What are the risk factors for Methicillin-resistant Staphylococcus aureus (MRSA) infections?

MRSA Risk Factors

Previous MRSA infection or colonization within the past year is the single most reliable predictor for future MRSA infection, and should be the primary factor guiding empiric antibiotic coverage decisions. 1, 2

Most Reliable Predictor

• Prior MRSA colonization or infection within the past 12 months carries a 9-10 times greater odds of developing MRSA infection and is consistently identified as the strongest risk factor across all clinical settings 3, 1, 4, 5

Healthcare-Associated Risk Factors

Recent Healthcare Exposures

• Recent hospitalization within the preceding 12-24 months significantly increases MRSA risk 1, 2, 6
• Long-term care facility or rehabilitation center residence within the last 18 months 1, 2
• Recent antibiotic exposure within 3-12 months, particularly beta-lactams, carbapenems, or quinolones 1, 7, 2
• Intravenous antibiotic treatment within the prior 90 days specifically for hospital-acquired pneumonia 2

Invasive Devices and Procedures

• Central venous catheters (60.4% of hemodialysis-associated MRSA cases had CVCs) 1, 7, 8
• Temporary dialysis access (66% increased risk compared to permanent access) 6
• Urinary catheters 1, 9
• Endotracheal tubes and nasogastric tubes 1, 9
• Surgical drains 1, 9

Open Wounds and Skin Breakdown

• Surgical wounds (2.9 times greater hazard ratio for progression from colonization to infection) 1, 9, 5
• Pressure ulcers (3.0 times greater hazard ratio) 1, 9
• Chronic skin lesions or ulcers 1, 7

High-Risk Comorbidities

Chronic Medical Conditions

• Diabetes mellitus, particularly in diabetic foot infections where MRSA prevalence ranges from 5-30% 3, 1, 7, 2
• Chronic kidney disease requiring hemodialysis (invasive MRSA incidence of 4.2 per 100 dialysis patients annually) 1, 7, 6, 8
• Chronic obstructive pulmonary disease (2.16 times greater odds) 7, 6
• Congestive heart failure 7, 2
• Chronic liver failure 7, 2
• Immunosuppression from any cause, including HIV infection and chemotherapy-induced neutropenia 7, 2

Nutritional and Laboratory Markers

• Lower serum albumin levels are associated with MRSA colonization in dialysis patients 6

Clinical Setting Risk Factors

Intensive Care Unit

• ICU admission carries a 26.9 times greater hazard ratio for developing MRSA infection within the first 4 days compared to medical ward patients 9, 5
• Respiratory failure requiring ventilatory support 2, 5
• Septic shock warrants empiric MRSA coverage regardless of other risk factors 1, 2

Surgical Patients

• Hip or knee replacement surgery (3.8 times greater odds of MRSA surgical site infection) 4
• Administration of three or more antibiotics perioperatively 9

Local Epidemiology Thresholds

Empiric MRSA coverage should be initiated based on local prevalence thresholds: 1, 2

• ≥50% of S. aureus isolates for mild soft tissue infections
• ≥30% of S. aureus isolates for moderate soft tissue infections
• ≥20% of S. aureus isolates in hospital-acquired pneumonia settings or specific ICU/hospital units

Community-Associated MRSA Risk Groups

• Children <2 years old 7
• Contact-sport athletes 7
• Injection drug users 7
• Military personnel 7
• Inmates of correctional facilities, residential homes, or shelters 7
• Veterinarians, pet owners, and pig farmers 7

Critical Clinical Implications

When to Provide Empiric MRSA Coverage

Empiric anti-MRSA therapy is warranted in three specific scenarios: 3, 1, 2

1. Patient has prior MRSA history within the past year
2. Local MRSA prevalence exceeds the threshold for the infection type and severity
3. Infection severity is high enough that treatment failure while awaiting cultures would pose unacceptable risk (septic shock, bacteremia, severe pneumonia)

Progression from Colonization to Infection

• 11.1% of MRSA-colonized patients progress to clinical infection 9
• 70% of dialysis patients with invasive MRSA had been hospitalized in the year prior to infection 8
• Intravenous catheterization carries a 4.7 times greater hazard ratio for progression to infection among colonized patients 9

Important Caveats

• The distinction between healthcare-acquired and community-associated MRSA has become increasingly blurred, making traditional classification less clinically useful 3, 7
• Some patients with MRSA-positive cultures may improve despite receiving antibiotics ineffective against MRSA, but this should never guide initial empiric therapy decisions 3
• For diabetic foot infections with concern for osteomyelitis, obtain a bone specimen when MRSA is suspected rather than relying solely on soft tissue cultures 3
• Healthcare-associated characteristics like hemodialysis or previous hospitalization, while important for colonization risk, may not independently predict surgical site infection in the absence of documented prior MRSA 4