Payne and colleagues derived this linear adjustment in 1973 from adult samples with a wide range of abnormal serum protein concentrations, normalising total calcium to an albumin of 40 g/L.
Later studies in older inpatients, haemodialysis patients and critically ill cohorts show that albumin-adjusted calcium often agrees poorly with ionised calcium and may misclassify both hypo- and hypercalcaemia, especially as albumin falls or in significant acid–base disturbance.
Recent reviews conclude that uncorrected total calcium performs at least as well as, and sometimes better than, adjusted calcium for detecting abnormal ionised calcium, and that correction formulas can underestimate hypocalcaemia in marked hypoalbuminaemia and mask hypercalcaemia when albumin is normal or raised.
Ionised calcium is the physiologically active fraction. It should be measured directly where available when precise assessment matters—for example in critical illness, advanced kidney disease, perioperative care, suspected primary hyperparathyroidism or malignancy, or when symptoms and total calcium disagree.
This tool is intended for adults with abnormal albumin when only total calcium is available. Interpret the corrected value against local adult total-calcium reference intervals (typically about 2.1–2.6 mmol/L, with severe hypercalcaemia at ≥3.5 mmol/L) and in the context of the clinical picture.

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.
Goltzman D. Approach to Hypercalcemia. In: Feingold KR et al., eds. Endotext [Internet]. MDText.com; ongoing updates.
Albert SG, Isbell TS. Reconsideration of “Albumin corrected total calcium” determinations: potential errors in the clinical management of disorders of calcium metabolism. Clin Chim Acta. 2023;544:117353.
Laboratory and endocrine references describing adult total-calcium reference intervals around 2.1–2.6 mmol/L and grading hypercalcaemia severity (mild <3.0 mmol/L, moderate 3.0–3.5 mmol/L, severe ≥3.5 mmol/L); local ranges vary.

Calcium correction for hypoalbuminaemia