Amoxicillin Dosing for a 3-Year-Old Weighing 13.6 kg
For this 3-year-old patient weighing 13.6 kg, prescribe amoxicillin 45 mg/kg/day divided every 12 hours (approximately 306 mg every 12 hours, which rounds to 300 mg or 6 mL of 250 mg/5 mL suspension twice daily) for most common infections, or 90 mg/kg/day divided every 12 hours (approximately 612 mg every 12 hours, which rounds to 600 mg or 12 mL of 250 mg/5 mL suspension twice daily) for pneumococcal infections or severe disease.
Dosing Algorithm Based on Indication
The appropriate dose depends critically on the suspected pathogen and infection severity:
For Streptococcus pneumoniae or Lower Respiratory Tract Infections
- High-dose regimen: 90 mg/kg/day in 2 divided doses (45 mg/kg/dose every 12 hours) 1, 2
- For this 13.6 kg child: 612 mg every 12 hours (rounds to 600 mg or 12 mL of 250 mg/5 mL suspension)
- This high-dose approach is specifically recommended for pneumococcal infections with penicillin MICs >2.0 µg/mL and is the preferred oral therapy for community-acquired pneumonia 1, 3
For Group A Streptococcus (Pharyngitis, Mild Skin Infections)
- Standard-dose regimen: 50-75 mg/kg/day in 2 divided doses 3, 1
- For this 13.6 kg child: 340-510 mg every 12 hours (rounds to 400 mg or 8 mL of 250 mg/5 mL suspension)
For Ear/Nose/Throat, Skin, or Genitourinary Infections (Mild-Moderate)
- Standard-dose regimen: 25 mg/kg/day in divided doses every 12 hours 2
- For this 13.6 kg child: 170 mg every 12 hours (rounds to 200 mg or 4 mL of 250 mg/5 mL suspension)
For Severe Ear/Nose/Throat, Skin, or Genitourinary Infections
- High-dose regimen: 45 mg/kg/day in divided doses every 12 hours 2
- For this 13.6 kg child: 306 mg every 12 hours (rounds to 300 mg or 6 mL of 250 mg/5 mL suspension)
Key Clinical Considerations
Duration of therapy: Continue for minimum 48-72 hours beyond symptom resolution or bacterial eradication. For Streptococcus pyogenes infections, treat for at least 10 days to prevent acute rheumatic fever 2.
Administration: Give at the start of meals to minimize gastrointestinal intolerance 2.
Suspension preparation: Use 250 mg/5 mL concentration for easier dosing in this weight range. Shake well before each use. Discard after 14 days. Refrigeration is preferable but not required 2.
Evidence Supporting High-Dose Therapy
The shift to high-dose amoxicillin (80-90 mg/kg/day) emerged from increasing pneumococcal penicillin resistance 4. Recent pharmacokinetic data confirms that high-dose amoxicillin achieves >93% probability of target attainment in serum for pneumococcal infections 5. The IDSA/PIDS guidelines specifically recommend 90 mg/kg/day for pneumococcal pneumonia with elevated MICs 1, 3.
Common Pitfalls to Avoid
Underdosing larger children: While not applicable to this 13.6 kg patient, be aware that children approaching adult weight often receive inappropriately low mg/kg doses when providers cap at "adult maximum doses" 6.
Switching to three-times-daily dosing: The FDA label lists both twice-daily and three-times-daily options, but twice-daily dosing (every 12 hours) improves adherence without compromising efficacy 2.
Premature discontinuation: Ensure full course completion even if symptoms improve rapidly, particularly for streptococcal infections 2.