Corrected Calcium Formula — Calcium Albumin Correction Explained
Understand the corrected calcium formula used to adjust for low albumin levels. Includes the Payne equation, worked examples, and clinical interpretation. Free calculator.
The Corrected Calcium Formula
Serum calcium is approximately 40–50% bound to albumin. When albumin is low (hypoalbuminemia), total measured calcium appears falsely low even when ionized (physiologically active) calcium is normal. The corrected calcium formula adjusts for this effect.
| Formula | Equation | Albumin Reference |
|---|---|---|
| Payne Equation (standard) | Corrected Ca = Measured Ca + 0.8 × (4.0 − Albumin) | 4.0 g/dL |
| Orrell Equation | Corrected Ca = Measured Ca + 0.6 × (4.0 − Albumin) | 4.0 g/dL |
| SI Units version | Corrected Ca = Measured Ca + 0.02 × (40 − Albumin) | 40 g/L |
Worked Example
A patient has a measured total calcium of 8.0 mg/dL and an albumin of 2.5 g/dL. Using the Payne equation:
Corrected Ca = 8.0 + 0.8 × (4.0 − 2.5) = 8.0 + 0.8 × 1.5 = 8.0 + 1.2 = 9.2 mg/dL
This corrected value falls within the normal range (8.5–10.5 mg/dL), indicating that the apparently low measured calcium was an artifact of hypoalbuminemia, not true hypocalcemia.
The Payne formula has known limitations — it performs poorly when albumin is very low (<2.0 g/dL) or in critically ill patients. In these cases, direct measurement of ionized calcium is preferred.
Frequently Asked Questions
Why do we correct calcium for albumin?
Approximately 40–50% of serum calcium is bound to albumin. In patients with low albumin (e.g., from liver disease, malnutrition, or nephrotic syndrome), total calcium appears falsely low. Correcting for albumin reveals whether physiologically active (ionized) calcium is actually deficient.
When should ionized calcium be measured directly instead of using the correction formula?
Direct ionized calcium measurement is preferred over the correction formula in critically ill patients, patients with acid-base disorders, those with very low albumin (<2.0 g/dL), and patients on heparin or with abnormal protein levels. The correction formula is least accurate in these populations.