Comparison of two commercial ionised calcium analysers, and values in South African blacks

Although research on the ionised fraction of serum calcium (Ca++) started some 50 years ago, normal values and, above all, the methods of determination are still controversial. The use of calcium ion selective electrodes greatly enhanced and improved the measurement of Ca++. A study was carried out to: evaluate two Ca++ analysers (Radiometer ICA1 (Copenhagen, Denmark) and Orion SS-20 (Orion Biomedical, Cambridge, Mass., USA) ); correlate serum ionised calcium, total calcium and albumin levels in various normal and pathological states; and determine the values of Ca++ in healthy black subjects. The two analysers gave results showing very good correlation over a wide range (r = 0.986). Results for ionised Ca++ v. total calcium (TCa) showed a significant linear correlation (r = 0.956). Correcting TCa for variations in serum albumin concentrations did not improve this correlation (r = 0.9114). Reference normal values for Ca++ were established for healthy blacks, cord blood values being highest (mean 1.356 +/- 0.139 mmol/l) and adult values lowest (mean 1.159 +/- 0.04 mmol/l). Although the results obtained on the analysers were similar, the Radiometer ICA1 requires less blood and measures pH simultaneously. Correcting TCa for the variation in albumin within the normal range seemed to be of little value in assessing Ca++. The determination of TCa in most clinical situations is a good reflection of Ca++.