What is the appropriate cephalexin dose for treating impetigo in a 30‑lb (≈13.6 kg) child administered three times daily?

Cephalexin Dosing for Impetigo in a 30-Pound Child (Three Times Daily)

For a 30-pound (13.6 kg) child with impetigo, prescribe cephalexin 167 mg three times daily (total 500 mg/day), which falls within the recommended 25–50 mg/kg/day range divided into 3–4 doses for a 7-day course. 1, 2

Dosing Calculation and Rationale

• Weight-based dosing: The standard pediatric dose for impetigo is 25–50 mg/kg/day divided into 3–4 doses 1, 2

• For this 13.6 kg child:

  • Minimum daily dose: 25 mg/kg × 13.6 kg = 340 mg/day
  • Maximum daily dose: 50 mg/kg × 13.6 kg = 680 mg/day
  • Three times daily regimen: 167 mg TID (total 500 mg/day) provides mid-range dosing 2
  • Alternative four times daily: 125 mg QID (total 500 mg/day) 1, 2

• Practical formulation: Using the 250 mg/5 mL suspension, administer approximately 3.3 mL (just over ½ teaspoon) three times daily 2

Treatment Duration and Monitoring

• Standard course: 7 days is typically sufficient for uncomplicated impetigo 1

• Clinical response expected: Lesions should begin clearing within the first week of therapy 3, 4

• Treatment failure indicators: If lesions persist after 7–10 days, consider MRSA coverage 1, 5

Critical Considerations for MRSA

Cephalexin is completely inactive against methicillin-resistant Staphylococcus aureus (MRSA) and should not be used if MRSA is suspected. 1, 5

• Switch to MRSA-active therapy if:

  • No clinical improvement after 48–72 hours 5
  • Known MRSA colonization or high local MRSA prevalence 5
  • Systemic signs (fever, extensive lesions, lymphadenitis) 5

• MRSA alternatives:

  • Clindamycin: 10–20 mg/kg/day divided TID (for this child: 45–90 mg TID) 1
  • Trimethoprim-sulfamethoxazole: 8–12 mg/kg/day (based on TMP component) divided BID 1

Evidence Supporting Three Times Daily Dosing

• Pharmacokinetic data: Recent studies demonstrate that TID dosing achieves adequate pharmacodynamic targets for methicillin-susceptible S. aureus (MSSA) with improved adherence compared to QID regimens 6, 7, 8

• Clinical efficacy: Cephalexin remains highly effective for staphylococcal impetigo when organisms are susceptible, with cure rates exceeding 95% in comparative trials 3, 4

• Dosing frequency advantage: TID dosing significantly improves medication adherence in pediatric populations compared to QID schedules 6, 8

Common Pitfalls to Avoid

• Do not use cephalexin for streptococcal pharyngitis coverage alone—while it covers group A streptococcus, penicillin remains preferred for pharyngitis 1

• Avoid underdosing: The 25 mg/kg/day lower limit should be reserved for mild infections; most impetigo cases benefit from 40–50 mg/kg/day dosing 2, 3

• Do not extend beyond 10 days without reassessment: Persistent lesions beyond this timeframe suggest treatment failure, resistance, or alternative diagnosis 1

• Storage: After reconstitution, refrigerate suspension and discard after 14 days 2