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(R) and TDx(R) immunoassay reagents for opiates and amphetamines. Mechanisms for the observed interference were also explored. DRI(R) 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(R) 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.