Amoxicillin 500 mg BID is NOT appropriate for a 6-year-old weighing 21.77 kg with a sinus infection
This child requires high-dose amoxicillin at 80-90 mg/kg/day divided into two daily doses, which equals approximately 870-980 mg twice daily (1,740-1,960 mg total daily dose), not the 500 mg BID (1,000 mg total daily) that was prescribed. 1, 2
Why Standard Adult Dosing is Inadequate
The prescribed dose of 500 mg BID provides only approximately 46 mg/kg/day for this 21.77 kg child, which falls well below the recommended high-dose threshold needed for acute bacterial sinusitis. 1
Standard-dose amoxicillin (45 mg/kg/day) is only appropriate for children ≥2 years who do not attend child care, have not received antibiotics in the past 4 weeks, and live in communities with <10% penicillin-nonsusceptible S. pneumoniae. 1
High-dose amoxicillin (80-90 mg/kg/day) is required for children <2 years, those attending child care, recent antibiotic exposure, moderate-to-severe illness, or areas with >10% resistant S. pneumoniae. 1, 2
Correct Dosing Calculation
For this 21.77 kg child:
- Minimum dose: 80 mg/kg/day = 1,742 mg/day ÷ 2 = 871 mg twice daily
- Maximum dose: 90 mg/kg/day = 1,959 mg/day ÷ 2 = 980 mg twice daily
- Maximum single dose cap: 2,000 mg per dose 2, 3
The practical prescription would be 875 mg twice daily (using the commercially available 875 mg tablet formulation), which provides 80.4 mg/kg/day. 3
Clinical Rationale for High-Dose Therapy
High-dose amoxicillin achieves sinus fluid concentrations adequate to overcome resistance in S. pneumoniae with altered penicillin-binding proteins, based on middle ear fluid data from acute otitis media studies. 1, 2
The three major bacterial pathogens in pediatric sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. 1
Increasing prevalence of penicillin-resistant S. pneumoniae and β-lactamase-producing H. influenzae necessitates higher dosing strategies. 4
When to Consider Amoxicillin-Clavulanate Instead
Switch to high-dose amoxicillin-clavulanate (90 mg/kg/day of amoxicillin component with 6.4 mg/kg/day clavulanate) if: 1, 2
- The child received antibiotics within the past 30 days
- Moderate-to-severe illness at presentation
- Age <2 years
- Attends child care
- Symptoms fail to improve or worsen within 48-72 hours
For this specific child, that would be approximately 975 mg of the amoxicillin component twice daily with appropriate clavulanate dosing. 1
Duration and Monitoring
Continue treatment for 7 days after the child becomes symptom-free, with a minimum total course of 10 days. 1, 5
Reassess within 48-72 hours if symptoms worsen or fail to improve, as this may indicate treatment failure requiring escalation to amoxicillin-clavulanate or consideration of complications. 1, 2
Administer at the start of meals to minimize gastrointestinal intolerance. 3
Common Pitfall to Avoid
Never use adult fixed-dose tablets (500 mg) for weight-based pediatric dosing without calculating mg/kg/day. Children are not small adults, and sinusitis treatment requires weight-based high-dose therapy to achieve adequate tissue penetration and overcome bacterial resistance. 1, 2