Corrected Calcium
With measured total calcium 8.2 mg/dL, serum albumin 2.5 g/dL, reference albumin 4 g/dL, corrected calcium comes to 9.40 mg/dL — corrected calcium. It is reached in 6 steps, the last of which is 8.2 + 1.2, and each one is printed on the page with its numbers filled in. The formula is the one published by Payne et al., not an approximation fitted to it.
Serum calcium adjusted for albumin by the Payne formula, in both conventional and SI units, with the case where the adjustment misleads.
Formula and sources checked · How we check
Measured total calcium 8.2 mg/dL, Serum albumin 2.5 g/dL, Reference albumin 4 g/dL
9.40 mg/dL
Corrected calcium for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- How far albumin is below reference
4 - 2.51.5 g/dL- Adjustment, 0.8 per g/dL
0.8 * 1.51.2 mg/dL- Corrected calcium
8.2 + 1.29.4 mg/dL- Corrected calcium in SI units
9.4 / 4.0082.345 mmol/L- Measured calcium in SI units
8.2 / 4.0082.046 mmol/L- Above the usual lower limit of 8.5
9.4 - 8.50.9 mg/dL
Worked example
A measured calcium of 8.2 mg/dL looks low. Albumin is 1.5 g/dL below the reference, and at 0.8 mg/dL per g/dL that is an adjustment of 1.2 — a corrected calcium of 9.4 mg/dL, comfortably normal. The apparent hypocalcaemia was low albumin, not low calcium.
How to work it out yourself
- 1.Subtract the measured albumin from the reference figure, usually 4.0 g/dL.
- 2.Multiply that deficit by 0.8. That is the adjustment in mg/dL.
- 3.Add it to the measured total calcium.
- 4.Divide by 4.008 for mmol/L, the unit used outside the United States.
The formula
- How far albumin is below reference
4 - 2.5 - Adjustment, 0.8 per g/dL
0.8 * 1.5 - Corrected calcium
8.2 + 1.2 - Corrected calcium in SI units
9.4 / 4.008 - Measured calcium in SI units
8.2 / 4.008 - Above the usual lower limit of 8.5
9.4 - 8.5
Source: Payne et al. — Interpretation of serum calcium in patients with abnormal serum proteins, BMJ 1973, NIH Office of Dietary Supplements — calcium
Questions people actually ask
- What is the corrected calcium formula?
- Corrected calcium = measured calcium + 0.8 × (4.0 − albumin), with calcium in mg/dL and albumin in g/dL. In SI units the same relation is measured calcium in mmol/L + 0.02 × (40 − albumin in g/L). Both come from Payne’s 1973 regression in the BMJ.
- Why does calcium need correcting for albumin?
- Because roughly 40% of circulating calcium is bound to albumin and only the free fraction is physiologically active. A total calcium assay measures both, so a patient with low albumin shows a low total while their free calcium is normal — the classic false hypocalcaemia in liver disease, nephrotic syndrome and critical illness.
- When should I not trust the correction?
- In critical illness, in acid-base disturbance and in end-stage renal disease. Multiple studies since Payne have found the formula misclassifies a substantial share of ICU patients in both directions, because binding depends on pH as well as albumin. Where the answer changes management, measure ionised calcium instead.
- What is ionised calcium and when is it measured?
- The free, unbound fraction — what the formula is trying to estimate. It is measured directly on a blood gas analyser, needs an anaerobic sample handled promptly, and is the standard of care in intensive care, during massive transfusion and in dialysis. Where it is available, the correction is unnecessary.
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