RIFAMPICIN CAUSES FALSE-POSITIVE IMMUNOASSAY RESULTS FOR URINE OPIATES

The treatment of tuberculosis usually includes the antibiotic rifampic in, especially in patients with concomitant human immunodeficiency vir us infection. Some of these patients are in withdrawal therapy for dru g abuse. When opiate screening is carried out in patients receiving ri fampicin, false positive results are detected with the kinetic interac tion of microparticles in solution method. We evaluated this interfere nce in a Cobas-lntegra analyzer and found a 12% cross-reactivity of ri fampicin for antibiotic concentrations ranging from 0.19 to 6.08 mu mo l/l (156 to 5000 mu g/l). This effect is not explained by the colour o f the rifampicin solutions. Calculations assuming first order kinetics of elimination show that more than 18 hours after a single oral dose of 600 mg of rifampicin, a false positive result for opiates could be obtained. This indicates that the risk of a false positive result must always be considered when urine samples from these patients are analy zed.