A case of false positive opiate immunoassay results from rifampin (rifampicin) treatment

Abstract

The drug screen test on a 12-year-old male patient was positive for opiates by a kinetic interaction of microparticles in solution (KIMS) immunoassay method on the Roche Cobas C502. The positive opiates result was not confirmed by the liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. A chart review revealed that the patient had tuberculosis and was on rifampin. We spiked rifampin into drug-free urine and tested opiates with the Cobas method. Once again, a positive result was obtained. This case showed that rifampin can still cause false positive opiate results measured with the KIMS method. We want to stress the importance of confirming positive screen results by more specific methods such as LC-MS/MS.

Keywords: False-positive; HPLC; LC-MS/MS; Opiate; Rifampin.

Figure 1

Structure comparison of various opiates and rifampin with structural differences highlighted in red.

Fig. 2

Rifampin dilution and spiking studies. Opiate concentrations were measured with Cobas OPI2 assay. (A) The apparent opiates results in diluted urine samples of the patient. The concentration of rifampin in each of the diluted urine samples was calculated using the neat rifampin concentration (52390 ng/mL) and the associated dilution factors. B) The apparent opiates results in urine specimens containing various spiked rifampin. Additionally, to achieve a positive result using the OPI2 from Roche required 2521 ng/mL of Rifampin.