Decreased signal in Emit assays of drugs of abuse in urine after ingestion of aspirin: potential for false-negative results. 
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 (delta A 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 delta A R < -4 (range +2 to -28 mA/min).  The rates decreased with increasing salicylate: delta A R = -0.057 x (salicylate, mg/dL) -0.22 mA/min (r = 0.85, n = 40, P < 0.01).  Urines from 100 control subjects (no salicylate) had mean +/- SD delta A R values of -1.05 +/- 2.2 mA/min (range +3 to -7; only two were < -4 mA/min).  Although direct addition of salicylic acid (200 mg/dL) to urine specimens did not reproduce the negative bias, ingestion of aspirin (acetylsalicylic acid) did by -0.09 mA/min per 1 mg/dL (72.4 mumol/L) salicylate.  Negative biases observed for other Emit d.a.u.  assays after salicylate ingestion lead us to conclude that ingestion of therapeutic doses of aspirin may cause false-negative results for drug screens in urines by this technology.