A manual spectrophotometric method for the measurement of serum sodium and potassium by enzyme activation

Manual procedures suitable for use on standard benchtop spectrophotometers have been developed for the enzymatic determination of Na+ and K+ in serum. Both assays require only minimal modification of reagents already available for BM/Hitachi analyzers and are performed in an endpoint mode, allowing up to 20 assays per run. The addition of a stop reagent is required--dipotassium EDTA for the Na+ assay and sodium dodecyl sulphate for the K+ assay. The most important criterion for achieving good assay performance is the precise pipetting of sample and reagent. Within-run imprecision is < 1% for Na+ and K+, and between-run imprecision < 1.5%, for both assays at all but the lowest concentrations of K+. Enzymatic electrolyte results compare well with flame photometry, however the assays are more prone to interference by very high concentrations of bilirubin or triacylglycerols than those performed on automated, dual-wavelength kinetic analyzers. It is possible to correct for most interferences by inclusion of appropriate sample and reagent blanks.