What is the recommended trimethoprim‑sulfamethoxazole (Bactrim) dosing for an adult with Serratia marcescens pneumonia that is susceptible, including renal‑function adjustments?

Bactrim Dosing for Serratia Pulmonary Infection

For susceptible Serratia marcescens pneumonia in adults, administer trimethoprim-sulfamethoxazole at 8-12 mg/kg/day (based on the trimethoprim component) divided into 3-4 doses intravenously, with each dose infused over 60 minutes. 1

Standard Dosing Regimen

• Severe pulmonary infections require parenteral therapy: The recommended dose is 8-12 mg/kg/day based on the trimethoprim component, divided into 4 equal doses given intravenously 1
• Practical dosing for average adults: This typically translates to 5 mg/kg/dose (trimethoprim) every 6-8 hours IV, which for a 70 kg patient equals approximately 350 mg trimethoprim (1,750 mg sulfamethoxazole) every 6-8 hours 1
• Infusion rate: Each dose should be infused over 1 hour to minimize adverse reactions 1

Duration of Therapy

• Treatment duration for pneumonia: Continue therapy for at least 10-14 days, though complicated cases may require longer courses based on clinical response 2
• Transition to oral therapy: Once clinical improvement occurs (defervescence, improved oxygenation, hemodynamic stability), switch to oral trimethoprim-sulfamethoxazole using the same total daily dose to complete the course 1
• Criteria for IV-to-oral conversion: The patient must have mild-to-moderate disease without malabsorption or diarrhea 1

Renal Function Adjustments

• Normal renal function (CrCl >30 mL/min): Use standard dosing as above 1
• Moderate renal impairment (CrCl 15-30 mL/min): Reduce dose by 50% or extend dosing interval to every 12 hours 1
• Severe renal impairment (CrCl <15 mL/min): Avoid use or consult nephrology, as trimethoprim-sulfamethoxazole is contraindicated in severe renal failure due to accumulation and increased toxicity risk 1

Susceptibility Considerations

• Verify susceptibility testing: Serratia marcescens shows variable susceptibility to trimethoprim-sulfamethoxazole, with resistance rates ranging from 37% (10 of 27 isolates) in some series 2
• Alternative agents if resistant: If the isolate is resistant to trimethoprim-sulfamethoxazole, carbapenems or aminoglycosides combined with third-generation cephalosporins are preferred 2
• Cotrimoxazole role: While cotrimoxazole (trimethoprim-sulfamethoxazole) should be considered for uncomplicated urinary Serratia infections, it is less reliable for invasive infections like pneumonia unless susceptibility is confirmed 2

Monitoring and Safety

• Hematologic monitoring: Obtain complete blood counts with differential and platelet count at baseline and monitor regularly (at least weekly) for cytopenias, particularly in prolonged therapy 3
• Hepatic monitoring: Check transaminases periodically, as elevations can occur 3
• Contraindications: Avoid in third-trimester pregnancy due to kernicterus risk, in nursing mothers, and in patients with documented sulfa allergies 1

Clinical Pitfalls to Avoid

• Do not use oral therapy initially: Serratia pneumonia is a severe infection requiring initial IV therapy; oral therapy is only appropriate after documented clinical improvement 1
• Do not underdose: The 8-12 mg/kg/day range is critical for adequate tissue penetration in pulmonary infections; lower prophylactic doses (e.g., one double-strength tablet daily) are insufficient 1
• Beware of intrinsic resistance: Serratia species have intrinsic resistance to multiple antibiotics including ampicillin, first-generation cephalosporins, and colistin; always verify susceptibility before continuing therapy 2, 4
• Consider combination therapy for resistant strains: Historical data suggest that combining trimethoprim-sulfamethoxazole with polymyxin may provide synergistic bactericidal activity against multiply-resistant Serratia, though this is rarely used in modern practice 5