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 ab sorbance (Delta A R) than the rate for a drug-free urine calibrator. M any of these urines contained calicylates. Among 40 urines with appare nt salicylate concentrations between 15 and 420 mg/dL tested for benzo ylecgonine (BE), 20 had Delta A R < -4 (range +2 to -28 mA/min). The r ates decreased with increasing salicylate: Delta A R = -0.057 x (salic ylate, mg/dL) - 0.22 mA/min (r = 0.85, n = 40, P <0.01). Urines from 1 00 control subjects (no salicylate) had mean +/- SD Delta A R values o f -1.05 +/- 2.2 mA/min (range +3 to -7; only two were < -4 mA/min). Al though direct addition of salicylic acid (200 mg/dL) to urine specimen s did not reproduce the negative bias, ingestion of aspirin (acetylsal icylic acid) did by -0.09 mA/min per 1 mg/dL (72.4 mu mol/L) salicylat e. Negative biases observed for other Emit d.a.u. assays after salicyl ate 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.