Mechanism and elimination of aspirin-induced interference in Emit II d.a.u. assays

The presence of salicylates in urine reduces the signal in Emit assays (Syva), potentially yielding false-negative drugs-of-abuse screening results. We demonstrate that the principal urinary metabolite of salicylate, salicyluric acid (SUA; 2-hydroxybenzoylaminoacetic acid), interferes with the measurement of NADH formed in the assay by reducing the molar absorptivity of NADH at 340 nm. Thus, for a given concentration of d.a.u. analyte the change in absorbance over the assay time interval is less in the presence of SUA. With the Emit cocaine assay on the Hitachi 704 analyzer, the rate of absorbance change (delta AR) monitored at 340 nm for a specimen containing approximately 270 micrograms/L benzoylecgonine (BE) was 57 +/- 1.9 mA/min without SUA and 29 +/- 2.7 mA/min with 5 g/L SUA (n = 20). In contrast, delta AR determined at 376 nm was 18.6 +/- 0.5 mA/min with and 17.9 +/- 0.8 mA/min without 5 g/L SUA (n = 20). Measuring the Emit assay signal at wavelengths where SUA has no absorbance (376 nm) eliminates the interference due to SUA while maintaining the precision of the assay near the cutoff concentration for BE (300 micrograms/L).