Interference by naproxen in the urinary 5-hydroxyindoleacetic acid assay is due to a metabolite, desmethylnaproxen

Interference by naproxen in the spectrophotometric assay for urinary 5-hydroxyindoleacetic acid has been investigated. Gas chromatography-mass spectrometry demonstrated that ingestion of naproxen was associated with the production of four urinary components, unchanged drug and three metabolites, the major one being desmethylnaproxen. Unlike naproxen, this metabolite reacted in the spectrophotometric assay giving a product with the same absorption spectrum as that observed in urine samples obtained after naproxen ingestion. Unlike 5-hydroxyindoleacetic acid, the colour due to desmethylnaproxen is thermolabile and so the interference may be overcome by performing the incubation at 100 degrees C.