What is the recommended dose of oral cloxacillin for a patient with a mild to moderate leg abscess, assuming normal renal function and no severe immunocompromisation?

Oral Cloxacillin Dosing for Leg Abscess

For a leg abscess, oral cloxacillin should be dosed at 500 mg four times daily (or 1 gram three times daily) for 5-7 days, but only after incision and drainage has been performed, as antibiotics alone are insufficient for abscess treatment. 1

Critical First Step: Incision and Drainage

• Incision and drainage (I&D) is the primary treatment for any abscess and must be performed before considering antibiotic therapy. 1, 2
• The IDSA guidelines explicitly state that "cellulitis" terminology is inappropriate for infections with pus collections like abscesses, and the primary treatment is surgical drainage, not antimicrobials. 1
• I&D alone achieves high clinical cure rates (>90%) in immunocompetent patients without additional interventions. 2

When to Add Oral Antibiotics After I&D

Antibiotics are indicated post-drainage if any of the following high-risk features are present: 1, 2

• Systemic signs of infection (fever, tachycardia, hypotension)
• Surrounding cellulitis extending beyond the abscess
• Multiple abscesses or recurrent infections
• Immunocompromised state (diabetes, neutropenia, HIV)
• Failed initial drainage or inability to achieve adequate drainage
• Presence of SIRS (severe infection)

Specific Cloxacillin Dosing Regimen

For mild to moderate infections after I&D: 1, 3, 4, 5

• Cloxacillin 500 mg orally four times daily for 5-7 days 1, 5
• Alternative: Cloxacillin 1 gram orally three times daily for 5-7 days 3
• Both regimens achieve adequate tissue concentrations above the MIC for methicillin-sensitive Staphylococcus aureus (MSSA), the most common pathogen in skin abscesses. 3, 4

Duration of therapy: 1

• 5 days is recommended for most cases of cellulitis/soft tissue infection, with extension only if infection has not improved. 1
• Recent evidence supports short-course therapy (5-6 days) as non-inferior to longer courses (10-12 days) for skin and soft tissue infections. 1

Microbiological Considerations

• Staphylococcus aureus accounts for 67% of organisms in primary skin infections and 46.7% in secondary infections. 4
• MSSA strains show 93% sensitivity to cloxacillin, making it the first-line oral agent. 4
• Cloxacillin achieves exudate concentrations above MIC for MSSA for approximately 7 hours after each dose, with bactericidal reduction to <0.01% of initial bacterial counts. 3

When Cloxacillin is NOT Appropriate

Switch to broader spectrum coverage if: 1

• MRSA is suspected (penetrating trauma, injection drug use, purulent drainage with systemic signs, known MRSA colonization) - use TMP-SMX, doxycycline, or clindamycin instead. 1
• Gram-negative organisms are likely (chronic wounds, diabetic foot infections, recent antibiotic use) - use amoxicillin-clavulanate or fluoroquinolones. 1
• Patient has penicillin allergy - use erythromycin, clindamycin, or cephalexin (if no anaphylaxis history). 1, 4

Common Pitfalls to Avoid

• Do not prescribe antibiotics without performing I&D first - this is the most common error and leads to treatment failure. 1, 2
• Do not use cloxacillin for diabetic foot infections - these require broader coverage for gram-negative organisms (amoxicillin-clavulanate or fluoroquinolones preferred). 1
• Do not combine ampicillin with cloxacillin routinely - this combination has little place except for cellulitis or infected eczema where streptococcal coverage is needed. 4
• Do not pack the abscess cavity routinely - recent evidence shows no benefit and increased pain compared to non-packed wounds. 2

Follow-Up Assessment

• Re-evaluate at 2-5 days initially to ensure clinical improvement (resolution of erythema, warmth, tenderness, and edema). 1
• If no improvement by day 5, obtain wound cultures and consider MRSA coverage or inadequate drainage. 1