Laboratory Testing for Shingles Diagnosis
PCR testing of vesicular lesions is the gold standard diagnostic test for shingles, with sensitivity and specificity approaching 100%. 1
Optimal Specimen Collection
Collect material from the base of unroofed vesicles by scraping or swabbing disrupted skin lesions for highest diagnostic yield. 1 The key technical points include:
- Sample vesicular lesions early in the disease course when vesicles are present, as this provides the highest yield 1
- Scrape or swab the base of disrupted vesicles to obtain adequate viral material 1
- Place specimens in appropriate transport medium immediately after collection 1
- PCR can still detect VZV DNA even in crusted lesions that are no longer infectious, providing diagnostic flexibility 1
Recommended Testing Methods
Nucleic acid amplification testing (NAAT/PCR) is the preferred diagnostic method, offering nearly 100% sensitivity and specificity for detecting varicella-zoster virus DNA. 1, 2 This represents a significant advantage over older methods:
- PCR confirmed clinical diagnosis of zoster in 95% of cases in validation studies 2
- Viral culture has only 20% sensitivity (though 100% specificity), making it inadequate as a primary diagnostic tool 2
- Direct immunofluorescent antigen staining has 82% sensitivity but only 76% specificity 2
Tests to Avoid
Do not use the Tzanck smear for shingles diagnosis, as it cannot differentiate between VZV and HSV infections and requires skilled microscopy interpretation. 1 Additional limitations include:
- Serological testing (VZV IgG/IgM) is not useful for diagnosing active shingles and should only be used to determine immunity status 1
- IgM and IgA antibody responses occur in only 48% of patients within 4 days of rash onset, making serology unreliable for acute diagnosis 2
Clinical Scenarios Requiring Laboratory Confirmation
Laboratory testing is particularly important in immunocompromised patients, as clinical presentation may be atypical. 1 Specific indications include:
- Atypical presentations where clinical diagnosis is uncertain 1
- Immunocompromised hosts who may have unusual manifestations 1, 3
- Cases where clinical features overlap with other vesicular conditions 1
- Suspected zoster sine herpete (zoster without rash) 4
- Suspected CNS involvement such as varicella meningitis, which requires PCR confirmation 3
Common Diagnostic Pitfalls
Clinical diagnosis alone can lead to both false positive and false negative diagnoses, making laboratory confirmation valuable even in seemingly straightforward cases. 1 Be aware that:
- Immunocompetent young adults can develop atypical presentations without typical preherpetic or postherpetic neuralgia 3
- Rapid diagnostic tests including varicella PCR help confirm diagnosis when typical zoster rash features are absent 3
- High index of suspicion is required for CNS complications like varicella meningitis 3