InterlaboratoryEvaluationof SalicylateInterferencein Colorimetric AcetaminophenMethodsand ItsClinicalSignificance Richard

Serum specimens with concentrations simulating an overdose of salicylate and acetaminophen were submitted to laboratoriesparticipatingin an external quality-controlprogram, to evaluate both the magnitudeof salicylateinterference In colonmetric acetaminophen methods and the clinical significanceof the interference. The apparent acetaminophen concentrationdetermined by nitration methods was increased by about 0.70 mg/L per milligramof salicylateper deciliter. Of those laboratoriesusing nitration procedures, 25% do not routinelycorrectfor salicylateand 66% use the (incorrect) correction factor provided by a kit manufacturer. Laboratory data, as they would have been reported to physicians, were used to estimate the acetaminophen halflife andwere alsoapplied to a nomogram used to assessthe probability of hepatotoxicity. Interference by salicylate in the simulatedoverdoseof 10 g (totaldose) of each drugfalsely Indicated impendinghepaticnecrosisunlessthe appropriate correction factorwas used. Laboratories usingnitrationprocedures should screen samples submittedfor acetaminophen assay for the presence of salicylate and, if present, either use a method specificfor acetaminophen or utilizea correction factor determinedin-house.