Measurement of Serum Ionized Calcium with the Ion-Exchange Electrode

Abstract

We found no differences in pH or ionized calcium concentration of serum from blood either collected into evacuated tubes or collected with syringes and separated under oil. Sera can be stored frozen in 1-ml plastic insulin-type syringes for one to three days without significant changes in pH or ionized calcium concentration, whereas storage under oil with or without re-equilibration with CO2 is associated with significant changes in both. Standards prepared with trypsin and triethanolamine give faster electrode response, and their ionized calcium concentrations remain unchanged for 6 h at room temperature. We suggest a not entirely satisfactory control solution of buffered protein, for use in assessing reproducibility of results.