Amoxicillin Dosing for Streptococcal Pharyngitis in a 45 kg Child
For a 45 kg child with confirmed Group A streptococcal pharyngitis, prescribe amoxicillin 1000 mg orally once daily for 10 days, or alternatively 500 mg twice daily for 10 days. 1
Weight-Based Dosing Calculation
- The recommended pediatric dose is 50 mg/kg once daily (maximum 1000 mg) or 25 mg/kg twice daily (maximum 500 mg per dose) for 10 days. 1
- For this 45 kg child, the calculated dose would be 2250 mg/day (50 mg/kg), but this exceeds the maximum adult dose of 1000 mg once daily. 1
- Therefore, use the maximum adult dose of 1000 mg once daily or 500 mg twice daily, as children ≥40 kg should follow adult dosing recommendations. 2
Evidence Supporting Once-Daily Dosing
- Once-daily amoxicillin (750-1000 mg) has been proven non-inferior to twice-daily dosing in multiple randomized controlled trials, with bacteriologic failure rates of 20.1% vs 15.5% at 14-21 days (difference 4.53%; 90% CI -0.6 to 9.7). 3
- A smaller study demonstrated that once-daily amoxicillin (750 mg) had a bacteriologic failure rate of only 5% compared to 11% with penicillin V three times daily. 4
- Real-time PCR analysis confirmed that once-daily amoxicillin achieves equivalent bacterial load reduction compared to multiple-daily dosing regimens, with 85.7% negative PCR results at 9-11 days for all regimens. 5
- The American Heart Association has recently endorsed once-daily amoxicillin dosing for streptococcal pharyngitis. 1
Critical Treatment Duration Requirement
- A complete 10-day course is mandatory to achieve maximal pharyngeal eradication of Group A Streptococcus and prevent acute rheumatic fever, even if symptoms resolve within 3-4 days. 1, 6
- Shortening the course by even a few days results in appreciable increases in treatment failure rates and rheumatic fever risk. 6
- The 10-day duration is specifically required to prevent acute rheumatic fever, which can occur up to 9 days after symptom onset if inadequately treated. 6
Why Amoxicillin is First-Line
- Amoxicillin remains the drug of choice due to proven efficacy, narrow antimicrobial spectrum, excellent safety profile, low cost, and the complete absence of documented penicillin resistance in Group A Streptococcus worldwide. 6, 1
- Amoxicillin has identical efficacy to penicillin V but offers better palatability and once-daily dosing convenience, which may improve adherence. 1, 7
- Compliance rates exceed 95% with amoxicillin regimens, with no significant difference between once-daily and twice-daily dosing. 3
Administration Instructions
- Administer amoxicillin at the start of a meal to minimize gastrointestinal intolerance. 2
- The child becomes non-contagious after 24 hours of antibiotic therapy. 1
- Patients should complete the full 10-day course even after becoming asymptomatic. 2
Common Pitfalls to Avoid
- Do not prescribe shorter courses (such as 5-7 days) except for azithromycin's unique 5-day regimen—this dramatically increases treatment failure and rheumatic fever risk. 6
- Do not use broad-spectrum antibiotics (such as second- or third-generation cephalosporins) when amoxicillin is appropriate, as they are more expensive and promote antibiotic-resistant flora. 6
- Do not order routine post-treatment throat cultures in asymptomatic patients who have completed therapy—testing should only be considered in special circumstances such as history of rheumatic fever. 6
Alternative Regimens if Amoxicillin Cannot Be Used
- For non-anaphylactic penicillin allergy: First-generation cephalosporins such as cephalexin 500 mg twice daily for 10 days (cross-reactivity risk only 0.1%). 6, 8
- For immediate/anaphylactic penicillin allergy: Clindamycin 300 mg three times daily for 10 days (~1% resistance rate in the US). 6, 8
- Azithromycin 500 mg once daily for 5 days is acceptable but has 5-8% macrolide resistance rates in the United States. 6, 8