MRSA Nasal Screening Collection Timing
Collect MRSA nasal screening swabs at hospital admission for high-risk patients, within 24-72 hours for ICU admissions, and 1-2 weeks preoperatively for elective surgical patients. 1
High-Risk Admission Screening
Screen immediately upon admission for the following patient populations:
- Patients with prior MRSA infection or colonization within the past year – this is the single most reliable predictor of current MRSA carriage 1, 2
- Direct transfers from other hospitals or nursing homes (odds ratio 6.4 for MRSA colonization) 3
- Patients hospitalized within the previous 3 months 3
- All ICU admissions – screening should occur within 72 hours of ICU admission 4, 1, 5
- Transplantation unit admissions 1
- Hemodialysis patients (p=0.012 for MRSA colonization association) 1
The rationale is straightforward: screening at admission allows implementation of contact precautions before nosocomial transmission occurs, which is critical since culture-based methods take 48-72 hours for results 4. PCR-based screening reduces this to 2-3 hours, enabling same-day isolation decisions 6.
Pre-Operative Screening
Collect screening swabs 1-2 weeks before elective surgery for:
This timing allows completion of 5-day mupirocin decolonization protocols before surgery if the patient screens positive 7. Patients with documented prior MRSA infection should be screened regardless of the time interval, as they remain high-risk 7.
Repeat Screening During Hospitalization
For initially negative patients who remain hospitalized in high-risk settings, collect repeat screening swabs at intervals based on:
- Weekly screening for ICU patients who remain admitted 4
- Screening before transfer to other healthcare facilities 4
- Screening if new risk factors develop: prolonged antibiotic therapy, new invasive devices, surgical procedures, or deteriorating underlying disease 4
The frequency depends on local MRSA prevalence and patient-specific risk factors 4. In endemic settings, repeat screening every 7 days captures ICU-acquired colonization early 5.
Outbreak Investigation Screening
During outbreaks, screen all patients in the affected unit immediately, regardless of risk factors 1. Repeat screening weekly until transmission is controlled 4.
Specimen Collection Technique
Collect nasal swabs using vigorous swabbing of both anterior nares 1. For comprehensive screening in high-risk settings:
- Nasal swabs detect 84% of MRSA carriers 5
- Adding throat swabs increases sensitivity 4
- Consider groin and perianal swabs, as 5% of carriers have negative nasal cultures but positive cultures at other sites 7
- Swab wounds and catheter sites in patients with invasive devices 4
Critical Timing Considerations
The diagnostic accuracy of nasal screening degrades over time: sensitivity is 79% when clinical cultures are obtained within 6 days of screening, but drops to 46% when cultures are obtained ≥7 days after screening (p<0.0001) 8. This means:
- Screening results remain valid for approximately 6 days 8
- PCR-based screening has 88% sensitivity within 6 days, compared to only 65.5% for chromogenic culture methods (p=0.006) 8
- Negative predictive value is 98% in low-prevalence settings, making negative results highly reliable for ruling out MRSA 9
Common Pitfalls to Avoid
- Do not delay screening until after admission – 36% of MRSA cases are imported and only detected through admission screening 3
- Do not rely solely on nasal swabs in outbreak settings – use multi-site screening (nasal, throat, groin, wounds) 4
- Do not assume negative screening remains valid indefinitely – repeat screening for prolonged hospitalizations 4
- Do not screen without concurrent infection control infrastructure – screening is worthless without >90% hand hygiene compliance and available isolation rooms 4, 6