What volume of amoxicillin/clavulanate suspension (125 mg/31.25 mg per 5 mL) should be given twice daily to an 18‑kg child?

Amoxicillin/Clavulanate Dosing for an 18‑kg Child

For an 18‑kg child requiring amoxicillin/clavulanate 125 mg/31.25 mg per 5 mL suspension twice daily, administer 16.2 mL per dose (810 mg amoxicillin component per dose, 1,620 mg total daily). This calculation is based on the high‑dose regimen of 90 mg/kg/day of the amoxicillin component divided into two doses, which is the guideline‑recommended first‑line therapy for presumed bacterial respiratory infections in children. 1, 2, 3

Weight‑Based Calculation

• Total daily amoxicillin requirement: 18 kg × 90 mg/kg/day = 1,620 mg/day 2, 3
• Per‑dose requirement (BID): 1,620 mg ÷ 2 = 810 mg per dose 2, 3
• Volume calculation: The 125 mg/31.25 mg per 5 mL suspension contains 125 mg amoxicillin per 5 mL, so 810 mg ÷ 125 mg × 5 mL = 32.4 mL per dose if using the 125/31.25 formulation 4

Critical Formulation Issue

• The 125 mg/31.25 mg per 5 mL suspension is NOT the appropriate formulation for high‑dose therapy in an 18‑kg child. This concentration would require an impractically large volume (32.4 mL twice daily). 3, 4
• Switch to the 400 mg/57 mg per 5 mL suspension (high‑dose formulation with 14:1 ratio), which would require only 10.1 mL per dose (810 mg ÷ 400 mg × 5 mL). 3, 4
• If only the 125/31.25 suspension is available and you must use it, the dose would be 32.4 mL twice daily, but this is not recommended due to poor palatability and adherence concerns. 3

Rationale for High‑Dose Regimen (90 mg/kg/day)

• The Infectious Diseases Society of America and Pediatric Infectious Diseases Society recommend 90 mg/kg/day in two divided doses for presumed bacterial community‑acquired pneumonia, acute otitis media with risk factors, and acute bacterial sinusitis in children. 1, 2, 3
• High‑dose therapy is specifically indicated when any of the following risk factors are present: age < 2 years, daycare attendance, recent antibiotic use within 30 days, incomplete Haemophilus influenzae type b vaccination, geographic area with > 10% penicillin‑resistant Streptococcus pneumoniae, or moderate‑to‑severe illness. 2, 3
• This regimen achieves middle‑ear and sinus fluid concentrations sufficient to overcome penicillin‑resistant S. pneumoniae with MICs up to 2–4 mg/L and provides coverage against β‑lactamase‑producing H. influenzae and Moraxella catarrhalis. 3

Treatment Duration and Monitoring

• Complete a full 10‑day course for acute otitis media, community‑acquired pneumonia, or acute bacterial sinusitis. 1, 2, 3
• Clinical improvement should be evident within 48–72 hours; fever typically resolves within 24–48 hours for pneumococcal infections, though cough may persist longer. 1, 2
• If no improvement or clinical worsening occurs after 48–72 hours, reassess the diagnosis, consider atypical pathogens (and potentially add a macrolide), and evaluate for complications or alternative diagnoses. 1, 2, 3

Common Pitfalls to Avoid

• Do not use the 125/31.25 suspension for high‑dose therapy in children weighing > 10 kg; the required volume becomes impractical and adherence suffers. 3, 4
• Do not substitute two 250 mg/125 mg tablets for one 500 mg/125 mg tablet in older children, as the clavulanate content differs and they are not interchangeable. 4
• Verify the suspension concentration before dispensing; the 125/31.25,200/28.5,250/62.5, and 400/57 formulations are NOT interchangeable and require different volume calculations. 3, 4
• Do not underdose; subtherapeutic amoxicillin levels fail to eradicate resistant organisms, promote antimicrobial resistance, and increase the risk of treatment failure and complications. 2, 3