Outcomes of Staphylococcus aureus bacteraemia attributed to blood culture contamination

Staphylococcus aureus is a common skin commensal and leading cause of bloodstream infection. Given the formidable mortality and morbidity associated with S. aureus bacteraemia (SAB), it is uncertain whether S. aureus can be viewed safely as a blood culture contaminant.

19 episodes of SAB over two years were identified that were prospectively attributed as contaminants using Health Protection Scotland criteria and compared these with 168 cases of genuine SAB. All cases were assessed by an infection specialist and underwent multidisciplinary review to agree the source. Follow-up was a minimum of six months.

Female sex, cognitive impairment and chronic skin conditions were more common in the contaminant group. No single clinical or laboratory feature reliably predicted contamination. No patients in the contaminant group subsequently developed a SAB or metastatic complications associated with SAB. Eighteen patients (95%) survived to hospital discharge. Compared to patients with genuine SAB surviving to discharge, the contaminant group had a shorter duration of hospital stay (median 14 days versus 19.5 days), a briefer exposure to intravenous antibiotics (median 1 day versus 14 days) and received less total antibiotic (median 7 days versus 19 days). Twelve patients received at least five days of antibiotics active against their SAB, though none would have been deemed appropriate SAB treatment under current guidelines.

The careful attribution of SAB as a contaminant by an infection specialist, combined with appropriate follow-up, is associated with favourable clinical outcomes, shorter hospital stay and reduced antibiotic use. A more robust definition of genuine SAB is needed.