The Ionized Calcium Test measures the level of free (unbound) calcium in the blood, which is the biologically active form of calcium. Unlike total calcium, which includes both bound and unbound calcium, ionized calcium is crucial for muscle function, nerve transmission, and bone health. Abnormal levels can indicate parathyroid disorders , kidney disease , vitamin D deficiency , or acid-base imbalances . It helps assess calcium metabolism and diagnose conditions related to calcium imbalance.