The differentiation between primary or tertiary (both hypercalcemic) and secondary (normocalcemic) hyperparathyroidism requires the identification of hypercalcemia. Calcium in the blood exists as bound, complexed and ionized fractions. Calcium sensors on parathyroid cells interact only with the ionized fraction (about 50% of the total calcium concentration). Many formulas using albumin, total protein or phosphate to correct or adjust total calcium to reflect the level of ionized calcium may be accurate only within a limited range. In addition, they can introduce errors based on inaccuracies in the measurement of these other metabolites. Clinical conditions, mainly those illnesses affecting acid-base balance, can alter the proportions of bound and free calcium. How and when the blood samples are drawn can alter the level of total calcium. Prolonged standing or prolonged venous stasis causes hemoconcentration, increasing the bound fraction. Preceding exercise can also affect blood calcium levels. Ingestion of calcium supplements or calcium-containing nutrients can cause transient elevations in blood calcium levels lasting several hours, leading to unnecessary further testing. Fasting total calcium levels may be sufficient for monitoring progress. However, for diagnostic purposes, fasting ionized calcium levels should be used. Therefore, for an isolated high total calcium level, we recommend obtaining a repeat fasting total and ionized calcium measurement before further investigations. Hypercalcemia may be diagnosed if there are persistent or frequent total or, preferably, ionized calcium levels >3 SD above the mean of the normal range or if there are progressively rising levels.
Keywords: Formulas; Hypercalcemia; Ionized calcium; Parathyroid hormone; Total calcium.