Determination of Serum Ionic Calcium with an Ion-Selective Electrode: Evaluation of Methodology and Normal Values

Ionized calcium in serum has been determined potentiometrically by using a calcium ion-selective electrode in a flow-through configuration. For 397 normal human adults the mean value for serum ionic calcium was 1.22 ± 0.09 (2 SD) mmol/liter. Factors affecting the measurement—such as the procedure for sample collection, sample storage, the preparation of standard solutions, the type of resin employed, and the choice of the reference electrode filling solution—were examined. Magnesium ions in physiological concentrations cause a slight depression of the electrode's response to calcium ions, indicating an interference by magnesium in the method. Under the conditions used, the relative standard deviation (CV) of the method is 0.8%.