Why calcium is corrected for albumin
Much of the calcium in blood is bound to proteins, mainly albumin. The rest is free (ionized) calcium, the active form. A standard blood test measures total calcium. When albumin is low, from illness or poor nutrition, total calcium reads low even if ionized calcium is normal. The correction estimates what total calcium would be with a normal albumin.
Formula
This adds 0.8 mg/dL of calcium for every 1 g/dL that albumin falls below 4.0 g/dL. In SI units, 1 mmol/L of calcium is 4.008 mg/dL (calcium's molar mass is 40.08 g/mol) and 1 g/dL of albumin is 10 g/L, so the same formula is about Ca + 0.02 × (40 − albumin in g/L). The calculator converts exactly rather than rounding to 0.02.
Worked example
Total calcium 8.0 mg/dL, albumin 2.5 g/dL: corrected calcium = 8.0 + 0.8 × (4.0 − 2.5) = 8.0 + 1.2 = 9.2 mg/dL, or 9.2 ÷ 4.008 = 2.30 mmol/L. In SI units: calcium 2.00 mmol/L with albumin 25 g/L also gives 2.30 mmol/L.
| Albumin (g/dL) | Added to calcium (mg/dL) |
|---|---|
| 3.5 | +0.4 |
| 3 | +0.8 |
| 2.5 | +1.2 |
| 2 | +1.6 |
| 1.5 | +2 |
Limitations
- It's an approximation. The UCSF Hospital Handbook notes the correction is unreliable far from the normal range and advises checking ionized calcium when albumin is below 2 g/dL or when the clinical picture is changing quickly.
- Measured ionized calcium is better. If an ionized calcium result is available, it reflects free calcium directly and doesn't need correcting.
- The factor varies. Payne and colleagues derived their 1973 adjustment from their own lab's data; their abstract gives adjusted calcium = calcium − albumin + 4.0 (mg/100 mL and g/100 mL). The 0.8 factor used here is the common US version. Labs that report an adjusted calcium may use their own formula.
- Acid–base changes, kidney failure and some proteins (such as paraproteins) shift calcium binding in ways albumin alone doesn't capture.
This is a general estimate from a published formula, not medical advice. Reference ranges differ between labs, so compare your result with the range on your report and ask your doctor what it means.