What is the recommended dosing of Bactrim (trimethoprim/sulfamethoxazole) liquid formulation for a 40kg patient with Methicillin-Resistant Staphylococcus Aureus (MRSA) infection?

Bactrim Liquid Dosing for MRSA in a 40kg Patient

For a 40kg patient with MRSA infection, administer trimethoprim-sulfamethoxazole (TMP-SMX) liquid at 4 teaspoonfuls (20 mL) every 12 hours, which provides trimethoprim 4-6 mg/kg/dose and sulfamethoxazole 20-30 mg/kg/dose. 1

Specific Dosing Calculation

• Standard dose for purulent cellulitis/skin infections: The IDSA guidelines recommend trimethoprim 4-6 mg/kg/dose with sulfamethoxazole 20-30 mg/kg/dose administered every 12 hours 1

• For a 40kg patient: This translates to 4 teaspoonfuls (20 mL) of the standard liquid formulation every 12 hours 2

• Liquid concentration: Standard TMP-SMX oral suspension contains 40 mg trimethoprim and 200 mg sulfamethoxazole per 5 mL 2

• Total daily dose: 160-240 mg trimethoprim and 800-1200 mg sulfamethoxazole divided into two doses 1

Type of MRSA Infection Matters

For uncomplicated purulent cellulitis or skin abscesses:

• Use the standard dose of 4 teaspoonfuls (20 mL) every 12 hours for 7-10 days 1
• Incision and drainage is the primary treatment; antibiotics are adjunctive 1

For complicated skin and soft tissue infections:

• Consider intravenous therapy with vancomycin or linezolid instead 1
• TMP-SMX is not recommended as first-line for complicated infections requiring hospitalization 1

For severe MRSA infections (bacteremia, pneumonia, osteomyelitis):

• TMP-SMX should NOT be used as monotherapy 3
• A 2015 randomized controlled trial showed TMP-SMX failed to achieve non-inferiority to vancomycin for severe MRSA infections, particularly in bacteremia (34% mortality vs 18% with vancomycin) 3

Critical Dosing Considerations

Higher doses are NOT more effective for skin infections:

• A 2011 prospective study found that high-dose TMP-SMX (320/1600 mg twice daily) had similar clinical resolution rates (73%) compared to standard dose (160/800 mg twice daily, 75% resolution) for MRSA skin infections 4
• Therefore, stick with the standard pediatric weight-based dosing 4

Age restrictions:

• TMP-SMX is contraindicated in children less than 2 months of age 2
• For a 40kg patient (typically 10-12 years old), there are no age-related restrictions 2

Duration of Therapy

• Uncomplicated skin infections: 7-10 days 1
• Purulent cellulitis: 7-10 days 1
• Treatment should continue until clinical resolution (absence of fever, improvement in erythema and swelling) 1

Common Pitfalls to Avoid

Never use TMP-SMX for:

• Severe systemic MRSA infections requiring hospitalization 3
• MRSA bacteremia as monotherapy (associated with higher mortality) 3
• MRSA pneumonia (poor lung penetration compared to alternatives) 1
• Pregnant women in third trimester (pregnancy category C/D) 1

Always ensure:

• Adequate incision and drainage has been performed for abscesses—antibiotics alone will fail 1
• Local MRSA susceptibility to TMP-SMX is confirmed or highly likely 1
• Patient is not on other medications that interact with sulfonamides 2

Alternative Agents if TMP-SMX Fails or is Contraindicated

• Clindamycin: 10-13 mg/kg/dose every 6-8 hours (not to exceed 40 mg/kg/day) if local resistance <10% 1
• Doxycycline: 2 mg/kg/dose every 12 hours for patients >8 years old and <45kg 1
• Linezolid: 10 mg/kg/dose every 8 hours (not to exceed 600 mg/dose) for severe infections 1