What laboratory tests are recommended for a patient with suspected shingles?

Laboratory Tests for Suspected Shingles

For suspected shingles (herpes zoster), laboratory confirmation should include viral detection from vesicle fluid samples through PCR testing, which has 95% sensitivity and is the most reliable diagnostic method. 1

Recommended Laboratory Tests

Primary Diagnostic Tests:

• PCR testing of vesicle fluid: The gold standard with 95% sensitivity for detecting VZV DNA 1
• Direct immunofluorescent VZV-specific antigen staining: 82% sensitivity with 76% specificity from vesicle samples 1
• Virus culture: Limited sensitivity (20%) but high specificity (100%) 1

Collection Method for Vesicular Lesions:

1. Open vesicles with a sterile needle
2. Collect vesicle content with a swab and either:
   • Apply to a microscope slide (for immunofluorescence staining)
   • Place in transport media for viral culture or PCR testing 2

Testing Algorithm

1. For typical presentation with characteristic rash:

   • PCR testing of vesicle fluid is preferred
   • Tzanck smear can be performed as a rapid office-based test (detects multinucleated giant cells but cannot differentiate between HSV and VZV) 3

2. For atypical presentations or immunocompromised patients:

   • PCR testing is essential
   • Consider adding serological testing (VZV IgM and IgA antibodies) which may show response within 4 days of rash onset 1

Clinical Considerations

• Laboratory confirmation is particularly important in:

  • Atypical presentations
  • Immunocompromised patients
  • Cases where treatment decisions depend on accurate diagnosis
  • Zoster sine herpete (zoster without rash) 4

• Timing matters: Collect samples from fresh vesicles before they crust over for optimal results

Pitfalls to Avoid

• Delayed sampling: Vesicle fluid yields decrease as lesions begin to crust
• Inadequate specimen collection: Ensure proper sampling technique to obtain sufficient vesicular fluid
• Relying solely on serology: Antibody tests alone have limited utility for acute diagnosis
• Misinterpreting Tzanck smear: Cannot differentiate between HSV and VZV infections 3

Remember that while laboratory confirmation is valuable, diagnosis of shingles is often made clinically based on the characteristic unilateral dermatomal distribution of vesicular lesions and associated pain. Laboratory testing is most important in atypical presentations, immunocompromised patients, or when confirmation would alter management decisions.