Objective: To confirm that the naproxen metabolite O-desmethylnaproxen interferes with the Jendrassik and Grof method for total bilirubin and was a possible cause of spurious hyperbilirubinemia in a case of naproxen overdose. Design and methods: Plasma was spiked with naproxen or O-desmethylnaproxen before total and direct bilirubin analysis. Results: O-desmethylnaproxen concentration showed a linear relationship with measured total bilirubin. Conclusion: O-demethylnaproxen interferes with the total bilirubin method. Interference of glucuronide metabolites of naproxen and O-demethylnaproxen is not excluded.
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