Clinical Chemistry, Vol 37, 656-661, Copyright © 1991 by American Association for Clinical Chemistry

Laboratory process specifications for assuring quality in the U.S. National Cholesterol Education Program

JO Westgard, PH Petersen and DA Wiebe
Department of Pathology and Laboratory Medicine, Medical School, University of Wisconsin, Madison 53792.

We have assessed the laboratory specifications necessary for ensuring that cholesterol testing processes satisfy the quality required by the U.S. National Cholesterol Education Program (NCEP). A model for setting process specifications has been developed to relate the NCEP guidelines for medical interpretation of a cholesterol test to the pre-analytical and analytical variables that can affect a test result. Using this model, we derived specifications for the imprecision (coefficient of variation, CV, or standard deviation, s) and inaccuracy (bias) that are allowable under stable operation, as well as the quality-control procedures (control rules and number of control measurements) that are necessary to detect unstable operation. The NCEP goals of an allowable CV less than or equal to 3% and an allowable bias no greater than +/- 3% are inadequate for assuring the quality of an individual or single cholesterol test when monitoring performance with many of the statistical quality-control procedures currently used in the U.S. With quality-control procedures having two control measurements per run, a CV of 3% is allowable only when bias is zero; a CV less than or equal to 2% is necessary if bias is +/- 3%. With quality-control procedures having four control measurements per run, a CV of 3% is allowable when bias is +/- 1.5%; a CV less than or equal to 2.5% is required if bias is as large as +/- 3%. For two serial tests, the NCEP 3% goals are adequate for current quality-control procedures having four control measurements per run.