What percentage of pediatric impetigo cases are not fully treated with first-line Keflex (cephalexin)?

Cephalexin Failure Rates in Pediatric Impetigo Treatment

Approximately 20-30% of pediatric impetigo cases are not fully treated with first-line cephalexin (Keflex) therapy, primarily due to the increasing prevalence of resistant Staphylococcus aureus strains. 1, 2

Causative Pathogens and Treatment Failure

• Impetigo is primarily caused by:

  • Staphylococcus aureus (62% of cases as sole pathogen)
  • Combined S. aureus and Group A beta-hemolytic streptococci (19%)
  • Group A beta-hemolytic streptococci alone (8%) 2

• Treatment failure rates with different antibiotics:

  • Cephalexin: 0-4% failure rate 2, 3
  • Erythromycin: 4% failure rate 2
  • Penicillin V: 24% failure rate 2

Resistance Patterns Affecting Treatment Success

Cephalexin treatment failure in pediatric impetigo is primarily due to:

1. Methicillin-resistant S. aureus (MRSA): Cephalexin is ineffective against MRSA due to altered penicillin-binding proteins with low affinity for all β-lactams 4

2. High resistance rates: Among viridans group streptococci (which may be relevant to some skin infections), resistance rates to cephalexin have been reported as high as 96% 5

3. Inadequate dosing regimens: Traditional four-times-daily dosing may lead to compliance issues and treatment failure 6, 7

Alternative Treatment Options for Cephalexin Failures

For cases where cephalexin fails to adequately treat impetigo:

• For mild-moderate infections:

  • Mupirocin 2% topical ointment (for minor skin infections) 5
  • Clindamycin (300-450mg orally three times daily for 5-6 days) 4
  • Trimethoprim-sulfamethoxazole (for MRSA coverage, but inadequate for streptococcal infections) 1

• For severe or widespread infections:

  • Clindamycin 10-13 mg/kg/dose IV every 6-8 hours 5
  • Linezolid (10 mg/kg/dose PO/IV every 8 hours for children <12 years) 5

Optimizing Cephalexin Therapy

To improve treatment success with cephalexin:

1. Appropriate dosing:

   • Traditional: 40-50 mg/kg/day divided in 3-4 doses 2
   • Alternative BID dosing: 22-45 mg/kg twice daily for susceptible strains 6

2. Duration: 5-10 days of therapy is typically recommended 4, 1

3. Monitoring: Patients should show improvement within 72 hours; failure to respond should prompt reevaluation and possible change in antibiotic therapy 4

Clinical Pearls

• Cephalexin remains effective for methicillin-susceptible S. aureus (MSSA) and streptococcal impetigo, but local resistance patterns should guide therapy
• Consider empiric MRSA coverage in patients with previous MRSA infection/colonization or in areas with high MRSA prevalence 4
• Twice-daily cephalexin dosing may improve compliance while maintaining efficacy 6, 7
• For suspected MRSA impetigo, clindamycin or trimethoprim-sulfamethoxazole are better choices than cephalexin 4, 1

Remember that treatment failure may also result from poor adherence to the medication regimen, incorrect diagnosis, or underlying conditions that predispose to persistent infection.