How Corrected Calcium is calculated
Roughly 40% of total serum calcium is bound to albumin, so low albumin lowers the total measured calcium without necessarily lowering the physiologically active, unbound (ionised) fraction. The correction formula adjusts for this.
The correction is only meaningful when albumin is below the reference 4.0 g/dL used in the formula; it should not be applied if albumin is high.
Normal values
| Corrected Ca (mg/dL) | Interpretation |
|---|---|
| < 8.5 | Hypocalcaemia |
| 8.5 – 10.5 | Normal |
| > 10.5 | Hypercalcaemia |
Common causes of hypocalcaemia
- Vitamin D deficiency or resistance
- Hypoparathyroidism (post-thyroidectomy/parathyroidectomy, autoimmune, congenital — e.g. DiGeorge syndrome)
- Chronic kidney disease (impaired vitamin D activation and phosphate retention)
- Acute pancreatitis (calcium saponification)
- Hypomagnesaemia (impairs PTH secretion and action)
- Massive blood transfusion (citrate chelates calcium)
- Rhabdomyolysis, tumour lysis syndrome
- Medications: bisphosphonates, calcitonin, cinacalcet, loop diuretics, some chemotherapy agents
- Sepsis / critical illness
Common causes of hypercalcaemia
- Primary hyperparathyroidism (most common cause in outpatients)
- Malignancy — PTHrP-secreting tumours, bony metastases, multiple myeloma (most common cause in inpatients)
- Vitamin D toxicity or excessive supplementation
- Granulomatous disease (sarcoidosis, tuberculosis — extrarenal vitamin D activation)
- Thiazide diuretics, lithium
- Hyperthyroidism, immobilisation, milk-alkali syndrome
- Familial hypocalciuric hypercalcaemia (FHH)
- Adrenal insufficiency
Clinical use
Used whenever total calcium is checked in a patient with abnormal albumin (e.g. critical illness, liver disease, malnutrition, nephrotic syndrome) to avoid mis-classifying hypocalcaemia or masking hypercalcaemia. When available — particularly in acid-base disturbance, critical illness, or when the corrected value is borderline or inconsistent with symptoms — a directly measured ionised calcium is more accurate and should be preferred over the albumin-corrected estimate.
Frequently asked questions
What is corrected calcium?
It's total serum calcium adjusted for a low albumin level, since roughly 40% of calcium is normally bound to albumin — low albumin can make total calcium look falsely low even when the physiologically active calcium is normal.
Why does albumin affect calcium levels?
Calcium circulates partly bound to albumin and partly as free (ionised) calcium. When albumin is low, less calcium is bound, so total calcium drops even though ionised calcium — the part that actually matters physiologically — may be unaffected.
When should ionised calcium be checked instead of corrected calcium?
In critically ill patients, those with acid-base disturbances, or when the corrected value is borderline or doesn't match the clinical picture — ionised calcium is measured directly and isn't affected by albumin-correction assumptions.
What is a normal corrected calcium level?
8.5–10.5 mg/dL is generally considered normal. Below 8.5 suggests hypocalcaemia and above 10.5 suggests hypercalcaemia, once corrected for albumin.
References
- Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J. 1973;4:643–646.
- Ridefelt P, et al. Should total calcium be adjusted for albumin? A reassessment. Clin Chem Lab Med. 2014.