Regional Antimicrobial Reference Laboratory, Department of Microbiology, Southmead Hospital, North Bristol NHS Trust, Bristol BS10 5NB, UK
Sir,
That samples for antibiotic assay should not be taken from the line through which the antibiotic is administered is a self-evident rule1 which gets broken regularly. Clinicians are reluctant to insert unnecessary needles, especially into very young or old patients, and often time constraints dictate that the quickest procedure be followed. Samples taken through the line may give falsely elevated results which, if not pharmacologically impossible, may result in inappropriate dosage reduction. Clinicians may, however, be unaware of other implications for the testing laboratory, as illustrated by the case below where teicoplanin assays were, by chance, performed with an almost new kit.
A pair of routine serum samples (one taken before and one after drug administration) was received for teicoplanin assay and assayed in a batch containing three additional tests and two control samples of 8 and 36 mg/L target concentration by fluorescence polarization immunoassay (FPIA) (Oxis International Inc., Portland, OR, USA). These two samples produced a HI result indicating that the teicoplanin concentrations in these samples were >100 mg/L. They were both diluted four-fold and re-assayed, again giving a HI result. Subsequent assay at high dilution with another kit revealed that the teicoplanin concentration in the pre-dose sample was >10 000 mg/L and that in the post-dose samples was 939 mg/L, clearly spurious results obtained as a result of contaminating the blood with residual drug from the line. However, the results of the other samples and controls assayed at the same time indicated that these two high-concentration samples had contaminated the reagent such that teicoplanin concentrations on the controls and samples were over-estimated.
The results of other samples assayed and the internal controls assayed with the high samples on three occasions (neat on the first and third occasions and diluted four-fold on the second occasion) are shown in the Table. The adjacent three samples were all over-estimated and the low controls were out of range after these samples were assayed the first time, and all controls were out of range after only three repeat assays.
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Notes
J Antimicrob Chemother 2000; 46: 10431044
* Corresponding author. Tel: +44-117-9595653; Fax: +44-117-9593217; E-mail: lesassays{at}ukneqasaa.win-uk.net
References
1 . MacGowan, A. P., Reeves, D. S. & Wise, R. (1999). Clinical interpretation of antimicrobial assays. In Clinical Antimicrobial Assays (Reeves, D. S., Wise, R., Andrews, J. M. & White, L. O., Eds), pp. 19. Oxford University Press, Oxford.