How is varicella diagnosed?

How to Diagnose Varicella (Chickenpox)

Varicella is primarily diagnosed clinically based on the characteristic presentation of an acute onset diffuse papulovesicular rash with crops of lesions in different stages of development, and laboratory confirmation is reserved for atypical presentations, immunocompromised patients, or when needed for public health purposes. 1

Clinical Diagnosis

Classic Presentation in Unvaccinated Individuals

• The hallmark is a generalized pruritic vesicular rash with 250-500 lesions appearing in successive crops over 2-4 days, with lesions progressing from macules to papules to vesicles to crusts 2, 1
• Look for lesions in multiple stages of development simultaneously (macules, papules, vesicles, and crusts present at the same time), which distinguishes varicella from other vesicular rashes 1
• The rash typically begins on the head and trunk, then spreads to the extremities 2
• Accompanying symptoms include fever, malaise, headache, and anorexia during the 4-5 day illness course 2
• New vesicle formation continues for 2-4 days in immunocompetent hosts 2

Modified Presentation in Vaccinated Individuals

• Vaccinated children often present with fewer lesions (often <50), predominantly maculopapular rather than vesicular, and milder systemic symptoms 1, 3
• The atypical nature of breakthrough varicella requires a high index of suspicion and consideration of varicella in the differential diagnosis of any maculopapular or vesicular rash 2, 3
• Disease in vaccinated persons is typically milder but can still occur 3

High-Risk Presentations Requiring Heightened Vigilance

• Adults, infants, adolescents, and immunocompromised persons are at higher risk for severe disease and complications including visceral dissemination and VZV pneumonitis 2, 4
• Immunocompromised patients may develop life-threatening forms with visceral dissemination affecting liver and brain 4
• Look for signs of complications such as progressive varicella (new lesions continuing beyond 7 days), which may indicate depressed immune response 5

Laboratory Confirmation

Indications for Laboratory Testing

Laboratory confirmation is indicated when:

• The clinical presentation is atypical or modified (common in vaccinated individuals) 1
• The patient is immunocompromised 1, 4
• Confirmation is needed for public health purposes 1
• Diagnosis is uncertain and would change management 1

Preferred Diagnostic Methods

• PCR (nucleic acid amplification) of vesicular fluid or scab scraping is the most sensitive and specific test, approaching 100% sensitivity and specificity 1, 6
• Direct fluorescent antibody (DFA) assay on vesicular fluid or scab material provides rapid results 2, 1
• Viral culture is specific but less sensitive and takes longer 2, 1
• Serology showing significant increase in VZV-specific IgG antibody between acute and convalescent serum samples can assist in diagnosis but is not useful for acute diagnosis of active infection 2, 1

Important Caveat About Serology

• Serology is NOT useful for diagnosing acute active varicella infection but is helpful for determining immunity status before exposure 1, 6

Immediate Actions After Diagnosis

Isolation Requirements

• Patients must be isolated immediately upon diagnosis as they are infectious from 2 days before rash onset until all lesions are crusted 1
• This typically means isolation for 4-7 days after rash onset in immunocompetent patients 5

Treatment Considerations

• Antiviral treatment with acyclovir or valacyclovir should be started promptly (within 72 hours of rash onset) for adults, immunocompromised patients, and those with severe disease or complications 1
• Treatment initiation should not be delayed while awaiting laboratory confirmation in high-risk patients 6

Monitoring for Complications

Watch for:

• Bacterial superinfection of skin lesions 1
• Pneumonia (especially in adults and immunocompromised) 1, 4
• Encephalitis 1
• Visceral dissemination in immunocompromised patients 1, 4

Common Pitfalls to Avoid

• Failing to consider varicella in vaccinated children with atypical maculopapular rash - breakthrough varicella is often modified and requires high clinical suspicion 1, 3
• Relying on serology for acute diagnosis - serology cannot diagnose active infection, only immunity status 1, 6
• Underestimating severity in adults - adults have 13 times higher hospitalization rates and 25 times higher mortality compared to children aged 5-9 years 1
• Delaying isolation - patients are contagious before rash appears and must be isolated immediately upon diagnosis 1
• Missing the narrow window for antiviral therapy - treatment must begin within 72 hours of rash onset for maximum benefit 1