Novel methods of predicting ionized calcium status from routine data in critical care: External validation in MIMIC-III

Abstract

Background: Low ionized calcium (I_Ca) is prevalent and prognostic in critical care, but poorly detected by either total calcium (T_Ca) or albumin-corrected T_Ca (cT_Ca). We recently derived models of I_Ca (Pred-I_Ca) and low I_Ca (Prob_HYPO) in critical care that adjust T_Ca for binding to albumin and small anions-represented by the anion gap's components. On internal validation, they outperformed cT_Ca in diagnosing low I_Ca. Two other new anion gap-based models of I_Ca, derived in renal patients, have not been validated. This study tested the external validity of these 4 new models in the Medical Information Mart for Intensive Care III (MIMIC-III) database.

Methods: We identified 4105 patients in MIMIC-III with I_Ca measured on an arterial blood gas panel within 20 min of chemistry panel measurements of T_Ca, albumin, sodium, chloride, and total carbon dioxide. The 4 models and cT_Ca were assessed by their diagnostic discrimination for low I_Ca (<1.10 mmol/l) and high I_Ca (>1.32 mmol/l), and by the agreement between predicted and observed values.

Results: Pred-I_Ca and Prob_HYPO had the best discrimination and agreement.

Conclusions: Pred-I_Ca and Prob_HYPO were externally validated in MIMIC-III. They can help clinicians efficiently decide when to order direct I_Ca testing in critical care.

Keywords: Albumin; Anion gap; Hypercalcemia; Hypocalcemia; Ionized calcium.