Clinical and Interpretive

Useful for assessing calcium states during liver transplantation surgery, cardiopulmonary bypass, or any procedure requiring rapid transfusion of whole blood in neonates and in critically ill patients. A second-order test in the evaluation of patients with abnormal calcium values.

Ionized calcium, which accounts for 50% to 55% of total calcium, is the physiologically active form of calcium. Low ionized calcium values are often seen in renal disease, critically ill patients, or patients receiving rapid transfusion of citrated whole blood or blood products. Increased serum ionized calcium concentrations may be seen with primary hyperparathyroidism, ectopic parathyroid hormone-producing tumors, excess intake of vitamin D, or various malignancies.

Nomograms have been used to calculate ionized calcium from total calcium, albumin, and pH values. However, calculated ionized calcium results have proven to be unsatisfactory.

Serum ionized calcium concentrations 50% below normal result in severely reduced cardiac stroke work. With moderate to severe hypocalcemia, left ventricular function may be profoundly depressed.

Ionized calcium values are higher in children and young adults. Ionized calcium values vary inversely with pH, approximately 0.2 mg/dL per 0.1 pH unit change. Proper specimen handling is necessary to ensure accurate results.