Which patients should be tested for Methicillin-resistant Staphylococcus aureus (MRSA)?

Which Patients Should Be Tested for MRSA

Patients at high risk for MRSA carriage should be screened, including those with prior MRSA infection/colonization within the past year, those admitted to high-risk units (especially ICUs), and those with specific risk factors such as previous hospitalization, long-term antibiotic use, chronic wounds, or hemodialysis. 1

High-Priority Screening Populations

Patients with Prior MRSA History

• Any patient with documented MRSA infection or colonization within the past year should be screened upon hospital admission 1
• Prior MRSA infection is the most reliable predictor for current MRSA carriage 1
• MRSA colonization can persist for extended periods, with a half-life of 549 days, and even longer (801 days) when multiple body sites are colonized 2

High-Risk Clinical Settings

• All patients admitted to intensive care units should undergo MRSA screening 1
• Patients admitted to transplantation units require screening 1
• Surgical patients, particularly those undergoing orthopedic, cardiac, thoracic, or neurosurgical procedures 1
• Patients in outbreak situations or when local MRSA prevalence is high 1

Specific Risk Factors for MRSA Colonization

• Hemodialysis patients (significantly associated with MRSA colonization, p=0.012) 1
• Patients with diabetic foot infections, especially those with:
  • Long duration of foot wounds 1
  • Presence of osteomyelitis 1
  • Previous hospitalization 1
  • Prior long-term or inappropriate antibiotic use 1
• Trauma patients admitted to trauma ICUs (10.1% colonization rate at admission) 3

Community vs. Healthcare Settings

• Community-acquired MRSA cases are increasing, making the distinction between healthcare-acquired and community-associated infections less clear 1
• Veterans Affairs nursing home residents show higher MRSA prevalence (30.3%) compared to community nursing homes (9.9%) 4

Screening Methodology

Optimal Specimen Collection

• Nasal swabs are the preferred screening method - they are non-invasive, well-tolerated, and do not require local anesthesia 5
• Vigorous swabbing of the nares is required for proper screening 5
• For comprehensive screening in high-risk settings, combine throat and nasal swabs 1
• Anal or rectal swabs for VRE surveillance when co-screening is indicated 1

Timing of Screening

• Screen at admission for high-risk patients 1
• For initially negative patients who remain at high risk, repeat screening at intervals during hospitalization 6
• Factors warranting repeat screening include: location in high-risk units, receipt of broad-spectrum antibiotics, invasive procedures, and underlying disease severity 6

Clinical Significance of Colonization

Infection Risk in Colonized Patients

• MRSA carriers have significantly higher infection rates compared to non-carriers 7, 3
• In long-term care: 25% of MRSA carriers developed infection vs. 4% of non-carriers (RR 3.8, p<0.01) 7
• In trauma patients: 33.3% of colonized patients developed MRSA infections vs. 6.6% of non-colonized (p<0.001) 3
• The rate of infection development among MRSA carriers is 15% per 100 days of carriage 7
• 73% of all MRSA infections occur among MRSA carriers 7

Impact on Outcomes

• MRSA colonization in trauma patients who develop infections is associated with higher mortality (22.2% vs. 5.0%, p<0.001) 3
• MRSA infection may increase time to wound healing, duration of hospitalization, need for surgical procedures including amputations, and likelihood of treatment failure 1

Common Pitfalls to Avoid

• Do not rely solely on clinical suspicion without screening in high-risk populations - colonization often precedes infection and is asymptomatic 7, 3
• Avoid inadequate swabbing technique - undirected nasal swabs show poor correlation with properly directed swabs due to contamination 5
• Do not assume negative screening eliminates risk - patients can acquire MRSA during hospitalization, with 23.8% of incident cases occurring during a hospital stay 4
• Do not screen only for clinical diagnosis specimens - dedicated screening specimens are necessary for surveillance purposes and cannot be replaced by routine diagnostic cultures 1
• Recognize that a single negative test does not guarantee continued MRSA-free status, especially in high-risk settings 6

Local Prevalence Considerations

Empiric MRSA coverage decisions should be based on local prevalence thresholds 1:

• For mild soft tissue infections: consider empiric coverage when local MRSA prevalence reaches 50% of all S. aureus isolates 1
• For moderate soft tissue infections: consider empiric coverage when local MRSA prevalence reaches 30% 1
• For severe infections: empiric MRSA coverage is warranted regardless of prevalence, as failing to cover MRSA while awaiting cultures poses unacceptable treatment failure risk 1