IM Ampicillin Dosing for a 5-Month-Old, 7kg Infant
For a 5-month-old infant weighing 7 kg, administer IM ampicillin at 50 mg/kg every 6 hours (350 mg per dose), which totals 200 mg/kg/day divided into 4 doses, based on the indication for treatment. 1, 2
Standard Dosing Guidelines
For Respiratory Tract and Soft Tissue Infections
- The recommended dose is 25-50 mg/kg/day divided every 6-8 hours for children weighing less than 40 kg. 2
- For this 7 kg infant, this translates to 175-350 mg per dose every 6-8 hours (total daily dose: 700-1400 mg/day). 2
For Gastrointestinal and Genitourinary Tract Infections
- The recommended dose is 50 mg/kg/day divided every 6-8 hours for children weighing less than 40 kg. 2
- For this 7 kg infant, this translates to 350 mg per dose every 6-8 hours (total daily dose: 1400 mg/day). 2
For Severe Infections (Bacterial Meningitis or Septicemia)
- For infants >28 days of postnatal age with gestational age >34 weeks, the dose is 150 mg/kg/day divided every 8 hours. 2
- For this 7 kg infant, this translates to 350 mg per dose every 8 hours (total daily dose: 1050 mg/day). 2
- For neonates ≤28 days, dosing is based on both gestational and postnatal age, ranging from 100-150 mg/kg/day. 2
Practical Dosing Calculation for This Patient
Most Common Indications (Respiratory/Soft Tissue)
- Start with 50 mg/kg/day divided every 6 hours = 87.5 mg per dose (round to 90-100 mg per dose). 2
- For more severe infections, use 100 mg/kg/day divided every 6 hours = 175 mg per dose. 1
Severe Systemic Infections
- Use 150 mg/kg/day divided every 8 hours = 350 mg per dose. 2
Critical Administration Details
Route-Specific Considerations
- IM ampicillin should be reconstituted with Sterile Water for Injection to achieve a concentration of 250 mg/mL. 2
- Administer within 1 hour of preparation, as potency decreases significantly after this period. 2
Duration of Therapy
- Continue treatment for a minimum of 48-72 hours beyond resolution of symptoms or evidence of bacterial eradication. 2
- For Group A beta-hemolytic streptococcal infections, treat for a minimum of 10 days to prevent acute rheumatic fever or glomerulonephritis. 2
Important Clinical Pitfalls
Dosing Errors to Avoid
- Do not use doses smaller than recommended, even for mild infections, as this promotes resistance. 2
- Higher doses should be used for stubborn or severe infections, and therapy may need to continue for several weeks. 2
Transition to Oral Therapy
- Switch to oral ampicillin when clinically appropriate, maintaining the same mg/kg/day dosing. 2
- Clinical improvement should be evident within 48-72 hours; if not, reassess the diagnosis and consider alternative pathogens or resistance. 1