What is the first line of testing for a patient suspected of having shingles (Herpes Zoster)?

Diagnostic Testing for Shingles (Herpes Zoster)

The first-line diagnostic test for suspected shingles is viral detection directly from skin lesions using PCR (polymerase chain reaction) testing, which offers the highest sensitivity and specificity for confirming Varicella Zoster Virus infection. 1, 2

Clinical Presentation and Initial Assessment

When evaluating a patient with suspected shingles, look for:

• Unilateral vesicular rash following a dermatomal distribution
• Prodromal pain or discomfort in the affected dermatome
• Local edema and erythema preceding the rash
• Evolution of lesions from maculopapular to vesicular to crusting stages
• Common locations: thoracic, lumbar, cervical dermatomes, trigeminal distribution

Diagnostic Testing Algorithm

1. Direct Viral Detection from Lesions (First-Line)

• PCR testing of vesicle fluid/scrapings: Highest sensitivity (approaching 100%) and specificity 1, 2

  • Can detect VZV DNA even in crusted lesions
  • Preferred method when available
  • Allows for viral typing and differentiation from HSV

• Viral culture: Alternative when PCR is unavailable 1

  • Less sensitive than PCR but highly specific
  • Requires viable virus (best from fresh vesicles)
  • Takes longer for results (days versus hours for PCR)

• Direct immunofluorescence (DIF): Alternative when PCR/culture unavailable 1

  • Detects viral antigens in lesion samples
  • Faster than culture but less sensitive than PCR
  • Can differentiate VZV from HSV

• Tzanck smear: Limited utility 2

  • Can identify multinucleated giant cells but cannot differentiate between VZV and HSV
  • Low sensitivity compared to molecular methods

2. Serological Testing (Limited Utility for Acute Diagnosis)

• VZV IgG/IgM: Not recommended for acute diagnosis of shingles 1
  • Cannot reliably distinguish between current infection and past exposure
  • May be useful in determining immunity status before immunosuppression
  • Not helpful in confirming active shingles

Important Considerations

• Timing matters: Sample collection should be done from active lesions, preferably vesicles
• Misdiagnosis risk: Studies show initial herpes zoster is frequently misdiagnosed as herpes simplex, highlighting the importance of laboratory confirmation 3
• Targeted testing: Avoid routine "shotgun" laboratory panels that are not indicated by specific clinical findings 1
• Special populations: In immunocompromised patients, testing is particularly important as atypical presentations are common 4

Common Pitfalls to Avoid

1. Relying solely on clinical diagnosis: While often sufficient, laboratory confirmation is important in atypical presentations, immunocompromised hosts, or when treatment decisions depend on accurate diagnosis 3

2. Ordering unnecessary serologic tests: Serology is not useful for diagnosing active shingles and should not be routinely ordered 1

3. Delaying treatment for test results: Initiate antiviral therapy promptly if shingles is suspected clinically, especially within 72 hours of rash onset, while awaiting confirmation 5, 6

4. Missing ocular involvement: Always assess for ophthalmologic involvement when rash affects the ophthalmic division of the trigeminal nerve 5

By following this diagnostic approach, clinicians can accurately confirm shingles and initiate appropriate treatment promptly to reduce complications like post-herpetic neuralgia and improve patient outcomes.