Rifampicin Cross-Reacts with Opiate Immunoassay

Published: Journal of Analytical Toxicology, Volume 23, Number 1, January/February 1999, pp.71.

Rifampicin Cross-Reacts with Opiate Immunoassay
H. van As and L.M.L. Stolk

To the Editor:
During routine workplace testing for drugs of abuse, a urine sample was screened for the presence of amphetamines, barbiturates, benzodiazepines, cannabinioids, cocaine, methadone, and opiates. The analysis was performed with a Cobas Integra 700 analyzer (Roche Diagnostics, Basel, Switzerland). The immunoassays used were Cobas Integra tests (Roche) that were based on KIMS (kinetic interaction of microparticles in solution). The test for cannabinoids was positive (56 µg/L), and the test for opiates was also positive (2245 µg/L). The other tests were negative. The analytical results of another sample collected 2 days later were as follows: cannabinoids 37 µg/L, opiates 2759 µg/L, others negative. The same samples were reanalyzed with another immunoassay. The second assay used was a fluorescence polarization immunoassay (Opiates reagent pack, Abbott Amstelveen, The Netherlands), and the apparatus was a TDxFLx analyzer (Abbott). Although the results for the other drugs were identical to those obtained with KIMS, the tests for opiates were negative.

Information about the subject’s medication revealed rifampicin use. No information about rifampicin cross-reactivity was available from the manufacturer. We spiked blank urine with different amounts of rifampicin. All samples tested positive for opiates (Table I).
The defined daily dose of rifampicin is 600 mg orally once a day. Half-life for elimination is 1.7 h. Approximately 10% of a rifampicin dose is excreted unchanged renally. Assuming a normal urine production of 1.5 L/day, a mean urine concentration of approximately 40 mg/L can be expected. This means that rifampicin will interfere with the KIMS opiates immunoassay.

Our conclusion is that medication with rifampicin causes a false-positive result of the KIMS test for opiates in urine.

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