What is the appropriate dosage and treatment duration of Bactrim (trimethoprim/sulfamethoxazole) for an adult patient with a mild to moderate skin infection, considering potential renal function impairment?

Bactrim for Skin Infections

For mild to moderate purulent skin infections (abscesses, furuncles, carbuncles) in adults, use Bactrim 1-2 double-strength tablets (160mg/800mg trimethoprim/sulfamethoxazole) twice daily for 7-10 days after incision and drainage, with dose adjustment required when creatinine clearance falls below 30 mL/min. 1, 2, 3

When to Use Bactrim

Purulent infections are the primary indication:

• Use Bactrim for purulent skin and soft tissue infections such as abscesses, furuncles, and carbuncles, particularly when MRSA is suspected or confirmed 1, 2
• Incision and drainage remains the primary intervention; antibiotics serve as adjunctive therapy 2
• For MRSA skin abscesses specifically, a 10-day course is superior to 3 days, reducing treatment failure by 10% and recurrent infection within 1 month by 10% 4

When NOT to Use Bactrim Alone

Critical limitation - poor streptococcal coverage:

• Do not use Bactrim as monotherapy for non-purulent cellulitis because Group A Streptococcus may have intrinsic resistance 2, 5
• For mixed infections or when streptococci are suspected, add a beta-lactam such as cephalexin or dicloxacillin 2
• Bactrim has poor activity against anaerobic bacteria, making it unsuitable for polymicrobial infections 2

Dosing Algorithm

Standard adult dosing:

• Mild to moderate infection: 1-2 double-strength tablets (160mg/800mg) twice daily for 7-10 days 1, 2, 3
• The higher dose (2 double-strength tablets = 320mg/1600mg twice daily) does not improve clinical resolution rates compared to standard dosing (1 double-strength tablet twice daily) for MRSA skin infections 6
• Treatment duration: 7-14 days based on clinical response, with 10 days preferred for MRSA abscesses 1, 2, 4

Pediatric dosing (age ≥2 months):

• 8-12 mg/kg/day based on trimethoprim component, divided into 2 doses 1
• Alternative: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours in two divided doses 3

Renal Dose Adjustment

Mandatory adjustments for impaired renal function:

• Creatinine clearance >30 mL/min: Use standard dosing 3
• Creatinine clearance 15-30 mL/min: Reduce to half the usual regimen 3
• Creatinine clearance <15 mL/min: Use not recommended 3

Monitoring and Adverse Effects

Common side effects to anticipate:

• Rash, gastrointestinal disturbances, and photosensitivity occur frequently 2
• Hematologic abnormalities including neutropenia and anemia may develop 5

Serious reactions requiring immediate discontinuation:

• Stevens-Johnson syndrome and toxic epidermal necrolysis are rare but life-threatening 2, 5
• Monitor for treatment failure, which may necessitate changing antibiotics 2

Clinical Pearls

Pharmacokinetic advantages:

• Trimethoprim achieves excellent tissue penetration with tissue/serum ratios of 1.2, and demonstrates bactericidal activity against both S. aureus and beta-hemolytic streptococci in diabetic foot infections 7
• Both standard and high doses achieve similar tissue concentrations 7

Recurrence prevention strategies:

• For recurrent S. aureus infections, consider a 5-day decolonization regimen with intranasal mupirocin twice daily, daily chlorhexidine washes, and decontamination of personal items 1
• Nasal mupirocin reduces recurrences by ~50%, while clindamycin 150mg daily for 3 months reduces recurrences by ~80% 2