What is the dose of amoxicillin (400 mg/5 ml) for a 35 kg child requiring 45 mg/kg?

Amoxicillin Dosing Calculation for 35 kg Child at 45 mg/kg

For a 35 kg child requiring 45 mg/kg of amoxicillin using the 400 mg/5 mL suspension, administer 19.7 mL (approximately 20 mL) per dose, given twice daily.

Step-by-Step Calculation

Total daily dose calculation:

• 35 kg × 45 mg/kg = 1,575 mg per day 1

Dose per administration (twice daily dosing):

• 1,575 mg ÷ 2 doses = 787.5 mg per dose 1

Volume calculation using 400 mg/5 mL suspension:

• 787.5 mg ÷ 400 mg = 1.97 (multiply by 5 mL)
• 1.97 × 5 mL = 9.84 mL per dose 1

However, this calculation assumes three times daily dosing. For the recommended twice-daily regimen at 45 mg/kg/day:

• Each dose should be: 787.5 mg
• Volume: 19.7 mL twice daily 1

Clinical Context and Dosing Rationale

The 45 mg/kg/day regimen is indicated for:

• Mild to moderate respiratory tract infections including community-acquired pneumonia 1
• Uncomplicated skin and soft tissue infections 1
• Genitourinary tract infections 2

For this 35 kg child, the practical dosing is:

• 20 mL of 400 mg/5 mL suspension twice daily (every 12 hours) 1
• This provides approximately 1,600 mg total daily dose 1
• Treatment duration should be 7-10 days for most respiratory infections 1

Important Dosing Considerations

Maximum dose limits:

• Although this child weighs 35 kg (under 40 kg), the calculated dose does not exceed the maximum single dose of 2,000 mg 1
• Total daily dose should not exceed 4,000 mg regardless of weight 1

When higher dosing (90 mg/kg/day) is required instead:

• Severe respiratory tract infections 1
• Areas with high pneumococcal resistance (>10%) 1
• Recent antibiotic exposure within 30 days 1
• Daycare attendance 1
• Age <2 years 1

Administration instructions:

• Give at the start of meals to minimize gastrointestinal intolerance 2
• Shake suspension well before each use 2
• Continue treatment for minimum 48-72 hours beyond symptom resolution 2
• For Streptococcus pyogenes infections, treat for at least 10 days to prevent acute rheumatic fever 2

Common Pitfalls to Avoid

Verify the suspension concentration before dispensing:

• The 400 mg/5 mL formulation requires different volume calculations than 250 mg/5 mL or 125 mg/5 mL suspensions 1
• Double-check that you're using the correct concentration to avoid dosing errors 1

Storage after reconstitution:

• Discard any unused suspension after 14 days 2
• Refrigeration is preferable but not required 2

Clinical monitoring:

• Expect clinical improvement within 48-72 hours of starting therapy 1
• If no improvement occurs within this timeframe, reevaluation is necessary 1