Albumin-Corrected Calcium
Serum Calcium Correction (Payne Formula)
Result
Corrected Calcium
Measured calcium:
Albumin:
About the Correction
Formula Used
Corrected calcium = Measured calcium + 0.8 × (4.0 − Albumin)
Where calcium is expressed in mg/dL and albumin in g/dL. When albumin is ≥ 4.0 g/dL, use the measured calcium value directly (the formula would produce a negative correction without relevant clinical significance).
Interpretation
| Corrected Calcium | Interpretation |
|---|---|
| < 8.5 mg/dL | Hypocalcaemia |
| 8.5–10.5 mg/dL | Normal |
| > 10.5 mg/dL | Hypercalcaemia |
Clinical Indication
Approximately 40–45% of total serum calcium is bound to albumin. In hypoalbuminaemia, the measured total calcium may be falsely reduced, whilst ionised calcium (the biologically active fraction) remains normal — a condition known as pseudohypocalcaemia. Albumin correction provides an estimate of physiologically active calcium without the need for direct ionised calcium measurement.
Clinical context: This formula is an estimate. Ionised calcium (free Ca²⁺) is the gold standard for assessing biologically active calcium, with reference values of 4.5–5.3 mg/dL (1.12–1.32 mmol/L). Direct measurement is preferable to corrected calcium whenever available, particularly in more complex clinical settings.
⚠️ Important limitations
- Never validated against ionised calcium: The original Payne formula (1973) was derived without direct ionised calcium measurement and using laboratory methodologies that differ from current practice.
- Unreliable in critically ill patients: Acid-base disturbances (acidosis increases the ionised fraction), sepsis and other critical states alter calcium–protein binding in ways not predictable by the formula.
- Paraproteinaemias: In multiple myeloma and other dysproteinaemias, abnormal proteins bind calcium differently from albumin — the correction is inappropriate in these cases.
- Severe hypoalbuminaemia (albumin < 2 g/dL): Accuracy of the correction decreases significantly; measure ionised calcium directly.