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Free calcium in serum. I. Determination with the ion-specific electrode, and factors affecting the results.

Free (ionic) calcium in human serum was measured with a commercially available ion-specific electrode, together with a saturated-KCl salt bridge, a saturated-calomel reference electrode, and a direct-reading electrometer. Effects of sodium, potassium, and magnesium on the electrode values for calcium were assessed and a rationale demonstrated for the use of calcium standards containing 140 mmol of NaCl, 5 mmol of KCl, and 0.55 mmol of MgCl2 per liter. Trypsin, triethanolamine, and heparin bind calcium; therefore, their use is to be avoided. Variability is greatly decreased if the serum is from a fasting subject and if the collection tube is completely filled, transported on ice, and handled anaerobically. The range for free calcium for 86 apparently healthy adults was 1.175-1.375 mmol/liter when measured at 25°C. The precision (coefficient of variation) of duplicates was 0.6%; the long-term precision ≃2.0%.