What are the diagnostic criteria for chickenpox in an adult?

Diagnostic Criteria for Chickenpox in Adults

The diagnosis of chickenpox in adults is primarily clinical, based on the acute onset of a diffuse generalized papulovesicular rash without other apparent cause, and laboratory confirmation is reserved for atypical presentations, immunocompromised patients, or public health purposes. 1, 2

Clinical Diagnostic Criteria

The CDC defines a clinical case of varicella as an illness with acute onset of diffuse (generalized) papulovesicular rash without other apparent cause. 1, 2 This clinical definition alone is sufficient for diagnosis in most immunocompetent adults with typical presentations.

Key Clinical Features to Identify:

• Rash characteristics: Crops of lesions in different stages of development (macules, papules, vesicles, crusts) appearing simultaneously 2
• Lesion distribution: 250-500 lesions in unvaccinated persons, diffusely distributed across the body 2
• Temporal pattern: New lesions continue erupting for 4-6 days in immunocompetent hosts 2
• Associated symptoms: Pruritic vesicular rash with fever and constitutional symptoms 2

Modified Presentation in Previously Vaccinated Adults:

• Fewer total lesions 2
• Predominantly maculopapular rather than vesicular 2
• Milder systemic symptoms 2

Laboratory Confirmation

Laboratory testing should be pursued when the clinical presentation is atypical, the patient is immunocompromised, or confirmation is needed for public health purposes. 1, 2

Preferred Diagnostic Methods (in order of preference):

• PCR (nucleic acid amplification): Most sensitive and specific test, approaching 100% for both parameters 1, 2
• Direct fluorescent antibody (DFA) assay: Rapid and reliable 2
• Viral culture: Specific but slower 1, 2
• Serology: NOT useful for acute diagnosis of active infection 1, 2

Specimen Collection:

• Sample from disrupted skin lesions: Scrape or swab material from blister base 1
• PCR can detect VZV DNA even in crusted lesions, though these are no longer infectious 1
• Avoid serology for acute diagnosis: Single serum samples cannot distinguish existing immunity from recent infection 1

Case Classification System

The CDC provides a formal classification system for surveillance purposes 1:

Probable Case:

• Meets the clinical case definition (acute onset diffuse papulovesicular rash without other cause) 1
• NOT laboratory confirmed 1
• NOT epidemiologically linked to another probable or confirmed case 1

Confirmed Case (either criterion):

• Laboratory confirmed (viral isolation or significant rise in varicella IgG antibody) 1
• OR meets clinical case definition AND is epidemiologically linked to a confirmed or probable case 1

Important note: Two probable cases that are epidemiologically linked are considered confirmed, even without laboratory testing. 1

Critical Pitfalls to Avoid

Do Not Rely on Serology for Acute Diagnosis:

• Serology is only useful for determining immunity status before exposure, not for diagnosing active infection 1, 2
• Paired acute and convalescent sera are rarely available during initial assessment 1

Recognize High-Risk Presentations Requiring Immediate Action:

• Adults have 13 times higher hospitalization rates and 25 times higher mortality compared to children aged 5-9 years 2
• Respiratory symptoms in adults suggest varicella pneumonia (occurs in 28.4% of hospitalized adults) 3
• Older age, male sex, and smoking increase pneumonia risk significantly 3
• Immunocompromised patients may present with atypical features making diagnosis more challenging 4

Differential Diagnosis Considerations:

• Rule out herpes simplex virus, herpes zoster, and enteroviral infections with standard laboratory testing 1
• In immunocompromised patients, consider disseminated zoster or other orthopoxviruses 1

Immediate Actions After Diagnosis

Once diagnosed, patients must be isolated immediately as they are infectious from 2 days before rash onset until all lesions are crusted. 2, 5

Antiviral treatment with acyclovir or valacyclovir should be initiated promptly (within 72 hours of rash onset) for all adults, as they are at significantly higher risk for complications. 2, 6 The recommended dose is acyclovir 800 mg orally 4-5 times daily for 7-10 days 6, 5