CLSI document C39-A A Designated Comparison Method for the Measurement of Ionized Calcium in Serum; Approved Standard provides a candidate designated comparison method to standardize the measurement of ionized calcium made by ion-selective electrode (ISE) potentiometry. This system can be used to assign ionized calcium concentrations to a commercially available, serum-based material to improve the traceability and transferability of results for the measurement of ionized calcium in the clinical laboratory. This standard addresses the principle of the test, the assignment of ionized calcium concentrations to NIST Standard Reference Material 956a, the materials and methods used, and the results and conclusions of an interlaboratory study to assign the ionized calcium concentrations.

This document emphasizes the use of stable, deep-frozen (-50 °C), pooled serum (NIST SRM 956a) with iCa2+ values assigned by a designated comparison method (DCM) as the key material which transfers accuracy for the measurement of ionized calcium. The substance concentration of iCa2+ in this human serum-based material is determined on the basis of potentiometric comparison to defined standard solutions made from high-purity reference materials. These standards are aqueous solutions whose compositions are established by convention to contain known concentrations of ionized calcium at an ionic strength of 0.160 mol/kg. In general, preparation of these standards follows the recommendations of the Working Group on Selective Electrodes of the International Federation of Clinical Chemistry (IFCC). 

The results of a multisite, interlaboratory study using NIST SRM 956a are reported. The objectives of this study were: 1) to show compatibility of the material with various commercial ionized calcium analyzers; and 2) to show usefulness of SRM 956a for providing uniformity to the measurement of ionized calcium in the clinical laboratory. 

This document likewise provides specifications for the data acquisition hardware and software components of the ionized calcium DCM. Detailed information is included on the design of the potentiometric ISE and reference half-cells, liquid-liquid junction, and fabrication of tubular, calcium ion-selective membranes. Operating steps for system calibration, sample measurement, and data reduction are also described. Analytical specifications are described in terms of intralaboratory “within-run” and “day-to-day” imprecision to be expected when this technology is mastered.