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Clinical Chemistry 48: 1963-1969, 2002;
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(Clinical Chemistry. 2002;48:1963-1969.)
© 2002 American Association for Clinical Chemistry, Inc.

Primary Standardization of Assays for Anticonvulsant Drugs: Comparison of Accuracy and Precision

John F. Wilson1a, Ian D. Watson2, John Williams1, Pat A. Toseland3, Alison H. Thomson4, Grace Sweeney5, Brian L. Smith6, Lance N. Sandle7, John D. Ramsey8 and Nigel E. Capps9

1 Department of Pharmacology, Therapeutics & Toxicology, University of Wales College of Medicine, Cardiff CF14 4XN, United Kingdom.

2 Department of Clinical Biochemistry, University Hospital Aintree, Liverpool L9 7AL, United Kingdom.

3 18 London Road, Burgess Hill RH15 8QX, United Kingdom.

4 Pharmacy Department and Department of Medicine & Therapeutics, Western Infirmary, North Glasgow Hospitals University NHS Trust, Glasgow G11 6NT, United Kingdom.

5 Department of Bacteriology, Southern General University Hospitals NHS Trust, Glasgow G51 4TF, United Kingdom.

6 Department of Pathology, Maudsley Hospital, London SE5 8AZ, United Kingdom.

7 Department of Chemical Pathology, Trafford General Hospital, Manchester M41 5SL, United Kingdom.

8 TICTAC Communications Ltd., St. George’s Hospital Medical School, London SW17 0RE, United Kingdom.

9 Department of Pathology, Princess Royal Hospital NHS Trust, Telford TF6 6TF, United Kingdom.

aAuthor for correspondence. Fax 44-29-2074-8316; e-mail wilsonjf{at}cardiff.ac.uk.

Background: The accuracy and precision of methods for the measurement of the anticonvulsants phenytoin, phenobarbital, primidone, carbamazepine, ethosuximide, and valproate in human serum were assessed in 297 laboratories that were participants in the United Kingdom National External Quality Assessment Scheme (UKNEQAS).

Methods: We distributed lyophilized, serum-based materials containing low, medium, and high weighed-in concentrations of the drugs. The 297 participating laboratories received the materials on two occasions, 7 months apart. Expected concentrations were determined by gas chromatography or HPLC methods in five laboratories using serum-based NIST reference materials as calibrators.

Results: In general, bias was consistent across concentrations for a method but often differed in magnitude for different drugs. Bias ranged from -1.9% to 8.6% for phenytoin, -2.7% to 3.1% for phenobarbital, -2.7% to 0.5% for primidone, -8.6% to 0.3% for carbamazepine, -5.6% to 2.0% for ethosuximide, and -7.2% to 0.1% for valproate. Intralaboratory sources of imprecision significantly exceeded interlaboratory sources for many drug/method combinations. The mean CVs for intra- and interlaboratory errors for the different drugs were 6.3–7.8% and 3.3–4.2%, respectively.

Conclusions: For these long-established and relatively high-concentration analytes, the closed analytical platforms generally performed no better than open systems or chromatography, where use of calibrators prepared in house predominated. To improve the accuracy of measurements, work is required principally by the manufacturers of immunoassays to ensure minimal calibration error and to eliminate batch-to-batch variability of reagents. Individual laboratories should concentrate on minimizing dispensing errors.




The following articles in journals at HighWire Press have cited this article:


Home page
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J. F. Wilson and K. Barnett
Variation with Time in Components of Variance for Measurements of Therapeutic Drugs
Clin. Chem., December 1, 2005; 51(12): 2385 - 2387.
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