Background/Aims: The aim of this study is to measure the difference of ionizedcalcium between heparinized whole blood and serum. Methods: We recruited 107 maintenance hemodialysis (HD) patients from ourhospital HD unit. The clinical and laboratory data included ionized calcium inserum and in whole blood (reference, 4.07 to 5.17 mg/dL). Results: The level of ionized calcium in serum was higher than that in wholeblood (p < 0.001). Bland-Altman analysis showed that difference for ionized calciumwas 0.5027. For the difference, the nonstandardized β was -0.4389 (p < 0.001)and the intercept was 2.2418 (p < 0.001). There was a significant difference in thedistribution of categories of ionized calcium level between two methods (κ, 0.279;p < 0.001). Conclusions: This study demonstrates that whole blood ionized calcium is underestimatedcompared with serum ionized calcium. Positive difference increasesas whole blood ionized calcium decreases. Therefore, significant hypocalcemia inwhole blood ionized calcium should be verified by serum ionized calcium.