How to use it
- Choose US units (mg/dL and g/dL) or SI units (mmol/L and g/L).
- Enter total serum calcium and albumin from the same sample.
- Leave the normal albumin at 4.0 g/dL (40 g/L) unless your lab uses another value.
- Read the corrected calcium, the same value in the other unit and how much was added.
Formulas
- US units (Payne): corrected Ca (mg/dL) = Ca + 0.8 × (normal albumin − albumin), albumin in g/dL, normal albumin 4.0 by default.
- SI units: corrected Ca (mmol/L) = Ca + 0.02 × (normal albumin − albumin), albumin in g/L, normal albumin 40 by default.
- Conversion between units: 1 mmol/L of calcium = 4.008 mg/dL.
The two forms are the same rule written in different units, so they agree to within rounding.
Worked example
Total calcium 8.0 mg/dL with albumin 2.0 g/dL:
corrected Ca = 8.0 + 0.8 × (4.0 − 2.0) = 8.0 + 1.6 = 9.6 mg/dL, which is 9.6 ÷ 4.008 = 2.40 mmol/L.
In SI units: calcium 2.00 mmol/L and albumin 20 g/L give 2.00 + 0.02 × (40 − 20) = 2.40 mmol/L.
Tips and common mistakes
- Do not mix units: calcium in mmol/L with albumin in g/dL gives a wrong answer. Choose the pair that matches your lab report.
- When albumin is above normal, the correction is negative and the corrected value is lower than the measured one.
- This is an estimate for information, not a diagnosis. Abnormal calcium results need a doctor’s review.