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Approximately 50% of the serum calcium is in the ionized form, which is the active component. The remainder is bound to albumin (40%) and organic anions such as phosphate and citrate (10%). The total serum calcium levels range from 8.5 to 10.5 mg/dL (2.1 to 2.6 mmol/L), and ionized calcium levels range from 4.4 to 5.2 mg/dL (1.1 to 1.3 mmol/L). Both concentrations are tightly regulated. The total serum calcium level must always be considered in its relationship to plasma protein levels, especially serum albumin. For each gram per deciliter of alteration of serum albumin above or below 4.0 mg/dL, there is a 0.8 mg/dL increase or decrease in protein-bound calcium and, thus, in total serum calcium levels.