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Predictive properties of a qualitative urine acetaminophen screen in patients with self-poisoning.

OBJECTIVE
Screening for acetaminophen toxicity is recommended in almost all cases of self poisoning. We compared a qualitative urine acetaminophen screen to the quantitative serum acetaminophen to test the hypothesis that a negative urine acetaminophen screen would be predictive of a negative serum acetaminophen level.


METHODS
All adults with intentional ingestions evaluated in our Emergency Department during 1995 were retrospectively identified based on Emergency Department International Classification of Disease--9th edition codes. Laboratory data from each patient including serum and urine toxicologic assays were examined. Predictive properties of urine acetaminophen screens for serum acetaminophen were evaluated.


RESULTS
A total of 88 patients were identified who had both a serum acetaminophen and a urine acetaminophen performed. The sensitivity of the urine acetaminophen screen was 100% (95% CI 72-100%) and the specificity was 87% (95% CI 80-95%). All patients with negative urine acetaminophen screens had negative serum acetaminophen levels (negative predictive value 100%; 95% CI 96-100%). Accuracy of the urine acetaminophen screen was 89%.


CONCLUSION
A negative urine acetaminophen screen was highly predictive of negative serum acetaminophen levels. It is possible that negative urine acetaminophen screens may obviate the need for 4-hour quantitative serum levels. Further validation in a prospective study is needed.