Diagnosis of Impetigo
Impetigo is diagnosed primarily through clinical examination based on characteristic lesion appearance, though cultures of vesicle fluid, pus, or erosions can confirm the causative organism. 1
Clinical Diagnosis
The diagnosis relies on recognizing two distinct presentations:
Nonbullous impetigo (70% of cases): Erythematous papules that rapidly evolve into vesicles and pustules, which rupture to form honey-colored crusts on an erythematous base 1, 2
Bullous impetigo (30% of cases): Fragile, thin-roofed vesicopustules caused by toxin-producing S. aureus that rupture, creating crusted erythematous erosions often surrounded by a collar of remnant roof 1
When to Culture
Gram stain and culture are recommended but treatment without these studies is reasonable in typical cases 1. Cultures help identify whether S. aureus and/or β-hemolytic Streptococcus is the cause, which guides antibiotic selection. Culture is particularly useful when MRSA is suspected or in treatment failures 1.
National Organizations for Treatment Guidelines
The Infectious Diseases Society of America (IDSA) provides the primary evidence-based treatment guidelines for impetigo, endorsed by the Pediatric Infectious Diseases Society (PIDS). 1
The 2014 IDSA Practice Guidelines for Skin and Soft Tissue Infections represent the authoritative national resource for impetigo management in the United States.
First-Line Treatment
For localized impetigo with few lesions, topical mupirocin 2% ointment or retapamulin 1% ointment applied twice daily for 5 days is the first-line treatment. 1, 2
Treatment Algorithm
For Limited Disease (Few Lesions):
- Topical mupirocin 2% ointment OR retapamulin 1% ointment
- Apply twice daily for 5 days
- Strong evidence, high quality recommendation 1
For Extensive Disease (Numerous Lesions) or Outbreaks:
Oral antibiotics are recommended to decrease transmission 1:
First-line oral therapy: Dicloxacillin or cephalexin for 7 days
- These target both S. aureus and streptococci
- Most staphylococcal isolates from impetigo are methicillin-susceptible 1
If MRSA suspected or confirmed: Doxycycline, clindamycin, or sulfamethoxazole-trimethoprim (SMX-TMP) 1
If cultures yield streptococci alone: Oral penicillin is the drug of choice 1
For Ecthyma (Deeper Infection):
Oral antimicrobials are required (topical therapy insufficient) 1
Key Clinical Pearls
The IDSA guidelines emphasize that topical treatment with mupirocin or retapamulin is as effective as oral antimicrobials for impetigo 1. However, systemic therapy is preferred for patients with numerous lesions or in outbreaks affecting several people to help decrease transmission of infection 1.
A common pitfall is prescribing oral penicillin V, which is seldom effective for impetigo 3. The antimicrobial must cover S. aureus, which requires penicillinase-resistant agents unless cultures confirm streptococci alone.
For penicillin-allergic patients: Macrolides or clindamycin are alternatives, though erythromycin resistance rates are rising 3.
Special Circumstances
During outbreaks of poststreptococcal glomerulonephritis, systemic antimicrobials should be used to help eliminate nephritogenic strains of Streptococcus pyogenes from the community 1.
Antimicrobial Stewardship Considerations
Recent evidence emphasizes the importance of antimicrobial stewardship in impetigo management 4, 5. Topical antibiotics should be prioritized for localized disease to reduce systemic antibiotic exposure and resistance development. Healthcare providers should be aware of local resistance patterns, particularly increasing fusidic acid and erythromycin resistance in some regions 6, 7.