Formulae
Payne Formula (SI units):
Corrected Ca (mmol/L) = Measured Ca (mmol/L) + 0.02 × (40 − Albumin g/L)
Payne Formula (conventional units):
Corrected Ca (mg/dL) = Measured Ca (mg/dL) + 0.8 × (4.0 − Albumin g/dL)
Ionised Ca estimation (if pH available):
Ionised Ca (mmol/L) ≈ Corrected Ca (mmol/L) × 0.46
(Approximation only — direct measurement preferred in ICU)
The Payne formula assumes a normal albumin of 4.0 g/dL (40 g/L). Each 1 g/dL fall in albumin below 4.0 g/dL lowers total serum calcium by approximately 0.8 mg/dL (0.02 mmol/L). In hyperalbuminaemia, total calcium is falsely elevated — the formula corrects in both directions.
Normal Ranges
| Parameter | SI Units | Conventional Units |
|---|
| Total calcium | 2.10–2.55 mmol/L | 8.5–10.2 mg/dL |
| Ionised calcium | 1.15–1.35 mmol/L | 4.6–5.4 mg/dL |
| Albumin | 35–50 g/L | 3.5–5.0 g/dL |
Clinical Features by Calcium Level
| Condition | Calcium Level | Clinical Features |
|---|
| Hypocalcaemia |
<8.5 mg/dL (<2.10 mmol/L) |
Tetany, perioral paraesthesia, Chvostek's sign, Trousseau's sign, prolonged QTc, seizures, laryngospasm |
| Mild Hypercalcaemia |
10.2–12 mg/dL (2.55–3.00 mmol/L) |
Often asymptomatic; fatigue, mild constipation, low-grade nausea |
| Moderate Hypercalcaemia |
12–14 mg/dL (3.00–3.50 mmol/L) |
"Bones, stones, groans, thrones" — bone pain, nephrolithiasis, nausea/vomiting, polyuria/polydipsia |
| Severe Hypercalcaemia |
>14 mg/dL (>3.50 mmol/L) |
Confusion, obtundation, coma, cardiac arrhythmias, cardiac arrest — hypercalcaemic crisis |
Clinical Application
- Always correct calcium for albumin before interpreting a low or borderline total calcium — hypoalbuminaemia (liver disease, malnutrition, post-surgery, critical illness) causes factitiously low total calcium
- In hyperalbuminaemia (dehydration, tourniquet effect), total calcium is factitiously high — the correction formula works in reverse
- Ionised calcium is the biologically active fraction — measured directly on blood gas analysers in ICU; formula-based estimation is an approximation only and should not replace direct measurement in ICU patients
- Post-thyroidectomy / post-parathyroidectomy: Monitor corrected calcium 6-hourly in the first 24–48h — hungry bone syndrome can cause precipitous hypocalcaemia requiring IV calcium replacement
- Causes of hypocalcaemia: Hypoparathyroidism (post-thyroidectomy/parathyroidectomy), vitamin D deficiency, acute pancreatitis (saponification), renal failure, hypomagnesaemia, massive transfusion
- Causes of hypercalcaemia: Primary hyperparathyroidism (commonest in outpatients), malignancy (PTHrP, osteolysis), vitamin D toxicity, sarcoidosis, thiazide diuretics, immobility
References
- Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J. 1973;4(5893):643–646.
- Ladenson JH, Lewis JW, Boyd JC. Failure of total calcium corrected for protein, albumin, and pH to correctly assess free calcium status. J Clin Endocrinol Metab. 1978;46(6):986–993.