Corrected Calcium Calculator
Calcium level adjusted for low albumin (Payne formula)
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Working out calcium level adjusted for low albumin (Payne formula) is easier with Corrected Calcium Calculator, which answers as you type. You supply Measured Calcium (mg/dL) and Albumin (g/dL) to Corrected Calcium; the answer follows at once. Every Corrected Calcium answer can be reproduced by hand. Anyone comparing options in Health can use Corrected Calcium to compare scenarios. Nothing is uploaded while Corrected Calcium works, and the figures stay put. Looking for corrected calcium calculator albumin payne formula hypocalcemia or a quick estimate? Corrected Calcium covers both. Corrected Calcium shows formula, assumptions and result together. Start with Corrected Calcium; the first number takes seconds.
What the defaults correct to
A measured calcium of 8 mg/dL with albumin of 2 g/dL corrects to 9.6 mg/dL.
About 40% of serum calcium is bound to albumin, so a low albumin makes total calcium read low even when the physiologically active ionised fraction is normal. The correction adds back what the missing albumin would have carried.
The Payne formula
Corrected calcium = measured calcium + 0.8 × (4.0 − albumin in g/dL).
The 4.0 is the reference albumin, in g/dL. Here the deficit is 4.0 − 2.0 = 2.0 g/dL, and 0.8 × 2.0 = 1.6 mg/dL added to the measured 8.0.
Worked example
Measured Ca 8.0 mg/dL
Albumin 2.0 g/dL
Deficit 4.0 − 2.0 = 2.0
Correction 0.8 × 2.0 = +1.6
Corrected = 9.6 mg/dL
An uncorrected 8.0 mg/dL looks like hypocalcaemia; 9.6 sits inside the normal range. The correction changes the interpretation, not just the number.
When the formula misleads
- Chronic kidney disease — albumin and calcium handling are both abnormal, and the correction overestimates.
- Acidosis and alkalosis — they shift calcium between bound and ionised fractions without changing total calcium.
- Very low albumin — below about 2 g/dL the linear correction becomes unreliable.
- Critical illness — albumin is both low and an unreliable marker, and ionised calcium is the measurement that matters.
The better measurement
When the answer will change management, measure ionised calcium directly. It is unaffected by albumin, it is the fraction that is physiologically active, and it avoids the assumptions this formula makes. The corrected value is a useful screen when ionised calcium is not available.
Units
The 0.8 coefficient belongs to mg/dL for calcium and g/dL for albumin. In mmol/L the equivalent correction uses a coefficient of about 0.02 per g/L of albumin, and mixing the two unit systems produces a wrong answer that still looks plausible.
Frequently Asked Questions
Why does 8.0 mg/dL become 9.6?
Because the albumin is 2.0 g/dL against a reference of 4.0, a deficit of 2.0. The Payne formula adds 0.8 mg/dL per g/dL of deficit: 8.0 + 1.6 = 9.6.
What is a normal corrected calcium?
Roughly 8.6–10.3 mg/dL, so 9.6 is mid-range. The reference interval varies slightly between laboratories, which is why the local range is the one to compare against.
When should I measure ionised calcium instead?
Whenever the result will change treatment, and in critical illness, advanced kidney disease or after major surgery. Ionised calcium is measured directly and does not depend on an albumin assumption.
Does the formula work in mmol/L?
Not with 0.8 and 4.0. In SI units the albumin reference and coefficient differ, so convert to mg/dL and g/dL first or use a validated SI version of the formula.
Why is albumin 4.0 the reference?
It is the mid-point of the normal albumin range in the population the formula was derived from. Patients with a different baseline, such as chronic liver disease, sit outside that assumption.
What does a calcium of 10 with albumin 2 correct to?
10 + 0.8 × (4.0 − 2.0) = 11.6 mg/dL, which is above the normal range. The correction adds 1.6 regardless of the measured value, so a high reading stays high and becomes more clearly abnormal.
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