This tool adjusts a measured total serum calcium result for a patient's albumin level, using the Payne formula. It's used because roughly 40% of circulating calcium is bound to albumin, so a low albumin can make total calcium look falsely low, and a high albumin can make it look falsely high.
Published by Payne et al. in 1973, the formula adds 0.02 mmol/L to the measured calcium for every 1 g/L that albumin falls below a reference value of 40 g/L (and subtracts the same amount for albumin above 40 g/L). In SI units: corrected calcium = measured calcium + 0.02 × (40 − albumin). When albumin is exactly 40 g/L, no correction is applied and the corrected and measured values are identical.
The corrected value is compared against standard clinical thresholds: below 2.10 mmol/L is hypocalcaemia, 2.10–2.60 mmol/L is the normal range, and above 2.60 mmol/L is graded mild (2.60–3.00), moderate (3.00–3.50), or severe (above 3.50) hypercalcaemia. These bands are for reference only and don't account for symptoms, trend over time, or the wider clinical picture.
The Payne formula was derived from a relatively small cohort and assumes a fixed relationship between albumin and protein-bound calcium that doesn't hold for every patient — it can be unreliable in critical illness, acid-base disturbance, and conditions like multiple myeloma, where a directly measured ionised calcium is more accurate. This tool is for reference only and is not a substitute for clinical judgement — interpretation and management decisions remain with the treating clinician.
Formula: Payne RB, Little AJ, Williams RB, Milner JR, "Interpretation of serum calcium in patients with abnormal serum proteins," Br Med J 1973;4(5893):643–646. Read the full paper on PMC.