Azithromycin Dose Adjustment for a 28‑lb (12.7 kg) Child
The Correct Dose Is 3 mL (200 mg) Once Daily, Not 5 mL
For a 28‑lb (12.7 kg) child, the British Thoracic Society weight‑band dosing guidelines recommend 200 mg azithromycin once daily, which corresponds to 3 mL of the 200 mg/5 mL suspension—not 5 mL. 1 The 5 mL volume would deliver 200 mg only if using the 200 mg/5 mL formulation, but the question implies a misunderstanding of either the concentration or the weight band. A 12.7 kg child falls into the 15–25 kg weight band, for which the recommended dose is 200 mg once daily. 1
Weight‑Band Dosing Algorithm (British Thoracic Society)
The British Thoracic Society uses a simplified weight‑band approach derived from the standard pediatric calculation of 10 mg/kg once daily (maximum 500 mg per dose) for the first day, followed by 5 mg/kg once daily for days 2–5 in the 5‑day regimen. 1 For practical dosing, the weight bands are:
| Weight (kg) | Recommended Dose | Volume (200 mg/5 mL suspension) |
|---|---|---|
| 15–25 | 200 mg once daily | 3 mL |
| 26–35 | 300 mg once daily | 7.5 mL |
| 36–45 | 400 mg once daily | 10 mL |
| ≥46 | 500 mg once daily | 12.5 mL |
A 28‑lb child weighs 12.7 kg, which is below the 15 kg threshold. However, the British Thoracic Society guidelines do not provide a specific band for children weighing less than 15 kg in the simplified table. 1 In this case, the FDA‑approved dosing for children ≥6 months is 10 mg/kg on day 1, then 5 mg/kg on days 2–5. 2 For a 12.7 kg child, this calculates to:
- Day 1: 10 mg/kg × 12.7 kg = 127 mg (approximately 3.2 mL of 200 mg/5 mL suspension)
- Days 2–5: 5 mg/kg × 12.7 kg = 63.5 mg (approximately 1.6 mL of 200 mg/5 mL suspension)
The 3 mL and 1.5 mL volumes are the practical, rounded doses that align with the FDA‑approved weight‑based calculation for a 12.7 kg child. 2
Why 5 mL and 2.5 mL Would Be Incorrect
If 5 mL of the 200 mg/5 mL suspension were given, the child would receive 200 mg, which is 15.7 mg/kg—significantly higher than the recommended 10 mg/kg on day 1. 2 Similarly, 2.5 mL would deliver 100 mg, or 7.9 mg/kg, which exceeds the 5 mg/kg target for days 2–5. 2 While azithromycin has a wide therapeutic margin, adhering to the FDA‑approved dosing minimizes the risk of adverse effects (primarily gastrointestinal) and avoids unnecessary drug exposure. 1, 2
Common Pitfalls to Avoid
- Do not confuse suspension concentrations. The 200 mg/5 mL formulation is distinct from the 100 mg/5 mL formulation; always verify the concentration before calculating volume. 1
- Do not underdose the initial 10 mg/kg dose on day 1. Full dosing is essential to achieve therapeutic tissue levels, particularly for atypical pathogens such as Mycoplasma pneumoniae and Chlamydia pneumoniae. 1
- Do not administer azithromycin simultaneously with aluminum‑ or magnesium‑containing antacids. Separate administration by at least 2 hours, as antacids reduce drug absorption. 1
Expected Clinical Response
Children on appropriate azithromycin therapy should demonstrate clinical improvement within 48–72 hours. 1 If no improvement or clinical deterioration occurs within this timeframe, reassess the diagnosis, consider alternative pathogens, resistance patterns, or complications, and adjust therapy accordingly. 1
Summary of the Dose Change
The change from 5 mL and 2.5 mL to 3 mL and 1.5 mL reflects adherence to the FDA‑approved weight‑based dosing of 10 mg/kg on day 1 and 5 mg/kg on days 2–5 for a 12.7 kg child. 2 This ensures therapeutic efficacy while minimizing the risk of adverse effects and avoiding unnecessary drug exposure. 1, 2