Amoxicillin Dosing for a 47-Pound (21 kg) Child
For a 47-pound (21 kg) child, administer 10.5 mL of amoxicillin 400 mg/5 mL suspension twice daily (total 1,890 mg/day = 90 mg/kg/day) for severe infections or high-resistance areas, or 5.25 mL twice daily (total 945 mg/day = 45 mg/kg/day) for mild-to-moderate infections without risk factors. 1
Dosing Algorithm Based on Clinical Scenario
High-Dose Regimen (90 mg/kg/day) – Use when ANY of the following apply:
- Age < 2 years 1
- Daycare attendance 1
- Recent antibiotic use (within past 30 days) 1
- Region with > 10% penicillin-resistant S. pneumoniae 1
- Moderate-to-severe illness 1
- Community-acquired pneumonia (presumed bacterial) 1, 2
- Acute bacterial sinusitis with risk factors 1
Calculation: 21 kg × 90 mg/kg/day = 1,890 mg/day ÷ 2 doses = 945 mg per dose = 10.5 mL of 400 mg/5 mL suspension twice daily 1
Standard-Dose Regimen (45 mg/kg/day) – Use for:
- Children ≥ 2 years without risk factors 1
- Uncomplicated respiratory tract infections 1
- No recent antibiotic exposure 1
- Not attending daycare 1
Calculation: 21 kg × 45 mg/kg/day = 945 mg/day ÷ 2 doses = 472.5 mg per dose = 5.25 mL of 400 mg/5 mL suspension twice daily 1
Group A Streptococcal Infections (Pharyngitis, Scarlet Fever)
- Dose: 50–75 mg/kg/day in 2 doses for 10 days 1
- Calculation: 21 kg × 50 mg/kg = 1,050 mg/day ÷ 2 = 525 mg per dose = 5.9 mL twice daily 1
- Maximum single dose: Do not exceed 1,000 mg per dose 1
- Duration: Complete the full 10-day course regardless of symptom improvement to prevent acute rheumatic fever 1
Practical Dosing Recommendations by Indication
| Indication | Dose (mg/kg/day) | Volume per Dose (400 mg/5 mL) | Frequency | Duration |
|---|---|---|---|---|
| Mild respiratory infection (no risk factors) | 45 | 5.25 mL | Every 12 hours | 7–10 days [1] |
| Severe pneumonia or high resistance | 90 | 10.5 mL | Every 12 hours | 10 days [1,2] |
| Group A Strep pharyngitis | 50–75 | 5.9–8.8 mL | Every 12 hours | 10 days [1] |
| Acute bacterial sinusitis (with risk factors) | 80–90 | 9.5–10.5 mL | Every 12 hours | 10 days [1] |
Critical Considerations
When to Use High-Dose Amoxicillin
- The 90 mg/kg/day regimen achieves middle ear and sinus fluid concentrations sufficient to overcome penicillin-resistant S. pneumoniae with MICs up to 2–4 mg/L 1
- High-dose therapy is specifically designed to overcome penicillin-resistant Streptococcus pneumoniae, the most common bacterial cause of community-acquired pneumonia 2
- The Infectious Diseases Society of America and Pediatric Infectious Diseases Society recommend 90 mg/kg/day for presumed bacterial pneumonia in children to ensure coverage of penicillin-resistant strains 1, 2
When to Switch to Amoxicillin-Clavulanate
- Suspected β-lactamase-producing organisms (H. influenzae, M. catarrhalis) warrant escalation to amoxicillin-clavulanate at 90 mg/kg/day of the amoxicillin component + 6.4 mg/kg/day clavulanate, divided twice daily 1
- Use amoxicillin-clavulanate for children with incomplete H. influenzae type b vaccination or concurrent purulent acute otitis media 1
- The high-dose 14:1 ratio formulation minimizes diarrhea compared with other ratios 1
Maximum Dosing Limits
- Maximum daily dose: 4,000 mg/day regardless of weight 1, 3
- For this 21 kg child, the calculated 1,890 mg/day (90 mg/kg/day) is well below the maximum 1
Treatment Duration and Monitoring
- Standard duration: 7–10 days for most respiratory infections; 10 days specifically for pneumonia 1
- Group A Strep infections: Must complete 10 full days to prevent acute rheumatic fever 1
- Clinical improvement should occur within 48–72 hours; if no improvement, reevaluate and consider atypical pathogens or resistant organisms 1
- Fever typically resolves within 24–48 hours for pneumococcal pneumonia, though cough may persist longer 1
Common Pitfalls to Avoid
- Do not underdose severe infections—always use 90 mg/kg/day for life-threatening infections, empyema, or documented resistant pneumococcus 1
- Do not use outdated 40 mg/kg/day dosing—current guidelines support 45–90 mg/kg/day to address contemporary resistance patterns 1, 4
- Do not prescribe antibiotics for Group A Strep based solely on clinical presentation—rapid antigen detection test or throat culture is required because approximately 70% of sore throats are not streptococcal 1
- Ensure twice-daily dosing is emphasized—this improves adherence compared with three-times-daily regimens and achieves superior pharmacokinetic profiles 2, 5, 6
Administration Instructions
- Take at the start of a meal to minimize gastrointestinal intolerance 3
- Shake suspension well before each use 3
- Refrigeration is preferable but not required; discard unused suspension after 14 days 3
- The suspension may be mixed with formula, milk, fruit juice, water, or cold drinks and taken immediately 3