Hypocalcemia laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and keywords: hypocalcaemia; low serum calcium level

Overview

Measurement of the serum albumin concentration is essential to distinguish pseudohypocalcemia from true hypocalcemia. Laboratory investigations that detect alterations in parathyroid hormone, vitamin D, and phosphate in the serum or urine aid in the differential diagnosis of the underlying etiologies. Other laboratory tests include serum alkaline phosphatase, liver function tests, coagulation profile, blood urea nitrogen, creatinine, and urinary cAMP levels at baseline or after PTH challenge.

Factors Interfere with Calcium Concentration Measurement

• In alkalosis, free calcium decreases due to enhanced binding of Ca⁺⁺ to albumin. A rise in pH of 0.1 unit will cause an approximately 0.05 mmol/L (0.1 mEq/L) fall in ionized Ca⁺⁺ concentration. Direct measurement of ionized calcium concentration may be required in critically ill patients with acid-base disorders.
• Hypoproteinemia may contribute to reduced total calcium levels.
• Excessive use of laxatives may lower blood calcium levels by increasing intestinal calcium loss.
• Hypocalcemia may be masked by calcium supplements taken shortly before specimen collection or the use of calcium-ion exchange resin in hemodialysis for hyperkalemia.

Adjustment for Hypoalbuminemia

• Adjustment of total serum calcium concentration for changes in plasma albumin level:

Adjusted total calcium in mg/dL = Total calcium in mg/dL + 0.8 * (4 - Albumin in g/dL)

Adjusted total calcium in mmol/L = Total calcium in mmol/L + 0.02 * (40 - Albumin in g/L)

Laboratory Differential Diagnosis of Hypocalcemia

Abbreviations: ADHP, autosomal dominant hypoparathyroidism; CKD, chronic kidney disease; Cr, creatinine; Def, deficiency; HP, hyperphosphatemia; HPTH, hypoparathyroidism; PHP, pseudohypoparathyroidism; VDDR, vitamin D-dependent rickets.

↓Ca

↑PTH

↔↓PTH

↑PO4

↔↓PO4

↔Mg

↓Mg

↑Cr

↔Cr

↓25(OH)D

↔↑25(OH)D

HPTH

ADHP or Mg def

CKD

PHP or HP

Vitamin D def

↓1,25(OH)2D

↑1,25(OH)2D

Type I VDDR

Type II VDDR