Hypocalcemia laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Vamsikrishna Gunnam M.B.B.S [2]

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.

Laboratory Findings

 CONFIRM HYPOCALCEMIA

• The first step in diagnostic approach to hypocalcemia includes "Repeat" measurement of calcium levels and confirming the levels.
• Patient previous levels for serum calcium should also be reviewed, if possible or available.

Hypoalbuminemia

• Calcium in blood is bound to albumin and calculation of the serum albumin is essential to distinguish true hypocalcemia.
• 1 g/dL decrease in the serum albumin levels will lower the total calcium levels by approximately 0.8 mg/dL (0.2 mmol/L).
• But these reduction in serum albumin levels does not affect serum ionized calcium levels and therefore no signs of hypocalcemia.
• So, a patient who has a serum albumin level that is 2 g/dL (20 g/L) below normal will have a fall in serum total calcium level of 1.6 mg/dL (0.4 mmol/L).

Laboratory findings consistent with the diagnosis of hypocalcemia include

• Serum PTH concentrations
• Magnesium concentrations
• Phosphate concentrations
• Vitamin D metabolites

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