The utility of risk factors to define complicated Staphylococcus aureus bacteremia in a setting with low MRSA prevalence

INTRODUCTION: Recommended duration of antibiotic treatment of Staphylococcus aureus bacteramia (SAB) is frequently based on distinguishing uncomplicated and complicated SAB, and several risk factors at the onset of infection have been proposed to define complicated SAB. Predictive values of risk factors for complicated SAB have not been validated, and consequences of their use on antibiotic prescriptions are unknown.

METHODS: In a prospective cohort, patients with SAB were categorized as complicated or uncomplicated through adjudication (reference definition). Associations and predictive values of 9 risk factors were determined, compared to the reference definition, as was accuracy of IDSA-criteria that include 4 risk factors, and the projected consequences of applying IDSA criteria on antibiotic use.

RESULTS: Among 490 patients, 296 (60%) had complicated SAB. In multivariable analysis, persistent bacteraemia (odds ratio (OR) 6.8 (95% CI 3.9-12.0)), community-acquisition of SAB (OR 2.9 (95% CI 1.9-4.7)) and presence of prosthetic material (OR 2.3 (95%CI 1.5-3.6)) were associated with complicated SAB. Presence of any of the four risk factors in the IDSA-definition of complicated SAB had Positive Predictive Value of 70.9% (95%CI 65.5-75.9) and Negative Predictive Value of 57.5% (49.1-64.8). Compared to the reference, IDSA criteria yielded 24 (5%) false-negative and 90 (18%) false-positive classifications of complicated SAB. Median duration of antibiotic treatment of these 90 patients was 16 days (IQR 14-19), all with favourable clinical outcome.

DISCUSSION: Risk factors have low to moderate predictive value to identify complicated SAB and their use may lead to unnecessary prolonged antibiotic use.