Nasal swab screening for methicillin-resistant Staphylococcus aureus--how well does it perform? A cross-sectional study.

OBJECTIVE: To determine the proportion of methicillin-resistant Staphylococcus aureus (MRSA) detections identified by nasal swabbing using agar culture in comparison with multiple body site testing using agar and nutrient broth culture.
DESIGN: Cross-sectional study. PATIENTS: Adult patients admitted to 36 general specialty wards of 2 large hospitals in Scotland.
METHODS: Patients were screened for MRSA via multiple body site swabs (nasal, throat, axillary, perineal, and wound/invasive device sites) cultured individually on chromogenic agar and pooled in nutrient broth. Combined results from all sites and cultures provided a gold-standard estimate of true MRSA prevalence.
RESULTS: This study found that nasal screening performed better than throat, axillary, or perineal screening but at best identified only 66% of true MRSA carriers against the gold standard at an overall prevalence of 2.9%. Axillary screening performed least well. Combining nasal and perineal swabs gave the best 2-site combination (82%). When combined with realistic screening compliance rates of 80%-90%, nasal swabbing alone probably detects just over half of true colonization in practice. Swabbing of clinically relevant sites (wounds, indwelling devices, etc) is important for a small but high-prevalence group.
CONCLUSIONS: Nasal swabbing is the standard method in many locations for MRSA screening. Its diagnostic efficiency in practice appears to be limited, however, and the resource implications of multiple body site screening have to be balanced against a potential clinical benefit whose magnitude and nature remains unclear.