What is the recommended Septrin (Co-trimoxazole) prophylactic dose for a patient with a Body Surface Area (BSA) of 1.1 m² according to the British National Formulary for Children (BNFC)?

Septrin Prophylactic Dose for BSA 1.1 m²

For a patient with BSA 1.1 m², the prophylactic dose of Septrin (co-trimoxazole) is 825 mg sulfamethoxazole with 165 mg trimethoprim per day, given as approximately 1.5 tablets twice daily (412.5 mg/82.5 mg per dose). 1

Dosing Calculation

The FDA-approved prophylactic dosing for children is 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim, divided into two equal doses given twice daily 1. This recommendation is supported by the American Academy of Pediatrics 2.

For BSA 1.1 m²:

• Sulfamethoxazole: 750 mg/m²/day × 1.1 m² = 825 mg/day (412.5 mg twice daily) 1
• Trimethoprim: 150 mg/m²/day × 1.1 m² = 165 mg/day (82.5 mg twice daily) 1

Practical Administration

According to the FDA dosing table, patients with BSA 1.06 m² receive 1 tablet (400 mg/80 mg) twice daily 1. Since your patient has BSA 1.1 m², which falls between the 1.06 m² and higher BSA categories, the appropriate dose is:

• 1.5 tablets (600 mg/120 mg) twice daily, or
• Alternatively, 3 single-strength tablets daily divided into two doses 1

This provides 1200 mg sulfamethoxazole and 240 mg trimethoprim daily, which slightly exceeds the calculated dose but remains within safe prophylactic ranges 1.

Important Clinical Considerations

Renal Function Monitoring

Assess creatinine clearance before initiating therapy 1. For patients with:

• CrCl >30 mL/min: Use standard dosing 1
• CrCl 15-30 mL/min: Reduce dose by 50% 1
• CrCl <15 mL/min: Co-trimoxazole is not recommended 2, 1

Duration and Monitoring

The typical prophylactic regimen is given on 3 consecutive days per week for Pneumocystis prophylaxis 1. However, for continuous daily prophylaxis (as in IBD patients on triple immunosuppression), 960 mg three times weekly or 480 mg daily are widely used alternatives 3.

Monitor for adverse effects in 25-33% of patients, though discontinuation is rarely necessary 2. Key adverse effects include:

• Dermatological reactions (rash, photosensitivity, Stevens-Johnson syndrome) 4
• Gastrointestinal symptoms (nausea, vomiting, diarrhea) 4
• Hematologic abnormalities (monitor CBC periodically) 4

Contraindications

Do not use in patients with 4:

• Hypersensitivity to sulfonamides or trimethoprim
• Severe renal impairment (CrCl <15 mL/min)
• Significant hepatic impairment
• Infants under 2 months of age

Drug Interactions

Monitor closely when co-administered with 4:

• Anticoagulants (increased bleeding risk)
• Methotrexate (increased toxicity)
• Phenytoin (altered levels)
• Cyclosporine (nephrotoxicity risk)

Common Pitfalls

Avoid using adult fixed-dose recommendations (one double-strength tablet daily) for pediatric or BSA-based dosing, as this may result in under- or overdosing 2, 1. Always calculate based on BSA when specified in the dosing guidelines, particularly for patients outside typical adult body habitus 3, 5.

BSA-based dosing is well-established in nephrology practice for peritoneal dialysis patients and other populations requiring precise dose adjustments 2, 6.