What is the recommended laboratory test for diagnosing shingles?

Laboratory Testing for Shingles (Herpes Zoster)

The recommended laboratory test for diagnosing shingles is nucleic acid amplification testing (NAAT) of material collected from vesicular lesions, which offers nearly 100% sensitivity and specificity for detecting varicella-zoster virus (VZV) DNA. 1

Specimen Collection and Testing Methods

Optimal Specimen Collection:

• Collect material from vesicular lesions by scraping or swabbing disrupted skin lesions 1
• For best results, sample the base of unroofed vesicles to obtain viral material 1
• The Kimura scraper can be used to sample the bases of lesions (either ulcers or burst vesicles) 1
• Specimens should be collected early in the course of the disease when vesicles are present for highest yield 1

Testing Methods (in order of preference):

1. Nucleic Acid Amplification Tests (NAATs):

   • PCR testing is the gold standard with sensitivity and specificity approaching 100% 1
   • Can detect VZV DNA even in crusted lesions that are no longer infectious 1
   • Superior to other methods, especially in atypical presentations 2, 3
   • Can distinguish between wild-type VZV and vaccine strains when needed 3

2. Viral Culture:

   • Less sensitive than PCR (approximately 53% sensitivity compared to PCR) 3
   • Requires viable virus and proper transport conditions 1
   • Results take longer (up to several days) 1

3. Microscopy (Tzanck Test):

   • Rapid but less sensitive (76.9% sensitivity compared to PCR) 2
   • Cannot differentiate between VZV and HSV infections 1
   • Shows multinucleate giant cells and eosinophilic intranuclear inclusions 4
   • Requires skilled microscopist for interpretation 1

4. Rapid Antigen Detection:

   • Faster than culture but less sensitive than PCR 1
   • Dependent on quality and timing of sample 1

Important Considerations

Transport Conditions:

• For PCR testing: Specimens should be placed in appropriate transport medium immediately after collection 1
• For viral culture: Transport on ice is preferable; material should not be kept for more than 24 hours at room temperature 1
• For microscopy: If storage is needed for more than 24 hours, fix smears with 96% ethyl alcohol for three minutes 1

Special Situations:

• For zoster sine herpete (shingles without rash), PCR testing of intact skin scrapings from the affected dermatome may confirm diagnosis 5
• In immunocompromised patients, testing is particularly important as clinical presentation may be atypical 1, 6

What Not to Use:

• Serology (VZV IgG/IgM) is not useful for diagnosis of active shingles or chickenpox 1
• Serology should only be used to determine immunity status, not for diagnosing active infection 1

Clinical Correlation

• Laboratory confirmation is important as clinical diagnosis alone can lead to both false positive and false negative diagnoses 1
• There is approximately 92% agreement between clinical diagnosis and PCR results in typical cases 3
• Testing is especially valuable in atypical presentations, immunocompromised hosts, or when clinical features overlap with other conditions 1, 6

Remember that prompt diagnosis allows for early initiation of antiviral therapy, which is crucial for reducing the duration and severity of symptoms and preventing complications such as postherpetic neuralgia 6, 5.