Amoxicillin Dosing for a 45‑Pound Child
For a 45‑pound (20.4 kg) child with normal renal function, the appropriate amoxicillin dose is 45 mg/kg/day divided into two doses (approximately 460 mg twice daily) for most common infections, or 90 mg/kg/day divided into two doses (approximately 920 mg twice daily) for severe infections or high‑resistance areas. 1
Weight‑Based Calculation
- A 45‑pound child weighs approximately 20.4 kg (45 lb ÷ 2.2 = 20.45 kg). 1
- For standard‑dose therapy (45 mg/kg/day): 20.4 kg × 45 mg/kg = 918 mg total daily, administered as 459 mg (≈460 mg) twice daily. 1, 2
- For high‑dose therapy (90 mg/kg/day): 20.4 kg × 90 mg/kg = 1,836 mg total daily, administered as 918 mg (≈920 mg) twice daily. 1
Indication‑Specific Dosing Algorithm
Standard‑Dose Regimen (45 mg/kg/day in two divided doses)
- Uncomplicated respiratory tract infections in children ≥2 years without recent antibiotic exposure warrant standard dosing. 1
- Skin and soft tissue infections of mild‑to‑moderate severity are appropriately treated with 45 mg/kg/day. 1, 2
- Genitourinary tract infections (uncomplicated) respond to standard‑dose therapy. 2
- Group A streptococcal pharyngitis requires 50–75 mg/kg/day divided into two doses for 10 days, with a maximum single dose of 1,000 mg. 1
High‑Dose Regimen (90 mg/kg/day in two divided doses)
- Community‑acquired pneumonia in any child requires 90 mg/kg/day to ensure adequate coverage of penicillin‑resistant Streptococcus pneumoniae. 1
- Severe infections or moderate‑to‑severe illness at presentation mandate high‑dose therapy. 1
- Recent antibiotic exposure (within the past 30 days) necessitates escalation to 90 mg/kg/day. 1
- Age < 2 years, daycare attendance, or residence in an area with >10% penicillin‑resistant S. pneumoniae are independent indications for high‑dose therapy. 1
- Acute bacterial sinusitis in high‑risk children (age <2 years, daycare, recent antibiotics) requires 80–90 mg/kg/day. 1
Practical Dosing Examples for a 20.4 kg Child
| Indication | Daily Dose | Per‑Dose Amount | Suspension Volume* |
|---|---|---|---|
| Uncomplicated infection | 918 mg (45 mg/kg/day) | 459 mg BID | 5.7 mL of 400 mg/5 mL BID |
| Pneumonia or severe infection | 1,836 mg (90 mg/kg/day) | 918 mg BID | 11.5 mL of 400 mg/5 mL BID |
| Strep throat (mid‑range) | 1,275 mg (62.5 mg/kg/day) | 637 mg BID | 8 mL of 400 mg/5 mL BID |
*Volumes are illustrative; verify suspension concentration before dispensing. 1, 2
Maximum Dose Limitations
- The maximum total daily dose of amoxicillin is 4,000 mg/day regardless of weight. 1
- For Group A streptococcal pharyngitis, the maximum single dose is 1,000 mg even if weight‑based calculation exceeds this amount. 1
Treatment Duration
- Respiratory tract infections (including pneumonia) require 10 days of therapy. 1
- Group A streptococcal pharyngitis mandates a full 10‑day course to prevent acute rheumatic fever, even if symptoms resolve earlier. 1, 2
- Therapy should continue for at least 48–72 hours after complete resolution of symptoms for all infections. 1, 2
When to Switch to Amoxicillin‑Clavulanate
- Suspected β‑lactamase‑producing organisms (Haemophilus influenzae, Moraxella catarrhalis) require amoxicillin‑clavulanate at 90 mg/kg/day of the amoxicillin component. 1, 3
- Treatment failure after 48–72 hours of amoxicillin monotherapy warrants escalation to amoxicillin‑clavulanate. 1, 3
- Incomplete H. influenzae type b vaccination or concurrent purulent otitis media in a child with pneumonia necessitates amoxicillin‑clavulanate from the outset. 1, 3
Clinical Monitoring
- Clinical improvement (reduced fever, improved respiratory status, better oral intake) should be evident within 48–72 hours of initiating appropriate therapy. 1
- If no improvement or worsening occurs after 48–72 hours, consider atypical pathogens (add a macrolide), obtain further imaging, or escalate to amoxicillin‑clavulanate. 1
Common Pitfalls to Avoid
- Do not underdose children in higher weight categories; obesity does not justify capping doses below weight‑based calculations unless the 4,000 mg/day maximum is reached. 4
- Do not use age‑based dosing for amoxicillin; weight‑based calculations (mg/kg/day) are the standard of care. 1, 5
- Do not prescribe standard‑dose therapy when high‑dose indications are present (age <2 years, recent antibiotics, daycare, severe illness, pneumonia); this leads to treatment failure with resistant organisms. 1
- Verify suspension concentration before calculating volume; common concentrations include 125 mg/5 mL, 250 mg/5 mL, and 400 mg/5 mL. 1, 2