Amoxicillin Dosing for Acute Otitis Media: Calculation in mg and mL
For an 18 kg child with acute otitis media, prescribe high-dose amoxicillin at 80–90 mg/kg/day divided into two doses, which equals 1,440–1,620 mg total daily (720–810 mg per dose), or 9–10 mL of the 400 mg/5 mL suspension per dose given every 12 hours. 1
Step-by-Step Dosing Calculation
Total Daily Dose Calculation
- Multiply the child's weight by 80–90 mg/kg to determine the total daily amoxicillin requirement 1
- For an 18 kg child:
- Lower range: 18 kg × 80 mg/kg = 1,440 mg/day
- Upper range: 18 kg × 90 mg/kg = 1,620 mg/day 1
Individual Dose Calculation
- Divide the total daily dose by 2 to determine each individual dose (given every 12 hours) 1
- For an 18 kg child:
- Lower range: 1,440 mg ÷ 2 = 720 mg per dose
- Upper range: 1,620 mg ÷ 2 = 810 mg per dose 1
Volume Calculation for 400 mg/5 mL Suspension
- The 400 mg/5 mL suspension contains 80 mg per mL 1
- To calculate mL per dose, divide the mg dose by 80 mg/mL:
- Lower range: 720 mg ÷ 80 mg/mL = 9 mL per dose
- Upper range: 810 mg ÷ 80 mg/mL = 10.1 mL per dose (round to 10 mL) 1
Treatment Duration and Administration
- Administer the calculated dose twice daily (every 12 hours) for 10 days in children younger than 6 years 1, 2
- The maximum single dose of amoxicillin is 2,000 mg, which is not a concern for this 18 kg patient 3
When to Switch to Amoxicillin-Clavulanate
- Switch to high-dose amoxicillin-clavulanate (90 mg/kg/day amoxicillin + 6.4 mg/kg/day clavulanate in a 14:1 ratio, divided BID) if the child: 1, 3
- Has received amoxicillin within the previous 30 days
- Has concurrent purulent conjunctivitis
- Attends daycare (higher risk of β-lactamase-producing organisms) 3
Monitoring for Treatment Response
- Clinical improvement should be evident within 48–72 hours of initiating therapy 1, 3
- If no improvement occurs within this timeframe, reassess the diagnosis and consider switching to amoxicillin-clavulanate or ceftriaxone 50 mg/kg IM/IV 1, 2
Rationale for High-Dose Therapy
- High-dose amoxicillin achieves middle ear fluid concentrations exceeding the minimum inhibitory concentration for approximately 87% of Streptococcus pneumoniae isolates (including intermediately resistant strains), compared to only 83% with standard dosing 1, 2
- This superior coverage is critical given the prevalence of drug-resistant S. pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis 1, 4
Common Pitfalls to Avoid
- Underdosing is the most common error—always use 80–90 mg/kg/day, not the older standard dose of 40–45 mg/kg/day 2, 5
- Ensure you are using the correct suspension concentration when calculating mL doses; the 400 mg/5 mL formulation is different from other available concentrations 1
- The 14:1 ratio formulation of amoxicillin-clavulanate causes significantly less diarrhea than older 7:1 formulations when combination therapy is needed 1, 3