Theophylline assay on Kodak Ektachem DTSC--performance and interference by structurally-related compounds and salicylate.

Theophylline was measured with a Kodak Ektachem DTSC using its property of uncompetitive inhibition of alkaline phosphatase. Within- and between-batch reproducibility was satisfactory. Agreement with consensus mean values on quality assessment samples was good as was agreement on patients' samples with a high performance liquid chromatography reference method and an automated fluorescence polarisation immunoassay. At therapeutic theophylline concentrations, no interference was seen with caffeine, theobromine, 1,7-dimethylxanthine, 1,3-dimethyluric acid or 3-propylxanthine. 3-methylxanthine (a theophylline metabolite) gave a positive bias but the concentrations of this metabolite found in serum are such that the clinical significance of this finding is questionable. Salicylate at concentrations which might be found during therapy for paediatric rheumatoid arthritis also gave a positive bias.