Chemistry
Corrected Calcium
Albumin-adjusted total calcium.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
Corrected (albumin-adjusted) calcium estimates what the total serum calcium would be if the albumin level were normal. Roughly half of circulating calcium is bound to albumin, so when albumin is abnormal the measured total calcium can misrepresent the physiologically active (ionised) fraction. A common formula adds 0.02 mmol/L to the measured calcium for every 1 g/L the albumin falls below 40 g/L. It is used to avoid missing hypercalcaemia, or over-diagnosing hypocalcaemia, in patients with abnormal albumin.
Interpreting the result
A raised corrected calcium points towards true hypercalcaemia and a low value towards true hypocalcaemia, after accounting for the albumin. The correction is most relevant in hypoalbuminaemia (for example in chronic illness or malnutrition), where a low total calcium may correct into the normal range. The corrected value is an estimate; where the result is critical or borderline, a directly measured ionised calcium is more reliable.
Worked example
A patient with a measured total calcium of 2.10 mmol/L and an albumin of 30 g/L has a corrected calcium of 2.10 + 0.02 × (40 − 30) = 2.30 mmol/L. The apparently low total calcium corrects into the normal range, reflecting the low albumin rather than true hypocalcaemia.
Pearls / pitfalls
- The correction is only an approximation; a directly measured ionised calcium is the gold standard when the result is critical or borderline.
- The formula assumes a reference albumin of around 40 g/L — check your local laboratory's reference value, which may differ.
- Acid-base disturbances alter the ionised fraction independently of albumin, so corrected calcium can still mislead in acidosis or alkalosis.
- Always interpret calcium alongside symptoms; severe symptomatic hypercalcaemia or hypocalcaemia needs urgent attention regardless of the corrected number.
Evidence & validation
Albumin-correction formulae derive from the Payne et al. work relating total calcium to albumin, and remain in routine UK laboratory and clinical practice, though guidance increasingly recommends ionised calcium where precision matters.
Frequently asked questions
Why correct calcium for albumin?
About half of serum calcium is bound to albumin, so an abnormal albumin distorts the total calcium. Correction estimates the calcium as if the albumin were normal, helping avoid missed or over-diagnosed calcium disorders.
When is correction most useful?
It matters most in hypoalbuminaemia, where a low total calcium may actually be normal once corrected. It is commonly applied in chronically unwell or malnourished patients.
Is corrected calcium as good as ionised calcium?
No. Corrected calcium is an estimate; directly measured ionised calcium is more reliable, especially when the result is critical, borderline, or in acid-base disturbance.
What reference albumin does the formula use?
A reference of around 40 g/L is typical, but laboratories vary. Use your local laboratory's stated reference value for the most accurate correction.
Can acid-base status affect the result?
Yes. Acidosis and alkalosis change the proportion of ionised calcium independently of albumin, so the corrected value may not reflect the true active calcium in those settings.
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.
- Association for Clinical Biochemistry and Laboratory Medicine guidance on calcium measurement and adjustment.
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