Corrected (or adjusted) calcium is a calculated value that accounts for the fact that around 40% of total serum calcium is bound to albumin. When albumin is low, total calcium appears falsely low even though the physiologically active ionised calcium may be normal. Correcting for albumin gives a truer picture of the patient's calcium status.

Payne's formula (1973) in mg/dL is: Corrected Ca = Total Ca + 0.8 × (4.0 − Albumin). In SI units (mmol/L) the formula is: Corrected Ca = Total Ca + 0.02 × (40 − Albumin in g/L). This is the most widely used correction formula in clinical practice, though several alternatives such as the Berry and Portale formulas also exist.

Corrected calcium should be calculated whenever the serum albumin is outside the normal range (below 35 g/L or 3.5 g/dL, or above 50 g/L or 5.0 g/dL). Common clinical situations include patients with liver disease, nephrotic syndrome, malnutrition, chronic illness, or dehydration. It is particularly important in the assessment of suspected hyperparathyroidism.

The generally accepted reference range for corrected calcium is 8.5–10.5 mg/dL (equivalent to 2.12–2.62 mmol/L). Values below 8.5 mg/dL indicate hypocalcaemia, while values above 10.5 mg/dL indicate hypercalcaemia. Local laboratory reference ranges may vary slightly, so always compare against the range provided on the lab report.

Ionised calcium is the physiologically active fraction and is the gold standard measurement. However, it requires a fresh arterial or venous blood sample analysed immediately on a blood gas analyser, which is not always available. Total calcium plus albumin is cheaper, widely available, and the correction formula gives a reliable approximation in most patients.

Payne's formula works well for most patients but can be inaccurate in chronic kidney disease (especially eGFR <30), severe hyperparathyroidism, and when calcium-binding paraproteins are present (e.g. multiple myeloma). In these cases, direct measurement of ionised calcium is strongly recommended instead of relying on corrected calcium.

To convert from mg/dL to mmol/L, divide by 4.0 (or multiply by 0.25). To convert from mmol/L to mg/dL, multiply by 4.0. For example, 10 mg/dL ≈ 2.5 mmol/L, and 2.0 mmol/L ≈ 8.0 mg/dL.

Low albumin (hypoalbuminaemia) is commonly seen in liver cirrhosis, nephrotic syndrome, malnutrition, protein-losing enteropathy, chronic inflammatory states, and severe burns. These patients often have a normal ionised calcium despite a low total calcium, which is why correction is essential to avoid unnecessary treatment.