What is the appropriate cephalexin dosing regimen for a 30‑lb (≈13.6‑kg) child with bullous impetigo?

Cephalexin Dosing for Bullous Impetigo in a 30-pound Child

For a 30-pound (13.6 kg) child with bullous impetigo, prescribe cephalexin 340-680 mg orally divided into 3-4 doses daily (25-50 mg/kg/day), which translates to approximately 85-170 mg per dose if given four times daily, for 10 days.

Dosing Calculation and Rationale

Standard Pediatric Dosing

• The FDA-approved pediatric dosage for cephalexin is 25-50 mg/kg/day in divided doses 1
• For a 13.6 kg child, this calculates to:
  • Minimum dose: 25 mg/kg/day = 340 mg/day
  • Maximum dose: 50 mg/kg/day = 680 mg/day 1

Practical Dosing Regimens

Four times daily (QID) dosing:

• Using 25 mg/kg/day: 85 mg per dose (approximately 2 mL of 125 mg/5 mL suspension) 1
• Using 50 mg/kg/day: 170 mg per dose (approximately 3.5 mL of 125 mg/5 mL suspension) 1

Twice daily (BID) dosing (alternative for improved compliance):

• The FDA label permits BID dosing for skin and skin structure infections: "the total daily dose may be divided and administered every 12 hours" 1
• Using 25 mg/kg/day: 170 mg per dose 1
• Using 50 mg/kg/day: 340 mg per dose 1

Evidence Supporting Cephalexin for Bullous Impetigo

Microbiologic Rationale

• Bullous impetigo is predominantly caused by Staphylococcus aureus (62-78% of cases as sole pathogen) 2, 3
• Cephalexin demonstrates excellent activity against methicillin-susceptible S. aureus (MSSA), the primary pathogen in bullous impetigo 4
• Penicillin resistance is common: only 2 of 64 staphylococcal strains (3%) were susceptible to penicillin G in one study 3

Clinical Efficacy Data

• Cephalexin showed 100% treatment success (0% failure rate) in a randomized trial of 73 children with impetigo, compared to 24% failure with penicillin V and 4% failure with erythromycin 2
• In a comparative study of 70 patients with staphylococcal skin infections (57 with bullous impetigo), cephalexin and dicloxacillin proved equally effective with only 1 treatment failure in each group 3
• A multicenter study of 394 pediatric patients with skin infections demonstrated 98.3% clinical cure rates with cephalexin 5

Dosing Frequency Considerations

• Twice-daily dosing is equally effective to four-times-daily dosing for skin and skin structure infections 3, 6
• A study of 154 patients found both BID and QID regimens were >97% effective when the total daily dose remained constant 6
• BID dosing enhances compliance and is explicitly approved by the FDA for skin infections in children 1

Treatment Duration and Monitoring

Duration

• Treat for 10 days to ensure adequate eradication of S. aureus 2, 3
• The FDA label recommends at least 10 days for β-hemolytic streptococcal infections, which applies to mixed infections 1

Expected Response

• Clinical improvement should be evident within the first week of treatment 3
• Persistence of lesions at 8-10 days constitutes treatment failure and warrants reassessment 2

Important Clinical Caveats

When Cephalexin May Not Be Appropriate

• Methicillin-resistant S. aureus (MRSA): If MRSA is suspected or confirmed, switch to clindamycin (30-40 mg/kg/day in 3-4 divided doses) or trimethoprim-sulfamethoxazole 4
• Severe systemic symptoms: Children with SIRS (fever, tachycardia, tachypnea) may require parenteral therapy 4
• Penicillin allergy with immediate hypersensitivity: Avoid cephalosporins; use clindamycin instead 4

Adverse Effects

• Diarrhea is the most common side effect (4-8% of patients) 5
• Gastrointestinal symptoms occur in approximately 4% of children 7
• Mild neutropenia can occur but is rarely clinically significant 7

Formulation and Administration

• Use 125 mg/5 mL suspension for this weight range 1
• Cephalexin may be given without regard to meals as it is acid-stable 1
• After reconstitution, store in refrigerator and discard after 14 days 1

Alternative High-Dose Regimen

Recent pharmacokinetic studies support a high-dose, reduced-frequency regimen of 45 mg/kg/dose (maximum 1.5 g) three times daily for MSSA infections 8, 7:

• For a 13.6 kg child: 612 mg per dose TID
• This regimen showed 99% cure rate in bone and joint infections 7
• May improve adherence compared to QID dosing 8, 7

However, this high-dose TID regimen exceeds standard FDA-approved dosing and should be reserved for situations where enhanced compliance is critical or when treating more severe MSSA infections 8, 7.