Sources of variability: a College of American Pathologists Therapeutic Drug Monitoring study.
OBJECTIVE: To determine the magnitudes and sources of analytic variation in testing for therapeutic drugs. Specifically, among laboratories using the same analytic method, to compare the within-laboratory variation (including both short- and long-term variation) with the between-laboratory variation. DESIGN: Four identical challenges were prepared from a lyophilized pool of spiked sera and were sent in pairs 4 months apart to laboratories participating in a nationwide proficiency-testing program. For each of 25 drugs, the variability in reported results from laboratories using the same method was investigated using nested analysis of variance. SETTING: The first 2 mailings of the College of American Pathologists Therapeutic Drug Monitoring Survey, 1996, sets Z and ZM. MAIN OUTCOME MEASURES: For each drug, total variance was partitioned into within- and between-laboratory components for common methods. The within-laboratory component was further partitioned into short- and long-term components. PARTICIPANTS: The approximately 5000 laboratories enrolled in the survey. RESULTS: For the 25 drugs, the average percentages of the total variance due to short-term, within-laboratory variance; long-term, within-laboratory variance; between-laboratory variance; and total laboratory variance were 25.0% (range, 8.8--50.6%), 57.8% (35.3--73.7%), 17.3% (5.0--35.4%), and 82.7% (64.6--95.0%), respectively. CONCLUSION: For all drugs tested, the within-laboratory component of variance was greater than the between-laboratory component of variance. Within laboratories, the magnitude of the long-term component was generally greater than the magnitude of the short-term component. This information will be helpful in determining the clinical utility of various drug assays and in evaluating the appropriateness of regulations involving therapeutic drug testing.
Steele, BW; Wang, E; Palomaki, G; Klee, GG; Elin, RJ; Witte, DL
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