Calcium, Ionized

The Calcium, Ionized test (free or unbound calcium) is used when total calcium does not reliably mirror the active fraction, particularly in hypoalbuminemia or acid–base disturbances that change protein binding. In these settings, ionized calcium supports diagnosis and management of calcium dysregulation. Serial ionized calcium measurements may be necessary in critically ill patients and during large-volume infusion or transfusion to guide replacement strategies and reduce risks such as arrhythmias, neuromuscular irritability, and altered mental status. When protein binding and acid–base balance are stable, total calcium often approximates ionized calcium; when they are not, ionized testing is preferred.

Calcium participates in bone mineralization, neuromuscular transmission, skeletal and cardiac muscle contraction, and the coagulation cascade. The ionized (free, unbound, dialyzable) fraction constitutes approximately 46%–50% of total circulating calcium and directly mediates biologic effects. Ionized calcium varies inversely with blood pH; for each 0.1 unit fall in pH, ionized calcium rises by about 1.5%–2.5%. Because ionized calcium is not dependent on serum protein concentration, it provides a more faithful indicator of calcium physiology when albumin is low. In hypoalbuminemic patients, ionized calcium can better reflect primary hyperparathyroidism than total calcium measurements.