1About the Corrected Calcium Calculator — Clinical Guide
Serum calcium exists in three forms: protein-bound (predominantly albumin-bound, ~40–45%), complexed to anions like phosphate and citrate (~15%), and free ionised calcium (~45%), which is the physiologically active fraction. Standard laboratory reports measure total serum calcium, which includes all three fractions. When albumin is low, the protein-bound fraction falls, reducing total calcium — but ionised calcium may remain entirely normal. This is why albumin correction is essential before interpreting serum calcium in any patient with hypoalbuminaemia.
The Payne formula (Payne RB et al., BMJ 1973) is the most widely used correction: Corrected Ca (mg/dL) = Measured Ca + 0.8 × (4.0 − Albumin g/dL). For SI units: Corrected Ca (mmol/L) = Measured Ca + 0.02 × (40 − Albumin g/L). Hypoalbuminaemia is common in Indian hospitals — seen in liver disease (cirrhosis), nephrotic syndrome, malnutrition, burns, and critical illness. In all these contexts, the uncorrected calcium will be misleadingly low.
Despite its widespread use, the Payne formula has well-documented limitations. It was derived in stable outpatients and performs poorly in critically ill patients, where acid-base disturbances independently alter calcium-albumin binding. It also underperforms in malignancy and nephrotic syndrome. For any patient in the ICU, HDU, or with significant acid-base disturbance, direct measurement of ionised calcium on a blood gas analyser is the gold standard and should be used instead of the corrected total calcium formula.
2Frequently Asked Questions
Correcting total serum calcium for abnormal albumin levels prevents misdiagnosing hypocalcaemia in hypoalbuminaemia, though direct ionised calcium measurement remains preferred in critical care settings.
Related calculators on RxMedCalc
Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.