What is the diagnostic approach for a patient, particularly a child, with suspected impetigo, considering their age, medical history, and overall health status?

How to Diagnose Impetigo

Impetigo is diagnosed clinically based on characteristic physical examination findings, and cultures are generally reserved for treatment failures, suspected MRSA, or recurrent infections. 1, 2

Clinical Diagnosis

Primary Diagnostic Approach

• The diagnosis is made clinically by recognizing the characteristic appearance of lesions on physical examination. 1, 3
• Obtain a focused history including age (peak incidence 2-5 years), recent trauma or insect bites, hygiene practices, geographic location, and exposure to other infected individuals. 1, 4
• Examine exposed areas of the body, particularly the face and extremities, where lesions most commonly occur. 1, 2

Distinguishing Between Types

Nonbullous Impetigo (70% of cases):

• Look for erythematous papules that rapidly evolve into vesicles, then pustules, and finally characteristic thick honey-colored crusts. 2, 4
• Pustules gradually enlarge and break down over 4-6 days before forming crusts. 2
• Caused by Staphylococcus aureus, Streptococcus pyogenes, or both in combination. 2, 4

Bullous Impetigo (30% of cases):

• Identify fragile, thin-roofed vesicopustules that form large, flaccid bullae. 2, 4
• More likely to affect intertriginous areas (skin folds). 4
• Caused exclusively by toxin-producing strains of S. aureus. 2, 4

Ecthyma (deeper variant):

• Recognize circular, erythematous ulcers with adherent crusts extending deeper than typical impetigo. 2, 5

Associated Findings

• Regional lymphadenitis may be present, but systemic symptoms (fever, malaise) are usually absent. 2
• Lesions remain well-localized but are frequently multiple. 1

When to Obtain Cultures

Microbiologic cultures are not routinely necessary for uncomplicated cases, but should be obtained in specific circumstances: 1

• Treatment failure after 48-72 hours of appropriate antibiotic therapy 6, 2
• Suspected methicillin-resistant S. aureus (MRSA) infection 6, 2
• Recurrent infections 6, 2
• Outbreak situations requiring identification of the causative strain 1

When cultures are indicated, obtain specimens from vesicle fluid, pus, or erosions. 2

Differential Diagnosis to Consider

Distinguish impetigo from other conditions that may appear similar:

• Folliculitis: More superficial with inflammation confined to hair follicles rather than extending through the dermis 2
• Furuncles (boils): Deeper infections presenting as inflammatory nodules in subcutaneous tissue, not superficial crusted lesions 2
• Impetiginized eczema: Secondary bacterial infection superimposed on underlying eczematous lesions 2
• Herpes simplex: May have similar vesicular appearance but typically more painful with grouped vesicles on an erythematous base 1

Common Pitfalls

• Do not delay treatment waiting for culture results in typical cases, as clinical diagnosis is sufficient and early treatment prevents spread. 1, 3
• Do not confuse resolving impetigo (flat, dry lesions) with active disease (purulent, crusted lesions) when assessing treatment response. 7
• Do not assume penicillin alone will be effective, as it lacks adequate coverage against S. aureus, which is the predominant pathogen. 6, 8
• Consider MRSA in patients who fail initial therapy or in areas with high MRSA prevalence. 1, 4

Documentation Recommendations

• Record the location, number, and appearance of lesions (vesicular, pustular, crusted, or ulcerated). 7
• Document whether lesions represent incident (new) or prevalent (existing) cases, particularly in endemic settings. 7
• Note any systemic symptoms, extent of disease (localized vs. extensive), and presence of complications. 1