Ionised Calcium 45% — FRCA Primary MCQ
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Correct answer: C — 45%
Total plasma calcium is approximately 2.2-2.6 mmol/L. Approximately 45% is ionised (physiologically active) / 40% is protein-bound (mainly albumin) / and 15% is complexed with anions (citrate / phosphate / bicarbonate). Only ionised calcium is physiologically active. Acidosis increases ionised calcium (reduced protein binding) / alkalosis decreases it.
Reference: Power Kam 2018 Principles of Physiology for the Anaesthetist