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Background: Anion gap has been used as a prognostic indicator in critical care settings as well as cardiovascular fields. However, few studies have demonstrated the association between anion gap in cardiac care units (CCU) and mortality. Objective: To assess a prognostic value of anion gap for in-hospital and 1-year all-cause mortality in CCU patients. Subjects and methods: This is a retrospective cohort study in Songklanagarind Hospital, a tertiary center in Southern Thailand. Patients admitted from the emergency department and outpatient department to CCU from July 2015 to December 2019 were included. Anion gap was calculated from serum electrolytes obtained at the emergency department or within 1 hour of CCU admission by the following formula; Na(mmol/L)-(Cl(mmol/L)+HCO3(mmol/L)). The associations between anion gap and either in-hospital or 1-year all-cause mortality were assessed by logistic and Cox regression models, respectively. Results: This study included 1,271 patients. There was a higher prevalence of diabetes mellitus and chronic kidney disease in patients with a wide anion gap (≥12) than inpatients with a normal anion gap. Compared with the normal anion gap group, hemoglobin level, blood urea nitrogen and serum creatinine were higher in the wide anion gap group, whereas the mean Braden skin score was lower. The 1-year survival rate of patients with an anion gap 14 was significantly lower than that of those with an anion gap <14. The survival rate of patients with an anion gap < 12 was not different from that of those with an anion gap between 12 and 14. There were linear relationships between baseline anion gap and in-hospital and 1-year all-cause mortality risk. Anion gap was an independent predictor for both in-hospital (odds ratio 1.13, 95% CI 1.07-1.19, p < 0.0001) and 1-year all-cause mortality (hazard ratio 1.09, 95%CI 1.07-1.12, p < 0.0001) in the multivariate analysis. Conclusions: Anion gap was an independent predictor for in-hospital and 1-year all-cause mortality in CCU patients. Calculating anion gap from three routine basic electrolytes can be used as a quick and reliable marker to predict the risk of CCU patients without extra cost.
Keywords: Anion gap, cardiac care unit (CCU), mortality predictors