Calcium is the most abundant mineral element in the body. About 98% of the 1200 g of calcium in the adult is in the form of hydroxyapatite in the skeleton. Hydroxyapatite is a lattice-like crystal composed of calcium, phosphorus, and hydroxide. The remaining calcium is in the extracellular fluid (50%) and in various tissues, especially skeletal muscle. Calcium is maintained within a fairly narrow range from 8.5 to 10.5 mg/dl (4.3 to 5.3 mEq/L or 2.2 to 2.7 mmol/L). Normal values and reference ranges may vary among laboratories as much as 0.5 mg/dl.