pseudohypocalcemia

Pseudohypocalcemia is a laboratory finding in which the total serum calcium appears low, but the physiologically active ionized calcium concentration is normal. It is not a true deficiency of circulating calcium, but rather an artifact usually caused by reduced serum albumin, since a large portion of calcium binds to albumin. When albumin is low, the total calcium falls while ionized calcium remains within normal limits. Less commonly, it can occur with abnormalities in calcium-binding proteins or assay interference, though hypoalbuminemia remains the main cause.

Diagnosis relies on assessing ionized calcium or correcting the total calcium for albumin. Direct measurement of ionized calcium from a blood gas or chemistry analyzer provides the most accurate assessment. If ionized calcium is normal, clinicians may report pseudohypocalcemia. A commonly used correction formula is corrected calcium = measured total calcium + 0.8 × (4.0 − albumin in g/dL). If the corrected calcium is normal and albumin is low, true hypocalcemia is unlikely. Re-evaluate calcium status if albumin normalizes or if symptoms suggest hypocalcemia.

Differential diagnoses include true hypocalcemia from hypoparathyroidism, vitamin D deficiency, renal failure, hypomagnesemia, or pancreatitis, particularly if ionized calcium is low. Management focuses on addressing the underlying cause of hypoalbuminemia or interpreting results in context; treatment aimed at increasing total calcium is not indicated when ionized calcium is normal.