New Population- and Laboratory-Specific Albumin-Adjusted Calcium Equation: Derivation and Internal Validation

Background: Albumin-adjusted calcium equations are influenced by both analytical methods and population characteristics. Existing albumin-adjusted calcium equations, such as the Payne formula, were derived using older assays and often overcorrect or undercorrect in modern laboratory settings. This study aimed to derive and internally validate a new locally appropriate correction equation.

Methods: A total of 3585 calcium–albumin pairs were analyzed. Serum calcium and albumin values were divided into derivation and validation groups. Linear regression, shrinkage assessment, and Bland–Altman analysis were performed. A Payne-format corrected calcium equation was derived.

Results: The regression model produced the equation Ca = 5.6812 + 0.8443×Albumin. The new Adjusted calcium formula was: Adjusted Ca = Measured Ca + 0.8443× (4 – Albumin). Shrinkage between derivation and validation groups was low (0.0228) which indicates good internal validity. Comparison with the traditional formula showed systematic differences.

Conclusion: The derived formula demonstrates higher internal validity and better agreement than the historical Payne equation. Laboratories should consider population-specific and laboratory specific equations for improved clinical accuracy