Prospective study of 424 cases of Staphylococcus aureus bacteraemia: determination of factors affecting incidence and mortality

Background: Staphylococcus aureus bacteraemia (SAB) is a common complicatio n of S. aureus infection and is associated with a high mortality. Aims: To document prospectively the pattern of ill- ness associated with SA B in New Zealand and, by recording patient demographic factors and clinical features, to identify risk factors associated with a poor outcome. Methods: From 1 July 1996 to 31 December 1997, adults with SAB were prospec tively studied in six tertiary care hospitals. All information obtained fro m patients' records was recorded on worksheets and transferred to a compute rized spreadsheet for analysis. Results: There were 424 patients with SAB. Maori (relative risk (RR) = 1.8, 95% confidence interval (CI) = 1.3-2.6) and Pacific Island people (RR = 4. 0, 95% CI = 3.1-5.3) were significantly more likely than people of European descent to acquire SAB, but not to die from the infection. Fifty per cent of cases were community acquired. A source was identified for 85%: intraven ous catheter (31%), primarily hospital acquired, and skin/soft tissue (22%) , primarily community acquired, were the most common foci. The 30-day morta lity was 19%, 83% of whom died within 2 weeks. Risk factors for a poor outc ome were: increasing age above 60, female sex (RR = 1.4, 95% CI = 1.0-2.1), diabetes mellitus (RR = 1.5, 95% CI = 1.0-2.4), immunosuppression (RR = 1. 5, 95% CI = 1.0-2.4), pre-existing renal impairment (RR = 1.8, 95% CI = 1.2 -2.7), malignancy (RR = 2.2, 95% CI = 1.4-3.5), lung as a source (RR = 2.8, 95% CI = 1.9-4.2) and unknown source (RR = 2.3, 95% CI = 1.5-3.3). Mortali ty was also accurately predicted by two multifactor scoring systems. There was a low rate of methicillin resistance (5%). Conclusions: Staphylococcus aureus bacteraemia is more likely to occur in c ertain ethnic groups, while mortality is associated with other identifiable risk factors and continues to be high. Intravenous catheters remain the mo st common and most preventable cause of SAB.