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.

Sodium measurements in the presence of paraproteins by four direct ISE methods and flame photometry compared.

Sodium was determined by flame photometry and by direct potentiometry using four different systems (Ektachem DT60, Corning 614, Orion 1020 and Nova-1) in serum samples from patients with normal proteins and paraproteins. Flame photometric and direct potentiometric differences (ISE-flame), showed the expected relationship with total protein whether or not the sample contained paraprotein, an exception being the Ektachem DT60 on normal protein. With the exception of the Ektachem DT60 with samples with normal protein levels, correlation between different ISE-flame estimates was good. Differences between direct potentiometric measurements are probably more attributable to efforts made by manufacturers to relate them to flame photometric measurements in 'normal samples', than to the volume occupied by increased protein content. The apparent lack of influence of protein on ISE-flame for the Ektachem DT60 was shown to relate to the original external reference electrode salt bridge used. A second study showed that changing this fluid to an equi-transferrant high salt content caesium chloride based reference fluid partially unmasked the protein effect.

Quality control in the simultaneous assay of anticonvulsants using an automated gas chromatographic system with a nitrogen detector.

An automated gas-liquid chromatographic technique for the routine determination of phenobarbitone, primidone and phenytoin using a nitrogen-detector is described. The high selectivity of the detector for nitrogen-containing compounds allows a direct extraction procedure and the use of a phenytoin analogue as an internal standard obviates the need for accurate aliquot measurement after extraction. The anticonvulsants are chromatographed as methyl derivatives produced by on-column methylation with trimethylphenyl ammonium hydroxide following automatic liquid injection. Quality control data from an internal quality control scheme is presented and the performance of the laboratory in an inter-laboratory control scheme is reported and discussed.