Interference of Selected Clinical Medicines on DRI^® and TDx^® Immunoassays of Morphine and Methamphetamine in Urine

Eleven commonly used antihistamines, nonsteroidal anti-inflammatory drugs, and vitamins were evaluated in vitro for potential interference with DRI® and TDx® immunoassay reagents for opiates and amphetamines. Mechanisms for the observed interference were also explored. DRI® reagents appear to be more susceptible to interference by the compounds studied. Regarding mechanistic aspects, tolmetin was found to be strongly absorbent at the detection wavelength (340 nm) causing a false negative response; diphenhydramine, pheniramine, and trimethobenzamide appeared to cause false positive interference through their affinity to the antibodies used in the DRI® reagents; while chlorpromazine was found to positively interfere with the assay through both cross reaction and UV absorption. Ascorbic acid, when in high concentrations (>2%, w/v), may inhibit the enzyme (G6PDH) activity and result in a false negative response.