Worked Example
Given: Total calcium 7.2 mg/dL, Albumin 2.0 g/dL
Formula: Corrected Ca = 7.2 + 0.8 × (4.0 − 2.0)
Calculation: 7.2 + 0.8 × 2.0 = 7.2 + 1.6 = 8.8 mg/dL
Interpretation: Normal range (8.5–10.5 mg/dL). Despite the low measured calcium, corrected calcium is within normal limits due to hypoalbuminaemia.
Reference Ranges (Adults)
Low corrected calcium
< 8.5 mg/dL | < 2.12 mmol/L
Normal corrected calcium
8.5–10.5 mg/dL | 2.12–2.62 mmol/L
High corrected calcium
> 10.5 mg/dL | > 2.62 mmol/L
Reference ranges vary by laboratory. These are approximate adult values.
Clinical Notes
- Approximately 40–50% of serum calcium is protein-bound, mainly to albumin.
- Hypoalbuminaemia lowers total calcium without changing ionized calcium.
- The correction factor 0.8 (mg/dL) or 0.02 (mmol/L) is an approximation.
- Ionized (free) calcium is the physiologically active fraction.
Ionized calcium is more reliable in critically ill patients, acid–base disturbance,
severe hypoalbuminaemia, major protein abnormalities, and rapidly changing clinical states.
This formula is an approximation only.
Limitations
- Not validated in all patient populations.
- Less reliable when albumin is very low (< 2 g/dL) or very high.
- pH affects calcium–albumin binding; acidosis increases free calcium.
- Does not account for other protein-binding (e.g., paraproteins).
References
- Payne RB et al. Interpretation of serum calcium in patients with abnormal serum proteins. BMJ. 1973.
- Bushinsky DA, Monk RD. Calcium. Lancet. 1998;352(9124):306–311.