Hypocalcemia has many causes (table 1). It can result from inadequate parathyroid hormone (PTH) secretion, parathyroid hormone resistance, vitamin D deficiency or resistance, abnormal magnesium metabolism, and extravascular deposition of calcium, which can occur in several clinical situations. (See "Etiology of hypocalcemia in adults" and "Etiology of hypocalcemia in infants and children".)
The diagnostic approach to hypocalcemia involves confirming, by repeat measurement, the presence of hypocalcemia and distinguishing among the potential etiologies. The diagnosis may be obvious from the patient's history; examples include chronic kidney disease and postsurgical hypoparathyroidism. When the cause is not obvious or a suspected cause needs to be confirmed, other biochemical tests are indicated.
This topic will review the evaluation of patients with hypocalcemia. The clinical manifestations and treatment of hypocalcemia are discussed separately. (See "Clinical manifestations of hypocalcemia" and "Treatment of hypocalcemia".)
INTERPRETATION OF SERUM CALCIUM
The first step in the evaluation of a patient with hypocalcemia is measurement of the serum albumin concentration. Calcium in serum is bound to proteins, principally albumin. As a result, the total serum calcium concentration in patients with low or high serum albumin levels may not accurately reflect the physiologically important ionized (or free) calcium concentration. Each 1 g/dL reduction in the serum albumin concentration will lower the total calcium concentration by approximately 0.8 mg/dL (0.2 mmol/L) without affecting the ionized calcium concentration and therefore without producing any symptoms or signs of hypocalcemia.
A patient who has a serum albumin concentration of 2 g/dL (20 g/L), which is 2 g/dL (20 g/L) below normal, will have a fall in serum total calcium concentration of 1.6 mg/dL (0.4 mmol/L). If the measured serum total calcium concentration is 8 mg/dL (2 mmol/L), then the corrected value will be 9.6 mg/dL (2.4 mmol/L), which is normal. Thus, in patients with hypoalbuminemia or hyperalbuminemia, the measured serum calcium concentration should be corrected for the abnormality in serum albumin (calculator 1) or for standard units (calculator 2).