Rifampin Interference with Opiate Immunoassays

Extract

To the Editor:

Clinical toxicology in emergency situations necessitates the availability of rapid analytical tests for different substances in a variety of settings. The primary question asked by clinicians is what compounds may have been ingested. In cases of suspected drug abuse or overdose, rapid screening by immunoassays has made this possible. However, many limitations are inherent in these methods, which necessitate confirmation by an alternative method such as gas chromatography-mass spectrometry (1). Here we present a case of drug interference with certain screening methods for opiates in urine.

A 7-year-old male (Table 1, patient 1) presented to the emergency unit at the American University of Beirut Medical Center with a progressive decrease in his level of consciousness over the previous 3 weeks. He was transferred from another hospital because of a suspicion of tuberculous meningitis. As part of the workup for this case, a urine drugs of abuse screen was ordered, which was positive for opiates by the Syva RapidTest (Dade Behring) on two occasions after being negative at the time of admission (Table 1). However, the patient was never on any opiates or opiate-like medication. After a review of the patient’s chart, the interfering substance in this positive opiate screen was suspected to be rifampin (rifampicin) (2). Confirmation of opiates in a urine sample collected on day 4 after admission by gas chromatography-mass spectrometry was negative. The actual opiates targeted by this analysis were pholcodine, codethyline, codeine, morphine, and monoacetylmorphine.