to the amount of albumin in the sample. The aggregation amount is measured optimetrically and converted to provide MA concentration.
**** RIA: Albumin in the urine sample displaces isotopically labeled human albumin that has been bound to an antibody to it. Consequently, the amount of labeled albumin that remains bound to the antibody is in inverse proportion to the amount of albumin in the sample. The "free" and "bound" labeled albumin can be separated in several ways for radioactivity measurement. Radioactive counts are compared to a calibration or standard curve to provide MA concentration.
Notably, comparisons of these laboratory methods for detecting and measuring albumin in the urine of persons with diabetes have demonstrated that the results from these methods can vary considerably from one another. In one study, immunonephelometry gave values that were approximately three-fold lower than immunoturbidimetry, meaning that an albumin concentration of about 30 mg/mL (MA range) with this immunoturbidimetry would only register as about 10 mg/mL (normoalbuminuric range) with immunonephelometry. (36) In other studies, RIA gave values that were 1.4-fold lower than immunonephelometry and over six-fold lower than immunoturbidimetry, (37) and immunonephelometry gave values that were 1.6-fold lower than immunoturbidimetry. (38)
The American Diabetes Association has recommended that if a laboratory is not readily available to screen for MA, dipstick testing may be used since it shows "acceptable sensitivity (95%) and specificity (95%) when carried out by trained personnel." (28) The National Academy of Clinical Biochemistry, however, has recommended that the sensitivity of qualitative or semiquantitative dipstick testing exceed 95% in order to minimize the false-negative rate and, consequently, the need to confirm negative as well as positive-negative results by a laboratory method. (39) Moreover, the Academy has suggested that this testing be based upon an upper limit of normal of 20 mg/L for albumin concentration to ensure detection of MA as measured by laboratory methods.
Data from evaluations of the two dipstick tests described above indicate that such tests may not fulfill efficacy requirements for detecting the early appearance of MA as a risk factor for diabetic nephropathy when its concentration is near the lower end of its recommended range. Moreover, these tests do not appear to fulfill the requirements for detecting MA as a risk factor for CVD at a concentration below the lower end of this range. Conventionally, then, precise detection and measurement of these critical risk factors could be assured only with laboratory testing.
Recently, however, as reported below, studies have found that dye-based and immunologically based dipstick tests and immunologically based laboratory methods have not been detecting and measuring all of the intact albumin in the urine, thus increasing their potential for giving false-negative results for MA. HPLC (high-performance liquid chromatography) substantially decreases this potential for misdetection and mismeasurement of MA, particularly in the urine of individuals with diabetes mellitus, by detecting and measuring all intact urinary albumin.
Researchers have recently made four important discoveries that force reconsideration of how incipient diabetic nephropathy can be diagnosed at the earliest possible stage of its development. First, albumin is excreted in the urine as a complex mixture of components, including immuno-reactive intact albumin, albumin fragments, polymer albumin aggregates, and immuno-unreactive intact albumin. (40-43) Second, immuno-unreactive albumin increases in incipient diabetic nephropathy. (44,45) Third, HPLC detects both immuno-reactive and immuno-unreactive intact albumin, whereas dye-based and immunologically based dipstick tests and immunologically based laboratory methods detect only immuno-reactive intact albumin fragments <12 kDa and polymer albumin aggregates. (36,44,45) Fourth, dye-based and immunologically based dipstick tests and immunologically based laboratory methods have been substantially underestimating urinary albumin concentrations in persons with diabetes, resulting in significant lag times for the diagnosis and treatment of incipient diabetic nephropathy. (36,46,47) The benefit of using HPLC rather than a conventional dipstick test or laboratory method for detecting MA in individuals with diabetes was demonstrated by two recently reported studies:
**** False-negative rates for the detection of MA (ACR [greater than or equal to]30 mg/g) by the Clinitek Microalbumin Reagent Strip versus HPLC were determined for urine samples from a group of 115 patients with diabetes and a group of 106 volunteers without diabetes. (46) The false-negative rate for the samples from the group with diabetes was 42.9%.
