Evaluation of commercial assays for vancomycin and aminoglycosides in serum: a comparison of accuracy and precision based on external quality assessment

Objective: To compare the accuracy and precision of commercial assay techniques in the measurement of gentamicin, tobramycin, amikacin, netilmicin and vancomycin in serum. Methods: Data from the measurement of 40 external quality assessment samples from 358 laboratories providing a therapeutic drug monitoring service were analysed. Results: Significant differences between techniques in accuracy and precision were observed for all drugs. Coefficients of variation ranged from 4.1% to 9.8% for the aminoglycosides and from 6.7% to 11.7% for vancomycin. The percentage difference in measurements from the weighed-in drug concentration ranged from -10.1% to +4.0% for the aminoglycosides and from -3.5% to +5.7% for vancomycin. The Dade Behring Emit immunoassay was notable in producing significantly more outliers (>4 s.d. from the weighed-in concentration) than other techniques in the measurement of gentamicin, amikacin and vancomycin. Conclusions: All assays performed to a satisfactory standard for measurement in non-renal patients, but none met the more stringent standards desirable for monitoring patients with renal impairment.