Resistance Rates of Staphylococcus Aureus in Relation to Patient Status and Type of Specimen

Objectives: 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.

Materials And Methods: 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.

Results: 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).

Conclusions: 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.