Derivation and validation of a locally adjusted plasma calcium equation in a tertiary care population

Abstract Objectives Measurement of ionized calcium is considered the gold standard for assessment of calcium status; however, it is not routinely available in many clinical laboratories. Total calcium measurement adjusted for serum albumin is therefore widely used, most commonly employing the Payne equation. Since adjusted calcium equations are influenced by analytical methods and population characteristics, locally derived equations may offer improved clinical accuracy. Methods This retrospective cross-sectional study included laboratory data from Dubai Health hospitals and ambulatory care centers between January and December 2023. Total plasma calcium and albumin were measured using NM-BAPTA (5-nitro-5′-methyl-(1,2-bis(o-aminophenoxy) ethan-N,N,N′,N′-tetraacetic acid) and bromocresol green (BCG) methods, respectively, on the Roche Cobas c platform. After applying predefined exclusion criteria, 1,313 paired calcium–albumin results were analyzed. Deming regression was used to derive a locally applicable albumin-adjusted calcium equation. Agreement between the newly derived equation and Payne’s equation was assessed using linear regression and Bland–Altman analysis. Results A total of 1,313 samples (906 females, 407 males) were included. Deming regression analysis demonstrated a slope of 0.553 and an intercept of 7.217 mg/dL between total calcium and albumin (λ=3.68; slope 95 % CI 0.479–0.627). The locally derived equation was: A.Ca (mg/dL)=T.Ca + 0.553 × (4.22−albumin [g/dL]). Comparison with Payne’s equation showed strong correlation (r=0.93), with Bland–Altman analysis demonstrating acceptable agreement. Use of the local equation reduced classification of hypocalcemia and increased identification of hypercalcemia compared with unadjusted total calcium. Conclusions A locally derived albumin-adjusted calcium equation demonstrated clinically meaningful differences from Payne’s equation and improved classification of calcium status. Laboratories should establish method- and population-specific adjusted calcium equations to enhance clinical interpretation and decision-making.