What dose of Bactrim (trimethoprim‑sulfamethoxazole) pediatric suspension should be given to a 6‑year‑old child weighing 21.3 kg, and how often?

Bactrim Liquid Dosing for a 21.3 kg, 6-Year-Old Child

For a 6-year-old child weighing 21.3 kg, administer Bactrim (trimethoprim-sulfamethoxazole) suspension at 8–12 mg/kg/day of the trimethoprim component (170–255 mg trimethoprim total daily), divided into 2 doses given every 12 hours. 1, 2

Practical Dosing Calculation

For standard infections (UTI, skin/soft tissue, otitis media):

• Total daily trimethoprim dose: 21.3 kg × 8 mg/kg = 170 mg trimethoprim per day (lower end) to 21.3 kg × 12 mg/kg = 255 mg trimethoprim per day (higher end) 1, 2
• Divided dose: Give 85–127.5 mg trimethoprim every 12 hours 1, 2

Using Bactrim pediatric suspension (40 mg trimethoprim/5 mL):

• Lower dose: 85 mg ÷ 40 mg per 5 mL = 10.6 mL every 12 hours 2
• Higher dose: 127.5 mg ÷ 40 mg per 5 mL = 16 mL every 12 hours 2

Practical recommendation: Start with 10–12 mL every 12 hours for mild-to-moderate infections, or 15–16 mL every 12 hours for more serious infections. 1, 2

Indication-Specific Adjustments

For mild-to-moderate infections (UTI, uncomplicated skin infections):

• Use the lower dosing range (8–10 mg/kg/day trimethoprim) divided every 12 hours for 7–10 days 3, 1
• This translates to approximately 10–12 mL of suspension every 12 hours 2

For serious infections (severe MRSA, complicated infections):

• Use the higher dosing range (10–12 mg/kg/day trimethoprim) divided every 12 hours 3, 1
• This translates to approximately 14–16 mL of suspension every 12 hours 2

For life-threatening infections (severe pneumonia, CNS infections):

• Consider 15–20 mg/kg/day trimethoprim divided every 6–8 hours (not every 12 hours) 3, 1
• This would require 4 divided doses per day rather than the standard twice-daily regimen 3

Treatment Duration

• Standard infections: 7–14 days 3
• UTI: 10–14 days 2
• Shigellosis: 5 days 2
• Otitis media: 10 days 2
• Severe MRSA osteomyelitis: >6 weeks (typically combined with rifampin) 1

Critical Monitoring Requirements

Obtain baseline and monthly laboratory monitoring during prolonged therapy:

• Complete blood count with differential and platelet count to assess for neutropenia, thrombocytopenia, and anemia 3, 1, 4
• Most adverse reactions develop within the first 2 weeks of therapy 5

Ensure adequate hydration:

• Patient should drink at least 1.5 liters of fluid daily to prevent crystalluria, especially with higher doses 1

Important Safety Considerations and Contraindications

Absolute contraindications:

• Age <2 months (risk of kernicterus from sulfonamide displacement of bilirubin) 6, 2
• Known hypersensitivity to sulfonamides or trimethoprim 6

Use with extreme caution in:

• G6PD deficiency: Screen before initiating therapy due to hemolytic anemia risk 1, 6, 4
• Renal insufficiency: Requires dose adjustment (see below) 1, 2
• Hepatic insufficiency: Avoid in severe hepatic impairment 1, 6

Renal Impairment Dose Adjustments

If creatinine clearance is reduced:

• CrCl 15–30 mL/min: Reduce dose by 50% 1, 2
• CrCl <15 mL/min: Reduce dose by 50% or use alternative agent 1, 2
• CrCl <10 mL/min: Use is not recommended per FDA label, though reduced dosing may be considered clinically 2

Management of Adverse Reactions

For mild rash:

• Temporarily discontinue and restart when resolved 3, 4
• Some patients can be successfully desensitized if rechallenge is necessary 3

For life-threatening reactions (anaphylaxis, Stevens-Johnson syndrome, urticarial rash, hypotension):

• Permanently discontinue the drug 3, 4

Common adverse effects (occur in ~15% of patients):

• Dermatologic reactions (rash) 6, 4
• Hematologic effects (neutropenia, thrombocytopenia) 6, 4, 5
• Gastrointestinal complaints (nausea, vomiting) 6, 5
• Hepatic effects (hepatitis) 4
• Renal effects (interstitial nephritis) 4

Key Drug Interactions

Exercise caution when co-administering with:

• Methotrexate: Increased methotrexate toxicity 1, 6
• Warfarin and other anticoagulants: Enhanced anticoagulant effect 1, 6
• Oral hypoglycemics: Increased hypoglycemia risk 1, 6
• Thiazide diuretics: Increased risk of thrombocytopenia 6
• Anticonvulsants (phenytoin): Altered phenytoin levels 6

Formulation Considerations

The liquid formulation is strongly preferred for this patient:

• Pediatric suspension allows accurate weight-based dosing for children <16 kg and provides flexibility for dose titration 1
• Standard Bactrim pediatric suspension contains 40 mg trimethoprim + 200 mg sulfamethoxazole per 5 mL 2
• Tablets are less practical for precise dosing in young children 1

Clinical Pearls

Pharmacokinetic data support the standard 8–12 mg/kg/day regimen:

• This dosing achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children 7
• The exposure matches adult dosing of 320 mg trimethoprim twice daily 7

For bacteria with higher MIC (up to 1 mg/L):

• Consider 12–15 mg/kg/day trimethoprim divided every 12 hours in children 6 years and older 7