Corrected Calcium: The Complete Clinical Guide
Why albumin correction matters, when to use ionised calcium, hypercalcaemia and hypocalcaemia management
Why Does Calcium Need Correcting?
Approximately 40–45% of serum calcium is bound to albumin. Standard laboratory calcium measurements reflect total calcium — both bound and free (ionised) fractions. When albumin is low, as is extremely common in unwell hospitalised patients, total calcium will appear falsely low even if the physiologically active ionised calcium is normal.
The corrected calcium formula adjusts the measured total calcium to what it would be if the albumin were a normal 40 g/L. This prevents both false diagnoses of hypocalcaemia in hypoalbuminaemic patients and missed diagnoses of true hypercalcaemia.
Interpreting Corrected Calcium
| Corrected Ca (mmol/L) | Classification | Action |
|---|---|---|
| < 1.75 | Severe hypocalcaemia | Urgent IV calcium. Cardiac monitoring. ITU review. |
| 1.75–2.19 | Hypocalcaemia | Investigate cause. Oral/IV calcium + vitamin D. |
| 2.20–2.60 | Normal | No action required for calcium. |
| 2.61–2.99 | Mild hypercalcaemia | Investigate cause (PTH, PTHrP, vitamin D, malignancy). |
| 3.00–3.49 | Moderate hypercalcaemia | IV fluids, bisphosphonates. Urgent investigation. |
| ≥ 3.50 | Severe hypercalcaemia | Medical emergency. IV fluids, calcitonin, HDU/ITU. |
Common Causes by Category
- Primary hyperparathyroidism (most common outpatient)
- Malignancy — PTHrP, bone mets, myeloma
- Vitamin D toxicity / granulomatous disease (sarcoid, TB)
- Thiazide diuretics, lithium
- Familial hypocalciuric hypercalcaemia (FHH)
- Milk-alkali syndrome, immobilisation
- Hypoalbuminaemia (pseudo-hypocalcaemia — correct first)
- Hypoparathyroidism — post-surgical, autoimmune
- Vitamin D deficiency (very common)
- Hypomagnesaemia (prevents PTH secretion)
- Acute pancreatitis, rhabdomyolysis
- Renal failure (phosphate retention)
When to Measure Ionised Calcium Instead
The corrected calcium formula is an estimate based on a linear albumin-calcium relationship that is not perfectly accurate in all patients. Measure ionised calcium directly (via blood gas analyser) in the following situations:
- Critical illness / ICU patients where accuracy is essential
- Significant acid-base disturbances (acidosis increases ionised Ca; alkalosis decreases it)
- Multiple myeloma or paraproteinaemia (protein binding differs)
- Discrepancy between symptoms and corrected calcium result
- Post-parathyroid or thyroid surgery (hungry bone syndrome monitoring)
- Neonates and paediatric patients
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View All Calculators →Frequently Asked Questions
What is the normal corrected calcium range?
The normal range for corrected (albumin-adjusted) calcium is 2.20–2.60 mmol/L (8.8–10.4 mg/dL). Laboratory reference ranges may vary slightly. Always interpret against your local laboratory's reference range as assay methods differ between institutions.
Why does the formula use 40 g/L as the standard albumin?
40 g/L represents the midpoint of the normal adult albumin range (35–50 g/L). The Payne formula (1973) established that for every 1 g/L that albumin falls below 40, total calcium falls by approximately 0.02 mmol/L. The correction therefore adds back this deficit. Some laboratories use 42 g/L as the standard — check your local guideline.
Can corrected calcium be used in hyperalbuminaemia?
Yes — the same formula applies in reverse. If albumin is above 40 g/L (as seen in dehydration), the correction will lower the corrected calcium below the measured value. This can unmask true normocalcaemia in an apparently hypercalcaemic patient. However, significant hyperalbuminaemia is uncommon and ionised calcium measurement is preferred in these cases.
References
- Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. BMJ. 1973;4(5893):643-646.
- NICE CKS: Hypercalcaemia. Updated 2023. National Institute for Health and Care Excellence.
- NICE CKS: Hypocalcaemia. Updated 2022. National Institute for Health and Care Excellence.
- Bushinsky DA, Monk RD. Calcium. Lancet. 1998;352(9124):306-311.
⚠️ Medical Disclaimer: This calculator is for clinical decision support only. Always interpret results in the context of the full clinical picture. In critical illness, measure ionised calcium directly. MedDraftPro accepts no clinical liability.