What is the appropriate volume in mL of amoxicillin suspension for a 12‑year‑old female weighing 85 lb (≈38 kg) with acute bacterial sinusitis?

Amoxicillin Dosing for a 12-Year-Old Female (85 lb / ≈38 kg) with Acute Sinusitis

For a 12-year-old female weighing 85 lb (≈38 kg) with acute bacterial sinusitis, prescribe high-dose amoxicillin-clavulanate 90 mg/kg/day of the amoxicillin component divided into 2 doses, which equals approximately 1,710 mg total daily (855 mg twice daily). Using a standard 400 mg/5 mL suspension, this translates to approximately 10.7 mL twice daily (round to 10–11 mL per dose for practical administration). 1

Dosing Calculation & Formulation Selection

• Weight-based calculation: 38 kg × 90 mg/kg/day = 3,420 mg/day ÷ 2 doses = 1,710 mg per day (855 mg per dose). 1

• Volume calculation using 400 mg/5 mL suspension: 855 mg ÷ 400 mg = 2.14 × 5 mL = 10.7 mL per dose, administered twice daily (every 12 hours). 1

• Alternative formulation (600 mg/5 mL suspension): 855 mg ÷ 600 mg = 1.43 × 5 mL = 7.1 mL per dose twice daily, which may improve compliance due to smaller volume. 1

• The high-dose regimen (90 mg/kg/day amoxicillin + 6.4 mg/kg/day clavulanate in 2 divided doses) provides a 14:1 ratio that achieves 90–92% predicted clinical efficacy against drug-resistant Streptococcus pneumoniae and β-lactamase-producing Haemophilus influenzae and Moraxella catarrhalis. 1, 2

Why High-Dose Therapy Is Indicated

• Age-based indication: Children under 18 years with acute bacterial sinusitis should receive high-dose amoxicillin-clavulanate as first-line therapy because pediatric patients have higher rates of antibiotic-resistant pathogens compared to adults. 1, 2, 3

• Standard-dose amoxicillin (45 mg/kg/day) is reserved only for uncomplicated cases without risk factors; however, the default recommendation for pediatric acute bacterial sinusitis is high-dose therapy (80–90 mg/kg/day) to ensure adequate coverage. 1, 2, 3

• High-dose therapy overcomes penicillin-resistant S. pneumoniae (MIC 0.12–1.0 µg/mL and many strains with MIC ≥2 µg/mL) by achieving middle-ear and sinus fluid concentrations that exceed bacterial resistance thresholds. 1

Treatment Duration & Monitoring

• Minimum treatment duration: 10–14 days for children, which is longer than the 5–7 day courses used in adults. Continue therapy until the patient is symptom-free for 7 consecutive days (typically 10–14 days total). 1, 2, 4

• Reassess at 72 hours (3 days): If no clinical improvement (persistent purulent nasal discharge, unchanged facial pain, or worsening symptoms), switch immediately to a respiratory fluoroquinolone (levofloxacin) or consider ENT referral. 1, 2

• Expected timeline: Noticeable improvement should occur within 3–5 days of appropriate therapy, with complete resolution by 10–14 days. 2

Essential Adjunctive Therapies

• Intranasal corticosteroids (mometasone, fluticasone, or budesonide) twice daily significantly reduce mucosal inflammation and accelerate symptom resolution; supported by strong evidence from multiple randomized controlled trials. 2

• Saline nasal irrigation 2–3 times daily provides symptomatic relief and enhances mucus clearance. 2

• Analgesics (acetaminophen or ibuprofen) for pain and fever control. 2

Common Pitfalls to Avoid

• Do not use standard adult dosing (875 mg/125 mg twice daily) for a 38 kg child; this would provide only 46 mg/kg/day, which is subtherapeutic and increases treatment failure risk with resistant organisms. 1

• Verify the suspension concentration (400 mg/5 mL vs 600 mg/5 mL) before calculating volume to avoid dosing errors. 1

• Ensure a minimum 10-day treatment duration for children to prevent relapse; shorter courses are only validated in adults. 1, 2, 4

• Gastrointestinal adverse effects (diarrhea) occur in 40–43% of patients on amoxicillin-clavulanate, but the 14:1 high-dose formulation causes less diarrhea than other preparations. 1, 2

When to Escalate or Refer

• No improvement after 72 hours: Switch to levofloxacin (dose-adjusted for weight) or obtain ENT consultation for possible sinus aspiration/culture. 1, 2

• Worsening symptoms at any time (increasing facial pain, fever, periorbital swelling, visual changes, severe headache, altered mental status) require urgent ENT referral to exclude complications such as orbital cellulitis or intracranial spread. 2

• Recurrent sinusitis (≥3 episodes per year) warrants evaluation for underlying allergic rhinitis, immunodeficiency, or anatomic abnormalities. 2