Enzymatic determination of sodium in serum

This is a kinetic assay for measuring serum Na+ concentration based on determination of Na+-dependent beta-galactosidase (EC 3.2.1.23) activity. The method, sufficiently sensitive to measure sub-millimolar concentrations of Na+, was modified by including a Na+-binding agent (cryptand) to provide a linear assay for serum Na+ concentrations between 110 and 160 mmol/L. The assay was developed with and evaluated in the Cobas Fara centrifugal analyzer (and has been used in other kinetic analyzers). Within-run and between-run CVs were less than 1%. The reaction rate for normal serum samples (0.20 delta A/min) is about 10-fold that of the reagent blank. Results correlated well with flame photometry. Interference from bilirubin, hemoglobin, lipemia, heparin, and other cations was negligible. The method offers a practical alternative to the use of ion-selective electrodes and flame photometry for measuring serum Na+ in high-throughput or "stat" biochemical analyzers.