Total serum calcium is partly bound to albumin — when albumin is low, total calcium underestimates the physiologically active (ionised) fraction. The Payne formula, published in 1973, adjusts for this: for every 1 g/L that albumin falls below 40 g/L, 0.02 mmol/L is added to the measured calcium. When albumin is normal the corrected and measured values are identical. The result is interpreted against standard clinical thresholds for hypocalcaemia, normal range, and hypercalcaemia severity.
This tool is for reference only. Clinical decisions should always involve a qualified healthcare professional.