Bactrim vs Keflex for Skin Abscesses
For uncomplicated skin abscesses after incision and drainage, use Bactrim (trimethoprim-sulfamethoxazole) rather than Keflex (cephalexin), as Bactrim provides superior outcomes while cephalexin offers no benefit over placebo in the MRSA era.
Primary Treatment Algorithm
Step 1: Incision and Drainage First
- Incision and drainage (I&D) is the definitive treatment for all skin abscesses 1, 2
- Antibiotics are adjunctive therapy only
Step 2: Decide if Antibiotics Are Needed
Use antibiotics after I&D if ANY of the following are present 1, 2:
- Multiple abscesses or carbuncles
- Systemic signs of infection (fever >38°C, tachycardia >90, WBC >12,000)
- Significant surrounding cellulitis (erythema extending >5 cm)
- Immunocompromised state
- Extremes of age
- Abscess in difficult-to-drain location
- Failed initial treatment
- Lack of response to I&D alone after 48 hours
Step 3: Choose the Right Antibiotic
Use Bactrim (TMP-SMX):
- Dosing: 1-2 double-strength tablets (160/800 mg) twice daily for 7-10 days 1
- Pediatric: 8-12 mg/kg/day (based on TMP component) divided twice daily 1
- Why: Moderate-to-high quality evidence shows TMP-SMX reduces treatment failure (OR 0.58), recurrence within 1 month (OR 0.48), hospitalization (OR 0.55), and late recurrence (OR 0.64) 3
- MRSA coverage: 55-87% of cultured abscesses grow MRSA in contemporary studies 4, 5, 6
Avoid Keflex (cephalexin):
- Cephalexin provides NO benefit over placebo for skin abscesses in the MRSA era 4, 3
- One high-quality RCT showed 90.5% cure with placebo vs 84.1% with cephalexin (no significant difference) 4
- Network meta-analysis confirms cephalosporins do not reduce treatment failure compared with placebo (moderate quality evidence) 3
Alternative: Clindamycin
- Use if TMP-SMX contraindicated (sulfa allergy, pregnancy, renal failure)
- Dosing: 300-450 mg four times daily for adults; 30-40 mg/kg/day divided three times daily for children 1, 2
- Caution: Higher risk of diarrhea (OR 2.71) and C. difficile infection compared to TMP-SMX 3
Duration of Therapy
For MRSA-positive abscesses: Use 10 days, not 3 days
- A pediatric RCT showed 10-day TMP-SMX reduced treatment failure by 10.1% and 1-month recurrence by 10.3% compared to 3 days in MRSA infections 5
- For non-MRSA or culture-negative: 7 days is adequate 1
Key Evidence Nuances
Why Not Keflex?
The 2014 IDSA guidelines 1 list cephalexin as an option for MSSA infections, but this predates high-quality RCT evidence showing cephalexin's ineffectiveness. The 2007 placebo-controlled trial 4 definitively demonstrated that in populations with high MRSA prevalence (87.8% of S. aureus isolates), cephalexin performs no better than placebo after I&D.
The Cellulitis Exception
For non-purulent cellulitis WITHOUT abscess, the evidence differs:
- A 2017 RCT showed cephalexin alone was non-inferior to cephalexin plus TMP-SMX for cellulitis 7
- This is because cellulitis is predominantly streptococcal, not staphylococcal
- Do not extrapolate cellulitis data to abscess management
Treatment Effect by Subgroup
TMP-SMX benefits are consistent across 6:
- All lesion sizes (even <5 cm)
- Patients with and without guideline-recommended antibiotic criteria
- Greatest benefit in patients with: prior MRSA infection, fever, or MRSA-positive cultures
Common Pitfalls to Avoid
Using cephalexin for abscesses - This is the most common error in ED practice, occurring in 68-71% of cases despite lack of efficacy 8
Prescribing antibiotics without adequate I&D - Antibiotics cannot substitute for surgical drainage 1, 2
Stopping TMP-SMX too early in MRSA infections - Use 10 days, not 3-5 days 5
Monitoring failures - Watch for hyperkalemia, especially in patients with renal insufficiency or on ACE inhibitors/ARBs 9
Ignoring adverse effects - TMP-SMX increases GI side effects (OR 1.28) but far less than clindamycin (OR 2.29 for GI effects, OR 2.71 for diarrhea) 3