What is the recommended dose of Bactrim (sulfamethoxazole and trimethoprim) for a 15-year-old female patient weighing 95 pounds with no known medical conditions or allergies?

Bactrim Dosing for a 15-Year-Old Female Weighing 95 Pounds

For this 15-year-old patient weighing 95 pounds (43 kg), administer Bactrim at 8-12 mg/kg/day of trimethoprim (344-516 mg/day) divided into two doses every 12 hours, which translates to 1.5 to 2 double-strength tablets twice daily for most infections. 1, 2

Weight-Based Calculation

• At 95 pounds (43 kg), this patient falls between the 40 kg and 66 lb dosing categories in the FDA-approved weight-based table 2
• The standard pediatric dose is 8-12 mg/kg/day of trimethoprim (40-60 mg/kg/day sulfamethoxazole) divided every 12 hours 1, 3
• For a 43 kg patient, this calculates to approximately 172-258 mg trimethoprim per dose (860-1290 mg sulfamethoxazole per dose) 1

Practical Tablet Dosing

• For mild-to-moderate infections (UTIs, skin/soft tissue infections): 1.5 double-strength tablets (240 mg TMP/1200 mg SMX) every 12 hours 2
• For serious infections (severe MRSA): 2 double-strength tablets (320 mg TMP/1600 mg SMX) every 12 hours 4, 1
• The FDA label specifically recommends 1.5 tablets for children weighing 30 kg (66 lbs) and 2 tablets or 1 DS tablet for 40 kg (88 lbs), placing this 43 kg patient appropriately in the 1.5-2 tablet range 2

Indication-Specific Adjustments

• MRSA skin and soft tissue infections: Use the higher end of dosing (2 DS tablets twice daily) for 7-10 days 4, 3
• Urinary tract infections: 1.5 DS tablets twice daily for 10-14 days is appropriate 2
• For severe MRSA osteomyelitis: Consider 4 mg/kg/dose (approximately 172 mg TMP) every 8-12 hours, typically combined with rifampin for >6 weeks 1

Critical Safety Monitoring

• Obtain baseline complete blood count with differential and platelet count before starting treatment 1, 3
• Repeat CBC monthly during prolonged therapy to monitor for thrombocytopenia, neutropenia, and anemia 1, 5
• Ensure adequate hydration (at least 1.5 liters daily) to prevent crystalluria 1

Important Contraindications and Precautions

• Screen for G6PD deficiency before initiating therapy due to hemolytic anemia risk 4, 3
• Use caution if concurrent methotrexate, warfarin, or oral hypoglycemics are prescribed, as Bactrim increases toxicity of these medications 1, 3
• Avoid use if severe hepatic impairment is present 1

Renal Dose Adjustment

• No adjustment needed if creatinine clearance >30 mL/min 1, 2
• For CrCl 15-30 mL/min: reduce dose by 50% 1, 2
• For CrCl <15 mL/min: use alternative agent 1, 2

Common Pitfalls to Avoid

• Do not use liquid formulation for this weight range—tablets provide more accurate dosing for patients >40 kg 1
• Do not underdose serious infections; the 8-12 mg/kg/day range achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children 1
• Trimethoprim can increase serum creatinine by 0.4-0.5 mg/dL through competitive inhibition of tubular secretion without affecting actual GFR—this is not true renal impairment 6