Measuring Calcium in Plasma

Abstract

It is generally recognized that the fundamental role ol calcium in many physiological processes is exerted mainly by the free, unbound calcium ions, the so-called ionized calcium (iCa2*). Before the development of calcium ion-selective electrodes (ISEs), no practical method for measuring iCa2* in plasma existed; various methods for measuring the concentration of diffusible calcium (1) in plasma require equilibrium conditions and are time-consuming (2). The development of calcium ISEs (3) has made it possible to assay serum iCa2*' rapidly and directly (4).Calcium ISEs have become widely used and, as reported by Bowers, Brassard, and Sena (5), iCa2* constituted 88% of all clinical calcium measurement requests compared with 11% for total calcium and 1% for urine calcium at Hartford

Measuring calcium in plasma. A new electrochemical reference cell shows promise for standardized measurements of ionized calcium in plasma

“…Good between-day reproducibility was obtained with both the Radiometer E 1 2 1 and the prepared ETHlOOl electrodes; the range of between-day variations for serum samples was only 0.01 mM for 10 days (12).…”

“…However, because sodium has been shown to have an effect on the electrode (8), the sodium concentration was decreased compared with that of the reference method. HEPES was added at l-mM concentration to buffer the calibration solutions, and the pH of the solutions was adjusted to 7.4.…”

“…The ion-exchange membrane (Radiometer F2121) showed a considerable pH dependence (8), and the ETHlOOl membrane prepared using the Dojindo oNPOE plasticizer showed a far stronger dependence. On the other hand, the AVL BP0360 membrane and the ETHlOOl membrane prepared using DOS plasticizer had no dependence on pH, whereas the ETHlOOl membrane prepared using Fluka oNPOE showed a small dependence, as indicated in Figure 2.…”

“…world have worked to fabricate a reference cell satisfying these criteria, problems such as lack of reproducibility, protein effect, and effects of membrane compositions and electrode configurations have been manifested (8)(9)(10)(11). We have developed an optimum reference cell design that not only meets the IFCC criteria, but also solves these problems (12).…”

“…We have developed an optimum reference cell design that not only meets the IFCC criteria, but also solves these problems (12). Factors that cause errors in iCa2+ measurements are clarified, and protein effects and discrepancies derived from membrane compositions are eliminated.…”