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

Background: Staphylococcus aureus bacteraemia (SAB) is a common complication of S. aureus infection and is associated with a high mortality. Aims: To document prospectively the pattern of ill- ness associated with SAB 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 prospectively studied in six tertiary care hospitals. All information obtained from patients' records was recorded on worksheets and transferred to a computerized 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%: intravenous catheter (31%), primarily hospital acquired, and skin/soft tissue (22%), primarily community acquired, were the most common foci. The 30-day mortality was 19%, 83% of whom died within 2 weeks. Risk factors for a poor outcome 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). Mortality 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 certain ethnic groups, while mortality is associated with other identifiable risk factors and continues to be high. Intravenous catheters remain the most common and most preventable cause of SAB.