Differential Diagnoses for Impetigo
The key differential diagnoses for impetigo include ecthyma, tinea faciei, folliculitis, immunobullous dermatoses (particularly for bullous impetigo), herpes simplex virus infection, contact dermatitis, and atopic dermatitis with secondary infection.
Primary Differentials Based on Clinical Presentation
For Bullous Impetigo
When evaluating bullous impetigo (characterized by fragile, thin-roofed vesicopustules that rupture leaving crusted erosions with a collarette of scale), consider:
- Immunobullous dermatoses - Bullous impetigo can clinically mimic rare immunobullous conditions, particularly when presentation is atypical 1. In pregnant women specifically, consider pemphigoid gestationis, herpes gestationis, and pustular psoriasis of pregnancy 2
- Herpes simplex virus - Presents with grouped vesicles but typically more painful and on an erythematous base
- Contact dermatitis - May have bullous lesions but history of exposure and distribution pattern differ
For Nonbullous Impetigo
When evaluating nonbullous impetigo (erythematous papules evolving to vesicles/pustules with characteristic honey-colored crusts), consider:
- Ecthyma - This is the most important differential as it represents a deeper infection than impetigo 3. Key distinguishing feature: ecthyma presents as circular, erythematous ulcers with adherent crusts and surrounding edema, and unlike impetigo, ecthyma heals with scarring 3
- Tinea faciei - Particularly when lesions are on the face, dermatophyte infections (especially Microsporum gypseum) can simulate impetigo 4. Look for annular scaling borders and perform mycologic studies if clinical suspicion exists
- Folliculitis - More superficial than impetigo, with pus limited to hair follicles rather than extending through dermis 3
Anatomic Location Considerations
Facial and perioral lesions require particular attention to differential diagnosis 5:
- Herpes simplex (more painful, grouped vesicles)
- Angular cheilitis
- Perioral dermatitis
- Tinea faciei (especially in infants) 4
Critical Diagnostic Pitfalls
Do not rely solely on clinical appearance - While impetigo diagnosis is primarily clinical 6, atypical presentations warrant additional workup. Cultures of vesicle fluid, pus, erosions, or ulcers establish the definitive cause 3.
In pregnant women, the differential expands significantly to include specific dermatoses of pregnancy, and bullous impetigo should always be considered when blisters appear 2.
For widespread or treatment-resistant cases, consider:
- MRSA infection (requires culture confirmation)
- Underlying immunocompromise
- Misdiagnosis of an immunobullous condition 1
Practical Approach to Differentiation
When uncertain between impetigo and other conditions:
- Assess depth of involvement - Superficial honey-colored crusts suggest impetigo; deeper ulceration with scarring suggests ecthyma 3
- Evaluate pain level - Impetigo is typically painless or only slightly pruritic 5; significant pain suggests herpes or other etiologies
- Check distribution pattern - Impetigo commonly affects face (especially perioral), extremities, and areas of skin trauma
- Obtain cultures when presentation is atypical, treatment fails, or in outbreak settings 3
- Consider mycologic studies for facial lesions that don't respond to standard impetigo treatment 4