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Diagnosis of hypocalcaemia is based on characteristic clinical signs and serum-ionized calcium concentration equal or lower than 0.8 mmol/l (3.2 mg/dl). Decreased serum total calcium should prompt assessment of serum ionized calcium. The use of corrective formulae to estimate ionized calcium concentration based on serum total calcium and total protein or albumin concentration is not recommended. These formulae do not accurately predict ionized calcium concentration and should not be used to make therapeutic decisions. Serum ionized calcium concentration is typically higher than ionized calcium concentration in heparinized plasma or whole blood (measured by means of a blood gas analyser) due to dilution with heparin (Schenck and Chew, 2008). Calcium concentrations should not be assessed on EDTA plasma as EDTA chelates calcium, giving artificially low calcium concentrations. Additional laboratory investigations (such as measurement of plasma PTH, magnesium, vitamin D metabolites, urinalysis) and diagnostic imaging are indicated depending on the suspected underlying cause of hypocalcaemia in individual patients.

Box 4.6. Causes of hypocalcaemia (modified from Schenck and Chew, 2008).

• Puerperal tetany (eclampsia);

• Renal failure (acute and chronic);

• Protein-losing enteropathies (hypoalbuminaemia);

• Acute pancreatitis;

• Ethylene glycol toxicity;

• Phosphate enema;

• Hypoparathyroidism:

• Primary;

• Idiopathic or spontaneous;

• Post-operative bilateral thyroidectomy;

• After sudden reversal of chronic hypercalcaemia;

• Secondary to magnesium depletion or excess;

• Nutritional secondary hyperparathyroidism;

• Citrate anticoagulant overdose with blood transfusion;

• Hypovitaminosis D.