Witte, David L.

Abstract Objective.—To identify the sources of analytical variation for cyclosporine and tacrolimus in a 3-year longitudinal study. Design.—Two pools of whole blood were spiked with cyclosporine and tacrolimus, respectively. One aliquot of cyclosporine and 2 of the tacrolimus pool were distributed in the first and last mailing for years 1999 to 2001. For both drugs, the total variance for each method was partitioned into within- and between-laboratory components. Setting.—The A and C mailings of the 1999, 2000, and 2001 AACC/CAP [American Association for Clinical Chemistry/College of American Pathologists] Immunosuppressive Drugs (CS) Monitoring Survey. Main Outcome Measures.—For each drug, total variance was partitioned into specimen, mailing, year, and interlaboratory effects for each analytical method. Participants.—The 292 laboratories for cyclosporine and 177 laboratories for tacrolimus enrolled in the survey from 1999 to 2001. Results.—For both cyclosporine and tacrolimus, the major source of imprecision came from within-laboratory factors, which accounted for nearly 85% (range, 77% to 90%) of the total variance. For cyclosporine, the major component of within-laboratory variance was between-mailing, within-year effect, whereas for tacrolimus it was the between-year, within-laboratory variation. Conclusion.—The major source of long-term survey imprecision for cyclosporine and tacrolimus is within-laboratory factors. The finding that 85% of the total variance was due to within-laboratory variation is similar to other therapeutic drugs.

Abstract Objective.—To determine if the levels of imprecision of the commonly used analytic methods for drug measurements are suitable for long-term therapeutic drug monitoring. Design.—In 1996, 4 identical lyophilized samples (2 in the first mailing and 2 in the second mailing 4 months later) were sent to laboratories participating in a nationwide proficiency testing program. Similarly, in 1999, replicates from a liquid pool of spiked sera were mailed 3 times, 4 months apart, to participating laboratories. For each of 11 drugs regulated under the Clinical Laboratory Improvement Amendments of 1988 and 1 metabolite, the total variance for each method was partitioned into within- and between-laboratory components. The total within-laboratory and the total survey coefficients of variation (CVs) for each method were then compared with the “acceptable” precision criteria of Glick, Burnett, and Fraser for each drug. Setting.—The first 2 mailings of the College of American Pathologists Therapeutic Drug Monitoring surveys for 1996, sets Z and ZM, and the 3 mailings of 1999, sets ZM, Z, and Z2. Main Outcome Measures.—For each drug studied, the CV of each method was compared with the various imprecision criteria, and if greater than any of the criteria, the method was then tabulated as not meeting that specific criterion. Participants.—The approximately 5000 participants of the survey. Results.—The number of methods deemed as not having acceptable total long-term within-laboratory precision by the various criteria ranged from 35% to 88% in 1996 and from 22% to 77% in 1999. Conclusion.—The number of failures possibly indicates that many of the reagent assays being utilized are not precise enough for long-term therapeutic drug monitoring of chronically administered drugs or that the published criteria used to evaluate the data in this study are too stringent.

Abstract 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.

Abstract Context. —Large disparities in prostate-specific antigen (PSA) results from different assays have been observed in the College of American Pathologists (CAP) Ligand Assay Survey, with interassay results varying severalfold. Survey specimens are predominately composed of free PSA and do not reflect the composition of typical patient specimens. Objectives. —To characterize a pilot material developed for CAP in which pooled sera samples were spiked with purified PSA and α 1 -antichymotrypsin–bound PSA at targeted concentrations and to compare it to CAP survey and reference materials. Design. —CAP survey, reference, and pilot materials were analyzed using 10 total PSA and 7 free PSA assays. These assays included Food and Drug Administration–approved assays and assays for research use only. Results. —Variability among the 10 total PSA methods was greatest for the 1997 ligand survey material (CV range, 56%–65%) followed by the pilot material (CV range, 10%–29%) and the reference material (CV range, 6%–13%). In contrast, interassay variability for the 7 free PSA methods was similar for the 3 preparations, with the exception of one specimen close to the limit of detection of the assays. As determined with the Hybritech Tandem-R method, the ligand survey specimens were essentially composed of all free PSA, whereas the reference and pilot materials were composed of approximately 10% and 35% free PSA, respectively. Conclusions. —The newly formulated pilot material prepared using a human base that contained defined concentrations of free PSA and α 1 -antichymotrypsin–bound PSA more closely resembled patient specimens and minimized differences among methods compared with the semen-supplemented original survey material.