Arterial and venous ionized calcium measurements: Is there a difference?

Abstract

Study objectives: In critically ill patients, the ionized calcium level has commonly been obtained from arterial blood gas samplings. Arterial puncture is associated with increased risks (ie, pseudoaneurysm) and is more painful compared with venipuncture. This study will assess the reliability of ionized calcium levels obtained by venous blood gas sampling compared with arterial blood gas samplings. Methods: A cross-sectional study of 102 patients in the emergency department of an urban, inner-city hospital was performed. Paired samples of arterial and venous blood were obtained within a 15-minute interval. Arteriovenous differences were analyzed for multiple variables (ie, Na, K, hemoglobin, glucose, and lactate), including ionized calcium. Statistical analysis was performed on the entire data set, as well as subsets, according to acidosis (pH <7.35) and alkalosis (pH >7.45). A paired sample <i>t</i> test was used, and Pearson correlation coefficient was determined for confirmation. Results: Within the entire data set (n=102), the mean difference between venous and arterial ionized calcium measurements was 0.015±0.045 (<i>P</i>=.001) with a Pearson correlation of <i>r</i>=0.94 (<i>P</i><.0001). When the arterial pH was acidotic (n=18), the mean difference between venous and arterial ionized calcium was 0.014±0.030 (<i>P</i>=.069), with a Pearson correlation of <i>r</i>=0.98 (<i>P</i><.0001). Under alkalotic states (n=20), the mean difference between venous and arterial ionized calcium was 0.036±0.039 (<i>P</i>=.0007), with a Pearson correlation of <i>r</i>=0.98 (<i>P</i><.0001). Conclusion: There is excellent correlation in the measurement of ionized calcium between venous and arterial blood gas samples, including both acidotic and alkalotic states. These data should lend support to the use of venous blood gas analysis for the determination of ionized calcium levels when arterial blood gas sampling is otherwise not required, is difficult, or is not desired by the patient.