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

Background: Low ionized calcium (I) is prevalent and prognostic in critical care, but poorly detected by either total calcium (T) or albumin-corrected T (cT). We recently derived models of I (Pred-I) and low I (Prob) in critical care that adjust T for binding to albumin and small anions-represented by the anion gap's components. On internal validation, they outperformed cT in diagnosing low I. Two other new anion gap-based models of I, 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 measured on an arterial blood gas panel within 20 min of chemistry panel measurements of T, albumin, sodium, chloride, and total carbon dioxide. The 4 models and cT were assessed by their diagnostic discrimination for low I (<1.10 mmol/l) and high I (>1.32 mmol/l), and by the agreement between predicted and observed values.

Results: Pred-I and Prob had the best discrimination and agreement.

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