About 40% of serum calcium is bound to albumin. In hypoalbuminemia, total calcium underestimates the physiologically active ionized calcium. The corrected calcium formula adjusts for this and provides a better estimate when ionized calcium is unavailable.
Formula
Corrected Ca (mg/dL) = Measured Ca + 0.8 × (4 − Albumin in g/dL)
Normal Range
8.5–10.5 mg/dL (2.12–2.62 mmol/L)
Clinical Context
The correction is meaningful when albumin is significantly low (<3.0 g/dL). However, recent literature suggests that ionized calcium is more accurate when available, particularly in critically ill patients, those with acid-base disorders, or after large transfusions.
Limitations
- Less accurate in critically ill or acidotic patients
- Does not account for changes in pH affecting calcium binding
- Fails in patients with multiple myeloma or paraproteinemias