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Clinical Chemistry 51: 2385-2387, 2005; 10.1373/clinchem.2005.056499
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(Clinical Chemistry. 2005;51:2385-2387.)
© 2005 American Association for Clinical Chemistry, Inc.

Technical Briefs

Variation with Time in Components of Variance for Measurements of Therapeutic Drugs

John F. Wilson1,a and Kathleen Barnett2

1 Department of Pharmacology, Therapeutics and Toxicology, Wales College of Medicine, Cardiff University, Cardiff, Wales, United Kingdom;2 Cardiff Bioanalytical Services Ltd., Cardiff, Wales, United Kingdom

aaddress correspondence to this author at: Cardiff Bioanalytical Services Ltd., 16 Mount Stuart Square, Cardiff CF10 5DP, Wales, United Kingdom; fax 44-29-2048-9003, e-mail wilsonjf@cardiff.ac.uk

The first 20% of the full text of this article appears below.

Analyses for therapeutic drugs are commonly made with automated, high-throughput, multichannel instruments that require minimal operator intervention. Performance monitoring of these assays is accomplished by means of internal and external (proficiency testing) quality-control procedures. Proficiency test schemes have demonstrated that the within-laboratory sources of variation are more important than between-laboratory sources (1)(2)(3), and the College of American Pathologists laboratory improvement program showed that the within-laboratory variance doubled for samples measured 4 months apart compared with measurements made at the same time (2). The present study was designed to track this decrease in within-laboratory precision over time to provide insights into the possible sources of imprecision in routine clinical measurements of therapeutic drugs in serum.

A lyophilized proficiency test sample was prepared by adding midtherapeutic concentrations of 14 drugs to 5.9 L of human serum (Scipac Ltd.). Drug concentrations were as follows: phenytoin, 15.2 mg/L; phenobarbital, 30.3 mg/L; primidone, 7.1 mg/L; carbamazepine, 7.7 mg/L; carbamazepine 10,11-epoxide, 1.9 mg/L; ethosuximide, 68.1 mg/L; valproate, 76.8 mg/L; clonazepam, 42.0 µg/L; lamotrigine, 3.1 mg/L; theophylline, 15.0 mg/L; caffeine, 7.8 mg/L; digoxin, 1.2 µg/L; gentamicin, 2.7 mg/L; and lithium 0.76, mmol/L. The CV of dispensing of test sample aliquots by weight was 0.08%. We distributed 5 differently coded aliquots of the proficiency test sample, on 4 occasions, for analysis by members of the United Kingdom National External Quality Assessment scheme for drug assays. The scheme has an international membership of mostly hospital- or clinic-based sites, with 60% of their participants from the United Kingdom, 30% from Europe, and 10% outside Europe. A pair of samples was sent for analysis 1 month, and 3 single samples were sent at intervals to . . . [Full Text of this Article]






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