Resistance rates of Staphylococcus aureus in relation to patient status and type of specimen

The aim of this study was to assess the impact of data stratification on the resistance rates of Staphylococcus aureus, with emphasis on the value of blood culture-based resistance data.

All S. aureus isolates from patients in the Vienna University Hospital (2140 beds), isolated between 1/1996 and 12/2002, were stratified by patient status (ICU patient, regular inpatient, outpatient). Four kinds of specimen [blood, respiratory tract (RT), wounds and urine] were defined for analysis. Oxacillin and eight other compounds were considered.

In total, 10,575 first isolates per patient were detected, derived from ICU patients (n=1464), inpatients (n=4152), and outpatients (n=4959). From blood, wounds, RT and urine, 610, 1464, 2716 and 3370 first isolates per patient, respectively, were available. The blood-MRSA-rate (19.93%) was similar to the MRSA-rate of RT- (OR: 0.98, 95% CI: 0.76-1.25), wound- (OR: 0.89, 95% CI: 0.71-1.12), and urine-isolates (OR: 0.91, 95% CI: 0.72-1.14). Isolates from inpatients (OR: 0.59, 95% CI: 0.47-0.74) and outpatients (OR: 0.16, 95% CI: 0.13-0.21), regardless of the specimen, showed lower MRSA-rates than blood-isolates, in contrast to isolates from ICU patients (OR: 1.12, 95% CI: 0.87-1.44). For other compounds, the resistance rates of blood-isolates were not always representative for RT- (six of eight rates similar), wound- (7/8), or urine-isolates (5/8). Most importantly, RT-, wound- and urine-isolates were significantly more often resistant to ciprofloxacin. Resistance rates of blood-isolates were more representative for isolates from inpatients (five of eight rates similar) than from ICU patients or outpatients (each 3/8).

The resistance rates of blood culture isolates enable a good overall assessment of the resistance of other clinically significant isolates. However, resistance data derived from selected specimens must not be equated with the overall resistance situation in the hospital.