Switching to Cefazolin for Cellulitis with Negative MRSA Nares Test
Yes, it is safe to switch to cefazolin for treating cellulitis if the MRSA nares test is negative, as first-line empiric treatment for uncomplicated cellulitis should target streptococci and methicillin-sensitive Staphylococcus aureus (MSSA). 1
Evidence-Based Rationale
MRSA Nares Testing and Antibiotic Selection
- A negative MRSA nares test has high negative predictive value, with a systematic review showing a specificity of 93.7% (95% CI 77.1-98.4) for predicting subsequent infections 2
- The Infectious Diseases Society of America (IDSA) guidelines support that first-line empiric treatment for uncomplicated cellulitis should target streptococci and methicillin-sensitive staphylococci 1
- Cefazolin is FDA-approved for skin and skin structure infections due to S. aureus (including beta-lactamase-producing strains) and S. pyogenes 3
Treatment Algorithm for Cellulitis
For patients with negative MRSA nares test:
- Switch to cefazolin for IV therapy
- For oral therapy options: cephalexin, dicloxacillin, or amoxicillin-clavulanate 1
When to consider MRSA coverage despite negative nares test:
- Purulent cellulitis (uncommon)
- Prior history of MRSA infection
- Local high prevalence of community-associated MRSA
- Immunocompromised status
- Treatment failure with beta-lactams
Efficacy of Cefazolin for Cellulitis
- Cefazolin has demonstrated effectiveness for skin and soft tissue infections caused by susceptible organisms, particularly MSSA and streptococci 3
- In a retrospective study of outpatients with cellulitis treated with once-daily IV cefazolin plus probenecid, the overall success rate was 78%, with chronic venous disease being the main risk factor for treatment failure 4
Important Considerations
- Causative organisms: Cellulitis is most commonly caused by streptococci and MSSA, which are effectively covered by cefazolin 1, 5
- Duration of therapy: Typically 5-7 days for uncomplicated cellulitis 1
- Monitoring: Reevaluation within 48-72 hours to assess treatment response 1
- Caveats:
- In areas with high prevalence of community-associated MRSA, treatment success rates with antibiotics active against MRSA (trimethoprim-sulfamethoxazole, clindamycin) may be higher than with cephalosporins alone 6
- However, a randomized controlled trial showed that adding trimethoprim-sulfamethoxazole to cephalexin did not improve outcomes for non-purulent cellulitis, supporting that beta-lactam monotherapy is appropriate for most cases 7
Risk Factors for Treatment Failure
- Chronic venous disease (odds ratio 4.4) 4
- Severity of cellulitis 6
- Therapy with an antibiotic not active against the causative organism 6
In conclusion, based on the most recent guidelines and evidence, cefazolin is an appropriate and safe choice for treating cellulitis in patients with a negative MRSA nares test, as it provides excellent coverage against the most common causative pathogens (streptococci and MSSA).