**** False-negative rates for the detection of MA (ACR [greater than or equal to]30 mg/g) by immunoturbidimetry versus HPLC were also determined for the urine samples from the group of 115 patients with diabetes and the group of 106 volunteers without diabetes. (46) The false-negative rate for the samples from the group with diabetes was 36.3%. Since the urine samples from the volunteer group would not be expected to contain immuno-unreactive albumin to be detected by HPLC, the false-negative rate of this group was 0%.
**** The differential lead time for the detection of MA (AER [greater than or equal to]20 [micro]g/min) with HPLC and conventional RIA was determined on groups of patients with type 1 and type 2 diabetes. (47) Analysis was performed on 511 urine samples collected over a 13-year period from 42 patients with type 1 diabetes, 17 of whom progressed from normoalbuminuria to MA, and 25 of whom continued to have normoalbuminuria. The mean lead time for HPLC versus RIA for these patients was 3.9 years (95% confidence interval of 2.1 to 5.6 years). Analysis was also performed on 634 urine samples collected over the same period from 49 patients with type 2 diabetes, 24 of whom progressed from normoalbuminuria to MA, and 25 of whom continued to have normoalbuminuria. The mean lead time for HPLC versus RIA for these patients was 2.4 years (95% confidence interval of 1.2 to 3.5 years).
The comparative effectiveness of HPLC to other conventional tests in the detection and measurement of MA as a risk factor for CVD remains to be determined. MA in individuals with and without diabetes appears to reflect a widespread vasculopathy that manifests as an increased renal endothelial permeability for albumin. (12,24-26) Therefore, unless biochemical processing of filtered albumin is different in persons with and without diabetes, one would expect that both immuno-reactive albumin and immuno-unreactive albumin would be excreted as risk markers for CVD as well as diabetic nephropathy.
Whether or not both immuno-reactive albumin and immuno-unreactive albumin are excreted in individuals at increased risk for CVD, the case can still be made for use of HPLC in the detection and measurement of MA as a risk factor for CVD. First, HPLC performs well, having an interassay coefficient of 2.4% at a urinary albumin level of 95.8 mg/L, and an albumin detection limit of 2 mg/L. (36) Second, HPLC measurement of MA has been shown to be invariably higher than that measured by RIA in patients with mild diabetes (48) and also higher when compared with immunonephelometry measurement in a community survey. (49)
Third, the low albumin-detection limit of HPLC appears crucial to detecting and measuring MA as a risk factor for CVD below the lower end of the MA range for the diagnosis of incipient nephropathy. (12,24,26)
Conventional dipstick testing with laboratory confirmation has been viewed as a cost-effective means of screening for MA in persons with diabetes. (11,50-52) Little attention has been given, however, to the negative impact of the dipstick test failing to detect MA in a substantial number of individuals with diabetes (false negatives). (36,39) These "missed" individuals are placed at increased risk of ESRD, which costs them greatly in quality and quantity of life and costs the healthcare system at least $37,000 per individual per year in the U.S., based on 2002 data. (53) The healthcare cost savings of early medical intervention, including drug therapy, to slow or stop progression of ESRD are quite apparent in light of the fact that in the year 2000, treatment for ESRD was initiated on over 41,000 Americans with diabetes, (54) and medication has been shown to be cost-effective. (55) Therefore, as with any dipstick test or laboratory method for the detection and measurement of MA, consideration of the cost benefits of the test must center on its ability to detect and measure MA at very low levels, especially when MA first appears as a risk factor for diabetic nephropathy.
