Evaluation of Caffeine Plasma Levels by an Automated Enzyme Immunoassay lpar;EMITrpar; in Comparison with a Highhyphen;Performance Liquid Chromatographic Method

摘要

Summary:A new enzyme immunoassay (EMIT) for the measurement of levels of caffeine in plasma was adapted to an automated centrifugal analyzer (Cobas Bio) and compared with a high-performance liquid chromatographic (HPLC) method. Precision of the EMIT test was similar to that of the HPLC method with intraassay coefficients of variation in the range of 2.0ndash;4.1percnt; (EMIT) and 1.5ndash;3.3percnt; (HPLC), respectively, depending on the concentration range tested. Day-to-day precision ranged from 2.7 to 5.6percnt; for EMIT and was 3percnt; for HPLC. Comparison of 69 patient samples assayed with both methods yielded the following equation: y = 1.06x plus; 1.25 mu;mol/L, r = 0.994 (x = HPLC, y = EMIT). Evaluation of the cross-reactivity of the three main human caffeine metabolites revealed no significant interference from theobromine and theophylline; however, there was significant interference (28percnt;) by paraxan-thine at a concentration range from 2.5 to 80 mu;mol/L. At low caffeine concentrations, up to 10 mu;mol/L, the level of this metabolite may be more than twice the corresponding caffeine concentration; therefore, the latter may be falsely elevated in the EMIT test. Despite this cross-reactivity, the new EMIT test proved to be suitable for use in a drug assay laboratory, as well as in the routine screening of outpatients for liver disease.