What it is
Roughly 40–45% of serum calcium is bound to albumin, so total calcium tracks albumin as much as it tracks physiology. In hypoalbuminemia the total falls while the biologically active ionized fraction may be normal. The Payne correction adjusts the measured total back to what it would read at a normal albumin, reducing false-positive hypocalcemia.
Formula
This calculator works in SI units (calcium mmol/L, albumin g/L):
$$ Ca_{corr} = Ca_{measured} + 0.02 \times (40 - albumin) $$
The 40 g/L term is the reference albumin; 0.02 mmol/L is added for every 1 g/L the albumin sits below 40 (and subtracted when above). A common legacy bug is reusing the conventional-unit coefficient (0.8 mg/dL per g/dL albumin) against SI values — that produces a ~40x error. This tool is deliberately fixed at 0.02 for SI inputs.
When to use
- Interpreting a total calcium when albumin is abnormal — sick, malnourished, nephrotic, or critically ill children.
- Screening for hypo- or hypercalcemia when an ionized calcium is not immediately available.
Worked example
Measured calcium 1.90 mmol/L, albumin 25 g/L.
$$ Ca_{corr} = 1.90 + 0.02 \times (40 - 25) = 1.90 + 0.30 = 2.20\ \text{mmol/L} $$
The raw value looks hypocalcemic; corrected to 2.20 mmol/L it sits at the low-normal edge — the “low” calcium was largely an albumin artefact. The tool flags <2.1 as hypocalcemia, 2.1–2.6 as normal, 2.6–3.0 as mild hypercalcemia, and >3.0 as significant hypercalcemia.
Pitfalls
- The correction is a population approximation, not a measurement. Individual albumin-binding varies widely; studies show albumin-adjusted calcium correlates poorly with ionized calcium in hospitalised patients. Where the answer is clinically load-bearing — tetany, arrhythmia, citrate load, acid-base derangement — measure ionized calcium.
- pH matters. Acidosis raises ionized calcium and alkalosis lowers it independent of albumin; the Payne formula ignores pH entirely.
- Mind your units. This tool is SI-only (mmol/L, g/L). For conventional units (mg/dL, g/dL) the coefficient and reference albumin differ — do not mix them.
- A normal corrected calcium does not exclude symptomatic ionized hypocalcemia in massive transfusion or citrate anticoagulation.
Run it: Corrected Calcium (Payne)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.