CE test on THE DETECTION AND MEASUREMENT OF MICROALBUMINURIA: A CHALLENGE FOR CLINICAL CHEMISTRY
MLO and Northern Illinois University (NIU), DeKalb, IL, are co-sponsors in offering continuing education units (CEUs) for this issue's article on THE DETECTION AND MEASUREMENT OF MICROALBUMINURIA: A CHALLENGE FOR CLINICAL CHEMISTRY. CEUs or contact hours are granted by the College of Health and Human Sciences at NIU, which has been approved as a provider of continuing education programs in the clinical laboratory sciences by the ASCLS P.A.C.E.[R] program (Provider No. 0001) and by the American Medical Technologists Institute for Education (Provider No. 121019; Registry No. 0061). Approval as a provider of continuing education programs has been granted by the state of Florida (Provider No. JP0000496), and for licensed clinical laboratory scientists and personnel in the state of California (Provider No. 351). Continuing education credits awarded for successful completion of this test are acceptable for the ASCP Board of Registry Continuing Competence Recognition Program. After reading the article on page 8 answer the following test questions and send your completed test form to NIU along with the nominal fee of $20. Readers who pass the test successfully (scoring 70% or higher) will receive a certificate for 1 contact hour of P.A.C.E.[R] credit. Participants should allow four to six weeks for receipt of certificates.
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Learning Objectives and CE questions prepared by Sharon M. Miller, professor emeritus, Northern Illinois University, DeKalb, IL.
1. Albumin excreted in the urine is described by all of the following EXCEPT that it
a. exists solely as an intact, undamaged molecule.
b. is a complex mixture of components.
c. consists of immunochemically reactive and unreactive forms.
d. includes polymeric aggregates of albumin.
2. Microalbuminuria (MA) is present if urinary albumin excretion is
a. 30 to 299 mg/24 hours.
b. 20 to 199 [micro]g/minute on a timed specimen.
c. 30 to 299 mg/g creatinine on a random spot collection.
d. Any of the above.
3. In the United States, the prevalence of MA among individuals without known risk factors is
b. less than 10%.
4. Among diagnosed diabetics, the overall prevalence of MA is
a. 90% to 95%.
b. 75% to 85%.
c. 50% to 60%.
d. 30% to 40%.
5. Once persistent MA is detected in type 1 or type 2 diabetes, overt nephropathy usually occurs within how many years?
a. 1 to 3.
b. 4 to 5.
c. 6 to 12.
d. 15 to 20.
6. For diabetics and nondiabetics, MA is a strong predictor of (1) kidney damage; (2) polycystic ovarian syndrome; (3) cardiovascular disease; or (4) celiac disease.
a. 1 and 2.
b. 1 and 3.
c. 3 and 4.
d. All of the above.
7. Independent risk factors for cardiovascular disease include all of the following EXCEPT
8. MA nearly doubles a person's risk for cardiovascular disease.
9. Of patients with mild or moderate hypertension, approximately what percentage has MA?
10. Treatment with ACE (angiotensin-converting enzyme) inhibitors or ARBs (angiotensin-receptor blockers) can be effective in slowing the rate of urinary albumin excretion.
11. Annual MA screening of patients with type 1 diabetes is recommended by the American Diabetes Association beginning
a. immediately upon diagnosis.
b. three to six months after diagnosis.
c. one year after diagnosis.
d. five years after diagnosis.
12. Persistent MA is diagnosed if
a. two of three specimens collected within a three- to six-month period are abnormal.
b. two consecutive specimens collected over 48 hours are abnormal.
c. a 24-hour specimen collected after a one-week restricted-protein diet is abnormal.
d. two of three abnormal specimens are obtained after all blood-pressure medications have been stopped for 72 hours.
13. One of the dipstick tests described is based on albumin binding to the dye
b. indocyanin green.
14. Dye-based and immunologically based tests detect
a. immuno-reactive intact albumin and polymeric aggregates.
b. immuno-unreactive intact albumin.
c. both immunochemically reactive and unreactive intact albumin.
d. All protein fragments in the urine.
