Determination of Calcium Fractions of Serum. III. Ionized Calcium in Ultrafiltrates of Normal Serum and Examination of Pertinent Variables

Mean normal values obtained by ultrafiltration and spectrophotometric methods were 5.58, 4.56, 1.02, and 4.24 mg/100 ml for ultrafiltrable, ionized, complexed, and protein-bound calcium in serum. Reproducibility of all fractions was 0.10–0.12 mg/100 ml. The effect of pH, temperature, and storage has been studied for both steps of the fractionation procedure. Significant amounts of calcium are complexed by heparin in plasma and by bicarbonate in physiological concentrations.