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Lipase method problems.

Q According to the clinicians at our facility, amylases and lipases performed on our new instrument, System B, seem to be less clinically reliable than those performed on our older instrument, System A, in cases of suspected pancreatitis. Some instances of false-positive lipase results have occurred. We have verified the accuracy of System B by internal studies and paired sample analyses at our reference laboratory. How do we convince our medical staff that System B is reliable and timely?

A Laboratories frequently come under heavy fire from clinicians when they change test methodologies. Sudden differences in units of measure, sensitivity and specificity of the assay, reference ranges, preanalytical variables, even sample type, become big issues for the end-user, and unexpected results focus their attention on "how much better it was in the good old days." For this reason, CLSI (formerly NCCLS) guidelines, (1) CLIA '88 regulations, (2) and prudent practice should lead laboratorians to follow a comprehensive protocol for new-test validation that addresses all of the issues that will be forthcoming when the assay is implemented.

When contemplating changing test instruments, an important issue for a lab to consider is how the new values will be reported compared to the old ones. To identify these differences without performing an experiment, consult the summary reports that accompany your laboratory's proficiency test results from CAP, AAB, AAFP, or whichever service you use to comply with CLIA '88 regulations. These reports tell you how all instruments and reagent systems tend to perform on the same samples. When the amylase and lipase results reported for System A instruments and System B instruments are compared, the System B amylase mean results are one-half the values of the System A results, and the System B lipase mean results are 50% to 300% higher than the System A results--no doubt due to the use of different substrates, measurement of different reaction products, and differing units of measure in the two test systems. That alone tells you that you face a terrific educational effort with the clinical staff, that you must conduct a reliable reference range study, and that you must prepare for user feedback and questioning.

Make sure that your reference range study is performed according to the guidelines established by our profession, notably CLSI. (3) A reference range based on the performance of the assay system in your particular laboratory on the population in your geographic area is extremely important in order to attach significance to values that are close to the upper reference limit (URL). Try to develop the concept of assessing the significance of these (and all) enzyme assay results in terms of "multiples of the URL." You should also document and publish your paired test quality-assurance study on the discrepancy cases so that your staff can come to believe your assay results and focus on other causes for the discrepancies.

You may want to compile a comprehensive list of known interfering substances, conditions, and circumstances for both tests so you can respond to queries about inconsistent and unexpected results. Such listings are available from the test system product manufacturers. Additionally, a reference I find immensely useful is Donald Young's three-volume series: Effects of Drugs on Clinical Laboratory Tests, Effects of Disease on Clinical Laboratory Tests, and Effects of Preanalytic Variables on Clinical Laboratory Tests. (4-6) No test has perfect sensitivity (ability to detect the disorder) and perfect specificity (lack of false positives); thus, to the extent that they are known, these factors should be available to the clinician so he can sort out the problematic cases.


1. NCCLS. Evaluation protocols; Specialty collection. NCCLS document SC1-L. Wayne, PA: NCCLS.

2. Code of Federal Regulations: 42 CFR Ch. IV (10-1-02 Ed), Subpart K--Quality Systems for Nonwaived Testing. Sec. 493, 1253. Standard: Establishment and verification of performance specifications. Available at: Accessed March 28, 2005.

3. NCCLS. How to define and determine reference intervals in the clinical laboratory; Approved guideline. 2nd ed. NCCLS document C28-A2. Wayne, PA: NCCLS; 2000.

4. Young DS. Effects of Drugs on Clinical Laboratory Tests. 5th ed. Washington, DC: AACC Press; 2000.

5. Young DS. Effects of Disease on Clinical Laboratory Tests. 4th ed. Washington, DC: AACC Press; 2001.

6. Young DS. Effects of Preanalytical Variables on Clinical Laboratory Tests. 2nd ed. Washington, DC: AACC Press; 1997.

--Louis Buettner, MD

Medical Director

Southern Diagnostic Laboratories Inc.

Birmingham, AL
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Title Annotation:Answering your questions
Author:Buettner, Louis
Publication:Medical Laboratory Observer
Geographic Code:1USA
Date:May 1, 2005
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