Drug screening with EMIT reagents: a quantitative approach to quality control

We investigated the precision, linearity, accuracy, and stability of quantitative results for five drugs of abuse [amphetamines, benzoylecgonine, opiates, phencyclidine, and the cannabinoid-tetrahydrocannabinol (THC)-9-acid metabolite], analyzed in control specimens by using EMIT d.a.u. reagents (Syva Co.) with a Monarch 2000 analyzer with a nonlinear interpolation curve-fitting algorithm. The within-day and between-days coefficients of variation (CVs) were less than 5% for all drugs except THC-9-acid, which had a CV between 10% and 20%. The drift of control values during a 30-day stability study was less than 10% from target values for three weeks after a single calibration, except for THC-9-acid control values, which were stable for only two to three days. Daily calibration reduced the drift away from target values during the 30-day stability study and produced optimum precision of all drug assays. Mean control values near the National Institute on Drug Abuse cutoff limits were within 10% of their target values.