MRSA Risk Score Calculator

MRSA (methicillin-resistant Staphylococcus aureus) is a strain of staph that resists most beta-lactam antibiotics.
Working out who is at elevated risk matters in both directions.
Empiric antibiotic choices for a serious infection need to cover MRSA when the pretest probability is high, and need to leave it out when the probability is low, because unnecessary vancomycin drives resistance and carries real toxicity.

This calculator adds up risk factors that recur across infection-control surveillance and the guidelines. Each factor present adds points to a running total. To be plain about it: the factors are well established, but the point weights are this page’s own, not a published and validated score. A higher total means more of the known risk factors are stacking up, which raises both the chance of MRSA carriage on a screening swab and the chance that a clinical infection (skin and soft tissue, bloodstream, pneumonia) turns out to be MRSA rather than methicillin-sensitive staph.

For pneumonia there is a guideline rule to set beside it. The 2019 ATS/IDSA (American Thoracic Society and Infectious Diseases Society of America) guideline for community-acquired pneumonia names two factors that justify covering MRSA from the start: MRSA grown from a respiratory sample before, and a hospital stay with intravenous antibiotics in the past 90 days, the second weighed against local data.

Risk factors and weights, out of a maximum of 14 points:

• Known prior MRSA colonization or infection (+3). The strongest single predictor by a clear margin. Previous carriers frequently recolonize even after a completed decolonization protocol, and a positive history from a year ago still shifts the odds.
• Hospitalization in the past 90 days (+2). The classic healthcare-associated exposure.
• Long-term care facility residence (+2). Colonization rates in nursing facilities commonly run above 10% of residents, and considerably higher in units with high antibiotic use.
• Hemodialysis (+2). Repeated vascular access plus frequent healthcare contact. Dialysis patients have among the highest documented MRSA bacteremia rates of any outpatient group.
• Injection drug use (+2). Community-associated MRSA strains circulate heavily in this population, and the entry route is direct.
• Antibiotics in the past 90 days (+1). Suppressing competing flora selects for what survives.
• Indwelling vascular catheter or other invasive device (+1). A direct portal for skin organisms.
• Surgery in the past 30 days (+1). Transient wound and skin exposure.

Score interpretation, and these thresholds are deliberately rough:

• 0 to 1 points, low risk. Empirical MRSA coverage usually unnecessary unless something else in the clinical picture points at it.
• 2 to 4 points, moderate risk. Consider MRSA coverage if the infection is serious. A screening culture is often worth sending.
• 5 or more points, high risk. Empirical coverage is often indicated for a significant infection, and a screening culture helps guide ongoing care.

This is a screening aid, not a diagnostic test, and it has a limitation worth stating plainly: local prevalence swings the answer more than the score does. A total of 3 in a hospital where 60% of staph isolates are methicillin-resistant means something quite different from a 3 in a community running at 8%. Always read the score alongside your own antibiogram and the clinical picture in front of you. A nasal PCR swab, where available, answers the colonization question in a couple of hours and beats any point score. Its main strength is a negative result: in a 2018 meta-analysis (Parente and colleagues) a negative nasal screen ruled out MRSA pneumonia about 96% of the time, which is why many hospitals use it to stop empiric vancomycin early.