Role of Comorbidity in Mortality Related to Staphylococcus aureus Bacteremia: A Prospective Study Using the Charlson Weighted Index of Comorbidity

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Abstract
Objective: To demonstrate the effectiveness of the Charlson weighted index of comorbidity (WIC) for controlling comorbidity in prospective studies focusing on mortality in patients with Staphylococcus aureus bacteremia (SAB). Design: Cohort study. Setting: Two tertiary-care, university-affiliated hospitals in France. Patients: One hundred sixty-six inpatients 18 years or older consecutively diagnosed with SAB from May 15, 2001, to May 15, 2002. Methods: Patients were prospectively assessed and cases were followed by the infectious diseases consult service at least 3 months after effective antibiotic therapy completion. The Charlson WIC was computed and dichotomized into scores of fewer than 3 points and 3 or more points. Bacteremia source, acute complication due to SAB acquisition in the ICU, and inappropriate empiric antibiotic therapy were recorded. The endpoint was death due to SAB and overall mortality. Results: In univariate analysis, the Charlson WIC was able to predict overall mortality and S. aureus-related death. The following variables were found to be independently predictive of mortality due to SAB using the Cox model: an acute complication due to S. aureus (OR, 8.9; CI95, 4 to 19.7; P < .001), a Charlson WIC score of 3 or more (OR, 3; CI95, 1.3 to 5.5; P = .006), and age (OR, 1.04; CI95, 1.009 to 1.07; P < .01). Conclusions: Comorbidity contributes to death in patients with SAB. The Charlson WIC is a good predictor of mortality in this population and may be a useful instrument to control comorbidity in studies aiming to investigate risk factors for death due to bacteremia.