What is the recommended dose and duration of amoxicillin (amoxicillin) for a pediatric patient weighing 18kg with a suspected bacterial infection?

Amoxicillin Dosing for an 18kg Pediatric Patient

For an 18kg child with a suspected bacterial infection, prescribe amoxicillin 45 mg/kg/day (810 mg/day total) divided into two doses of 405 mg every 12 hours for mild to moderate infections, or 90 mg/kg/day (1,620 mg/day total) divided into two doses of 810 mg every 12 hours for severe infections or high-resistance areas. 1, 2

Dosing Algorithm Based on Clinical Severity

Standard-Dose Regimen (Mild to Moderate Infections)

• For uncomplicated respiratory tract infections in children ≥2 years old without risk factors, use 45 mg/kg/day divided every 12 hours, which equals approximately 405 mg twice daily for this 18kg patient 3
• This standard dose provides adequate coverage for susceptible Streptococcus pneumoniae, non-β-lactamase-producing Haemophilus influenzae, and Streptococcus pyogenes 2
• Risk factors that would exclude standard dosing include: age <2 years, daycare attendance, or antibiotic use within the past 30 days 3

High-Dose Regimen (Severe Infections or Resistance Risk)

• For moderate to severe illness, children <2 years, daycare attendance, or recent antibiotic exposure, prescribe 80-90 mg/kg/day divided every 12 hours, which equals approximately 720-810 mg twice daily for this 18kg patient 3
• High-dose therapy is specifically designed to overcome penicillin-resistant S. pneumoniae by achieving adequate sinus and middle ear fluid concentrations 3, 4
• In communities with >10% penicillin-resistant S. pneumoniae, initiate treatment at the high dose 3

Indication-Specific Dosing

Group A Streptococcal Infections

• Prescribe 50-75 mg/kg/day divided into 2 doses for 10 days, which equals 450-675 mg twice daily (not exceeding 1000 mg per dose) for this 18kg patient 1, 2
• The full 10-day course is mandatory to prevent rheumatic fever, even if symptoms resolve earlier 1, 5
• Patients become non-contagious after 24 hours of therapy 1

Acute Bacterial Sinusitis

• For children ≥2 years without risk factors: 45 mg/kg/day (405 mg twice daily) 3
• For children <2 years, in daycare, or with recent antibiotic use: 80-90 mg/kg/day (720-810 mg twice daily) 3

Community-Acquired Pneumonia

• For mild to moderate outpatient cases: 45 mg/kg/day (405 mg twice daily) 2, 4
• For severe cases or high pneumococcal resistance: 90 mg/kg/day (810 mg twice daily) 2, 4

Treatment Duration

• Most respiratory infections require 7-10 days of therapy, with pneumonia specifically requiring 10 days 1, 2
• An alternative approach is to continue antibiotics for 7 days after symptom resolution, resulting in a minimum 10-day course 3
• Clinical improvement should occur within 48-72 hours; if no improvement by 72 hours, reassess the diagnosis and consider alternative antibiotics 1, 2

When to Use Amoxicillin-Clavulanate Instead

• Switch to amoxicillin-clavulanate (90 mg/kg/day of amoxicillin component) if β-lactamase-producing organisms are suspected, including 3:
  • Nearly 100% of Moraxella catarrhalis produce β-lactamase 3
  • 10-42% of H. influenzae produce β-lactamase (varies by region) 3
• The clavulanate component at 6.4 mg/kg/day adequately inhibits all β-lactamase-producing organisms 3

Practical Prescribing Details

Formulation and Administration

• Prescribe amoxicillin oral suspension 250 mg/5 mL 5
• For 45 mg/kg/day regimen: 8.1 mL (approximately 8 mL) twice daily 5
• For 90 mg/kg/day regimen: 16.2 mL (approximately 16 mL) twice daily 5
• Administer at the start of meals to minimize gastrointestinal intolerance 5
• Shake suspension well before each use; refrigeration is preferable but not required 5
• Discard unused suspension after 14 days 5

Critical Prescription Components

• Every prescription must include: total daily dose in mg/kg/day, number of divided doses, duration of therapy, indication, and the child's weight 1
• Maximum daily dose is 4000 mg (4 g) regardless of weight 1, 2

Common Pitfalls to Avoid

• Twice-daily dosing improves adherence compared to three-times-daily regimens, making the every-12-hour schedule preferred over more frequent dosing 1, 2
• Do not use the 875 mg tablet formulation in patients with severe renal impairment (GFR <30 mL/min) 5
• Amoxicillin alone will not cover MRSA; if methicillin-resistant Staphylococcus aureus is suspected, alternative antibiotics must be considered 1
• The evidence shows that standard 40 mg/kg/day dosing is inadequate for resistant organisms, particularly during viral coinfection 6

Monitoring and Adverse Effects

• The most common adverse effects are gastrointestinal disturbances (diarrhea, nausea, vomiting) 1, 2
• Rash, urticaria, and hypersensitivity reactions may occur 1, 2
• If the patient remains significantly febrile or symptomatic at 24 hours after starting therapy, consider parenteral ceftriaxone (50 mg/kg as a single dose) 3

Penicillin Allergy Considerations

• Recent evidence indicates that the risk of serious allergic reactions to second- and third-generation cephalosporins in patients with penicillin allergy is almost nil 3
• For non-anaphylactic penicillin allergy, cefdinir or cefuroxime are appropriate alternatives 2
• For Type I hypersensitivity reactions, consider azithromycin or clarithromycin, though these have inferior bacteriologic efficacy 2