Decreased signal in Emit assays of drugs of abuse in urine after ingestion of aspirin: Potential for false-negative results

Abstract

During routine drug analysis with the Syva d.a.u. Emit immunoassays we observed a high frequency of urines with lower rates of changes in absorbance (ΔA R) than the rate for a drug-free urine calibrator. Many of these urines contained salicylates. Among 40 urines with apparent salicylate concentrations between 15 and 420 mg/dL tested for benzoylecgonine (BE), 20 had ΔA R <0.01). Urines from 100 control subjects (no salicylate) had mean ± SD ΔA R values of -1.05 ± 2.2 mA/min (range +3 to -7; only two were