Clinical Relevance of Calcium Measures: QT-Based Comparison of Ionized, Total, and Albumin-Corrected Calcium

Background: Ionized calcium (iCa) is the physiologically active fraction of circulating calcium, whereas total calcium (tCa) and albumin-corrected calcium (cCa) remain widely used. Despite growing concerns over the accuracy of correction formulas, most evaluations rely on concordance with iCa rather than meaningful physiologic outcomes. We compared iCa, tCa, and cCa and assessed their clinical validity using ventricular repolarization−reflected by the QT interval−as a physiologic endpoint. Methods: This single-center cross-sectional observational study included adults who visited our outpatient department with simultaneous measurements of iCa, tCa, electrolytes, albumin, and electrocardiography. iCa, tCa and seven albumin-correction formulas were examined. Associations with the QT interval were evaluated using linear regression and restricted cubic splines; logistic regression assessed QTc prolongation. Sensitivity analyses included alternative QT-correction methods and analyses in hypoalbuminemia. Results: Among 647 participants (median age 68 years; 54% male), common comorbidities included hypertension (33%), diabetes (18%), and cardiac disease (11%). Correlations between iCa and other calcium measures varied widely. In fully adjusted analyses, iCa demonstrated the strongest association with QT interval (standardized β = −0.22; 95% CI −0.29 to −0.14). tCa and all cCa exhibited substantially weaker associations. Although some tCa and cCa demonstrated relatively higher correlation with iCa, they exhibited weaker associations with QT. In hypoalbuminemia (albumin <3.0 g/dL, n = 118), agreement with iCa declined for all cCa, and associations with QT were attenuated; only iCa (standardized β = −0.25; 95% CI −0.44 to −0.06) and limited formulas (notably Payne) retained modest associations. Conclusions: iCa is the physiologically and clinically valid measure of calcium status consistently demonstrating the strongest associations with ventricular repolarization. In contrast, tCa and cCa demonstrate substantially reduced clinical relevance and numerical concordance with iCa does not reflect clinical validity. These findings support outcome-based validation of calcium measure and reinforce the importance of direct iCa measurement.