Measurement of ionized calcium in human plasma with a calcium selective electrode

Abstract

The application of a calcium-selective electrode in the determination of ionized calcium in human plasma and serum has been investigated. A linear relation between calcium ion potential and log calcium ion concentration was obtained with standards prepared in excess neutral salt (0.15 moles per 1 NaCl). Stable and reproducible data could be obtained on samples collected in heparinized Vacutainer tubes if they were quickly centrifuged and kept cold prior to measurement. At a calcium concentration of 4.0 mg/100 ml, electrode sensitivity was about 0.4 mg/100 ml/mV. Reproducibility was about ± 0.1 mV. Greater variation in electrode potential readings was encountered however over periods of several hours or from day to day. Therefore, standards were always included with each group of samples to be measured. Total calcium in paired plasma and serum samples was identical, but ionized calcium in plasma was reduced by 0.24 mg/100 ml. This appeared to be related to the presence of heparin. Since the magnitude of this effect was small, use of plasma was continued in order to permit more rapid measurement than was possible with serum. Plasma samples obtained from 50 normal individuals yielded the following mean data: total calcium 10.60 ± 0.82 mg/100 ml; ionized calcium, 4.64 ± 0.44 mg/100 ml; and bound calcium, 5.97 ± 0.87 mg/100 ml. The proportion of ionized and bound calcium as a percentage of total calcium was 43.9 ± 5.00% and 56.1 ±5.00%, respectively. It was concluded that the electrode method is a simple and convenient technique for determination of ionized calcium in plasma. The Vacutainer technique for sample collection appeared acceptable provided that subsequent measurement was not unduly delayed.