Predicting ionized hypocalcemia: External validation of an ionized calcium prediction model in patients with COVID-19 and renal failure

Abstract

Background Ionized hypocalcemia is common in critically ill patients with COVID-19 and is associated with adverse outcomes. We previously developed a linear model that estimates ionized calcium (ICa) by adjusting total calcium (TCa) for the three components of the anion gap and albumin. On internal validation, it outperformed the popular method that corrects TCa for albumin alone (cTCa) in diagnosing low ICa. In this study, we sought to externally validate our ICa model in hospitalized COVID-19 positive patients. Methods We retrospectively studied all 200 patients with COVID-19 who were admitted to the State University of New York Downstate Medical Center between March 11th and April 30th 2020 and referred to the nephrology service for renal failure, and who had ICa measured on a venous blood gas within 25 min of a comprehensive metabolic panel. We compared the performance of the ICa model and cTCa in diagnosing low ICa by ROC analysis, and also examined the accuracy of the absolute values predicted by the two methods relative to measured ICa. Results On ROC analysis, the ICa model was better than cTCa (area under ROC curve: 0.872 [0.025] vs. 0.835 [0.028]; p = 0.045). The ICa model estimated ICa accurately, but the cTCa method seemed to overcorrect TCa, as a substantial number of patients with clearly normal cTCa values had low ICa. Conclusions In an external validation cohort, the ICa model estimated ICa accurately and was better than cTCa in the diagnosis of low ICa. This finding can be useful in guiding direct ICa testing.