15. The technique with the highest detection level for albumin is
c. RIA (radioimmunoassay).
16. Immunochemically reactive and unreactive albumins are measured by
d. HPLC (high-performance liquid chromatography).
17. In incipient diabetic nephropathy, urinary excretion of immuno-unreactive albumin
c. stops completely.
d. remains unchanged.
18. In the detection of MA, a false-negative result is _____ likely when dipstick testing or immunoturbidimetry is used as compared with an HPLC-based method.
19. Overt nephropathy develops in what percentage of individuals with diabetes?
a. 65% to 80%.
b. 25% to 50%.
c. 10% to 20%.
d. Less than 10%.
20. Delayed identification of MA in a diabetic increases the patient's risk of eventually developing
a. chronic pulmonary disease.
b. retinitis pigmentosa.
c. progressive liver failure.
d. end-stage renal disease.
Table 1. Range for MA recommended by the American Diabetes Association (27,28) Category* Spot collection ([micro]g/mg creatinine) Normoalbuminuria <30 Microalbuminuria 30-299 Macroalbuminuria [greater than or equal to]300 Category* 24-h collection (mg/24 h) Normoalbuminuria <30 Microalbuminuria 30-299 Macroalbuminuria [greater than or equal to]300 Category* Timed collection ([micro]g/min) Normoalbuminuria <20 Microalbuminuria 20-199 Macroalbuminuria [greater than or equal to]200 *At least two of three urine specimens collected within a period of three to six months should be abnormal before a patient is considered to have MA. Table 2. Performance characteristics of immunonephelometry, immunoturbidimetry, and radioimmunoassay methods used for the detection and measurement of microalbuminuria in persons with diabetes (36) Method Inter-assay Coefficients Detection limit of Variation for albumin Immunonephelometry 4.2% at 12.1 mg/L 2 mg/L (Beckman Array Analyzer) 5.3% at 45 mg/L Immunoturbidimetry 4.1% at 10.6 mg/L 6 mg/L (Dade-Behring Turbidimeter) 2.2% at 77.9 mg/L Radioimmunoassay 9.2% at 12.2 mg/dL 16 [micro]g/L 4.8% at 33 mg/L
* The sensitivity (true-positive rate) of a test for MA is its ability to detect those who are known to have MA (expressed as true positives/true positives plus false negatives). The specificity (true-negative rate) of this test is its ability to detect those who are known not to have MA (expressed as true negatives/true negatives plus false positives). The false-negative rate is equal to one minus the sensitivity, and the false-positive rate is one minus the specificity.
** Type 1 diabetes, formerly called insulin-dependent diabetes or juvenile diabetes, is due to an absolute insulin deficiency. It most commonly occurs in childhood or adolescence. Type 2 diabetes, formerly called non-insulin-dependent diabetes or adult-onset diabetes, is due to insulin resistance and, usually, a relative insulin deficiency. It is associated with older age, obesity, inactivity, and race/ethnicity (e.g., Native American, African American, Hispanic/Latino). Because it develops gradually, it often goes undiagnosed for many years.
*** By definition, ESRD patients require renal dialysis or transplant.
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By Douglas E. Busby, MD, MSc, and Robert C. Atkins, MD
Douglas E. Busby, MD, MSc, is an independent medical writer and a longtime member of the American College of Preventive Medicine. Robert C. Atkins, MD, Professor of Medicine, Department of Nephrology, Monash Medical Center, Melbourne, Australia is past president of the International Society of Nephrology and serves on the Advisory Board of AusAm Biotechnologies. AusAm Biotechnologies has developed the Accumin Total Intact Albumin Assay, the only FDA-cleared diagnostic that uses high-performance liquid chromatography equipment.
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|Author:||Busby, Douglas E.; Atkins, Robert (American physician)|
|Publication:||Medical Laboratory Observer|
|Article Type:||Cover Story|
|Date:||Feb 1, 2005|
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