Corrected Calcium Calculator
Corrected calcium
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Enter a calcium and an albumin.
- Measured
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- Correction added
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The working
This works out the arithmetic. The order, the product in your hand and local policy still govern what is given.
The corrected calcium formula
Corrected Ca (mg/dL) = measured Ca + 0.8 × (4.0 − albumin in g/dL)
Roughly half the calcium in serum is bound to albumin and is biologically inert. A low albumin therefore lowers the total calcium a laboratory reports without the active fraction having changed at all, and a patient can look hypocalcaemic on paper while being biochemically normal.
The correction adds back what the missing albumin would have been carrying. The 0.8 factor comes from Payne and colleagues in 1973, and the SI form uses 0.02 against an albumin in g/L for the same reason.
How much the correction moves the number
| Measured calcium | Albumin | Correction | Corrected calcium |
|---|---|---|---|
| 8 mg/dL | 2 g/dL | +1.6 | 9.6 mg/dL |
| 8.5 mg/dL | 2.5 g/dL | +1.2 | 9.7 mg/dL |
| 9 mg/dL | 3 g/dL | +0.8 | 9.8 mg/dL |
| 9.2 mg/dL | 4 g/dL | 0 | 9.2 mg/dL |
At a normal albumin the correction does nothing, which is the check that the formula is behaving. The further albumin falls, the more it adds.
Where the correction is unreliable
The Payne correction was derived in a general hospital population in 1973 and assumes a fixed relationship between albumin and calcium binding. That relationship shifts with pH, with other binding proteins and with the acute phase response, so the correction performs least well in exactly the patients it gets applied to most: the critically ill and those with chronic kidney disease.
An ionised calcium measures the active fraction directly and needs no correction. Where it is available and the answer matters, it is the better test, and a corrected figure should not be preferred over it.
The anion gap calculator carries a related albumin correction, for the same underlying reason: albumin is itself an unmeasured anion.
Questions about corrected calcium
- What is the corrected calcium formula?
- In conventional units, corrected calcium in mg/dL equals the measured calcium plus 0.8 times the difference between 4.0 and the albumin in g/dL. In SI units it is the measured calcium in mmol/L plus 0.02 times the difference between 40 and the albumin in g/L.
- Why does calcium need correcting for albumin?
- About half the calcium in serum is bound to albumin and is not physiologically active. When albumin falls, the total calcium reported falls with it even though the active ionised fraction has not changed. The correction estimates what the total would have read at a normal albumin.
- Is corrected calcium reliable?
- It is an approximation and a contested one. Studies have found it performs poorly in critical illness and in chronic kidney disease. Where the question matters and an ionised calcium is available, the measured ionised value is the better figure.
- What is a normal corrected calcium?
- Roughly 8.5 to 10.2 mg/dL, or 2.2 to 2.6 mmol/L, though reference ranges differ between laboratories and the range printed on your own report is the one that applies.