What is the appropriate cephalexin dosage for a 45‑lb (≈20 kg) child with bullous impetigo?

Cephalexin Dosing for Bullous Impetigo in a 45-pound Child

For a 45-pound (approximately 20 kg) child with bullous impetigo, administer cephalexin 25–50 mg/kg/day divided into 3–4 doses, which translates to 500–1000 mg total daily dose, typically given as 125–250 mg four times daily or 167–333 mg three times daily for 7–10 days. 1, 2

Dosing Calculation and Administration

For this 20 kg child, the standard dosing approach is:

• Standard dosing: 25–50 mg/kg/day divided into 3–4 doses 1, 2
• Calculated range: 500 mg/day (at 25 mg/kg) to 1000 mg/day (at 50 mg/kg)
• Practical regimens:
  • 250 mg four times daily (1000 mg/day total) 1
  • 250 mg three times daily (750 mg/day total) for improved adherence 2

The FDA label specifically states that for skin and skin structure infections in pediatric patients, the total daily dose may be divided and administered every 12 hours, though traditional four-times-daily dosing remains standard 2.

Evidence Supporting Cephalexin for Bullous Impetigo

Cephalexin is highly effective for staphylococcal impetigo, which is the primary pathogen in bullous impetigo:

• Cephalexin demonstrated 100% treatment success in one randomized trial of pediatric impetigo, superior to penicillin V (76% success) and comparable to erythromycin (96% success) 3
• Staphylococcus aureus is isolated in 62–81% of impetigo cases, making anti-staphylococcal coverage essential 3, 4
• Clinical cure rates of 98.3% have been documented in pediatric skin infections treated with cephalexin 4

Dosing Frequency Considerations

Three-times-daily dosing may be preferable to four-times-daily dosing to improve adherence while maintaining efficacy:

• Twice-daily cephalexin (same total daily dose) proved equally effective to four-times-daily dosing in dermatologic infections, with >97% efficacy for both regimens 5, 6
• Recent pharmacokinetic modeling supports 45 mg/kg three times daily (maximum 1.5 g per dose) for achieving adequate drug exposure 7, 8
• The FDA label permits divided dosing every 12 hours for skin infections in children over 1 year of age 2

For this 20 kg child, practical options include:

• 250 mg four times daily (standard approach) 1
• 250 mg three times daily (750 mg/day total, enhances compliance) 2, 7
• 500 mg twice daily (if using higher end of dosing range) 2, 5

Treatment Duration and Monitoring

• Duration: 7–10 days is standard for skin and soft tissue infections 1, 2
• Most patients with bullous impetigo show clearing within the first week of treatment 6
• Treatment should be continued for the full course even if clinical improvement occurs earlier 2

Important Caveats

Consider MRSA coverage if:

• The patient has failed initial cephalexin therapy 1
• There is known MRSA colonization or high local prevalence 1
• The patient has systemic signs of infection (fever, tachycardia, elevated WBC) 1

In these scenarios, switch to clindamycin (20–40 mg/kg/day in 3–4 divided doses) or trimethoprim-sulfamethoxazole 1.

First-generation cephalosporins like cephalexin are inactive against MRSA and should not be used if methicillin-resistant organisms are suspected 1.

Safety Profile

Cephalexin is generally well-tolerated in children:

• Diarrhea occurs in approximately 4–8% of pediatric patients 4
• Mild neutropenia may occur but is typically asymptomatic 8
• The suspension formulation should be refrigerated after mixing and discarded after 14 days 2