EFFECT OF VARIABILITY OF PLASMA INTERFERENCES ON THE ACCURACY OF DRUGIMMUNOASSAYS

Most immunoassays applied to drugs in human plasma do not use an extra ction of analyte. To compensate for interferences due to plasma protei ns or salts, standards are prepared in drug-free plasma. Because the c oncentration of plasma components varies from one subject to another, it is likely that the drug-free plasma is not representative of the po tential interference in each plasma. Using two immunoassays, for a ste roid (nomegestrol acetate) and a heptapeptide (BN 52080), the authors have shown that tracer binding to the antibody may vary significantly between plasma from different subjects. Intersubject variability of tr acer-antibody binding was 21.6% (coefficient of variation for 25 subje cts) for nomegestrol acetate. When the same plasma were spiked with th e steroid at a concentration corresponding to the central part of the standard curve, the recovery was between 39 and 215%. Intersubject var iability in tracer binding was lower (7.7%) for the peptide immunoassa y, but still affected accuracy. The authors show that this problem is common to direct immunoassays for other drugs and must be solved in as say development.