Calcium Metabolism
Introduction
Calcium is an essential divalent cation with key roles in bone structure, neuromuscular function, intracellular signalling, coagulation and cardiac electrophysiology. In emergency practice the common problems are hypercalcaemia and hypocalcaemia. Emergency clinicians need to:
- interpret calcium results correctly,
- recognise clinically important derangements and ECG consequences,
- initiate appropriate acute management, and
- know when to involve endocrinology, nephrology or surgical teams.
Physiology and distribution
- Total body calcium in adults ≈ 1 kg (≈ 25 000 mmol); >99% is in bone as hydroxyapatite.
- Only a small fraction is extracellular. Plasma calcium exists in three fractions:
- Ionised (free) calcium - physiologically active; influenced by pH.
- Protein‑bound calcium (mainly albumin).
- Complexed calcium (citrate, phosphate).
- Ionised calcium is the best physiological measure but is not always available; routine practice uses total calcium adjusted for albumin unless ionised calcium is indicated (critical illness, acid-base disturbance, major hypo/hyperalbuminaemia).
Normal values and definitions
- Typical total serum calcium reference range: 2.2-2.6 mmol/L.
- Definitions (total calcium unless otherwise stated):