What is a reliable laboratory test for diagnosing Herpes Simplex Virus (HSV) infections?

Optimal Laboratory Tests for Herpes Simplex Virus Diagnosis

Nucleic acid amplification testing (NAAT/PCR) from active lesions is the gold standard diagnostic test for herpes simplex virus infections, offering superior sensitivity and specificity compared to other available methods. 1

Diagnostic Testing Algorithm

For Patients with Active Lesions:

1. First-line test: NAAT/PCR

   • Sensitivity >90%, specificity >90% 1
   • Can differentiate between HSV-1 and HSV-2 2
   • Collect specimen by swabbing the base of an unroofed vesicle
   • Perform testing before lesions begin healing for optimal sensitivity 2
   • Results available within 4 hours 3

2. Second-line test: Viral culture (if NAAT/PCR unavailable)

   • Less sensitive than PCR (detects approximately 34% of cases vs. 57% with PCR) 3
   • More labor-intensive and time-consuming (requires 7-28 days) 4
   • Can still differentiate between HSV-1 and HSV-2 1

3. Third-line test: Antigen detection

   • Direct immunofluorescence (DIF) or enzyme immunoassay (EIA) 1
   • Lower sensitivity than PCR or culture 1
   • Only use if NAAT/PCR and culture are unavailable 1

For Patients Without Active Lesions:

1. Type-specific serologic testing (limited indications)
   • Appropriate for:
     • Asymptomatic individuals with high-risk exposures
     • Pregnant women at risk of acquiring HSV near delivery
     • Men who have sex with men
     • HIV-positive individuals 1, 2
   • Not recommended for general screening 1
   • Wait at least 12 weeks after suspected exposure (window period) 1

Important Considerations for Serologic Testing

• HSV-1 serologic tests: Limited sensitivity (~70%) 1
• HSV-2 serologic tests: High sensitivity (92%) but lower specificity 1
• Interpretation of results:
  • HSV-2 index values ≥3.0 may be sufficient for diagnosis without confirmation
  • Low positive HSV-2 results (index values <3.0) should ideally be confirmed with a second test 1
  • False positives are more common in patients with HSV-1 infection 1

Common Pitfalls and How to Avoid Them

1. False-negative results:

   • Testing healing lesions (collect specimens from fresh lesions) 1
   • Testing during window period for serologic tests (wait 12 weeks after exposure) 1
   • Using less sensitive methods like Tzanck smear (avoid these tests) 1

2. False-positive results:

   • Low index values in HSV-2 serologic testing (confirm with second test) 1
   • Cross-reactivity in patients with HSV-1 infection (use type-specific assays) 1

3. Inappropriate testing:

   • Testing asymptomatic sites (not recommended due to intermittent shedding) 2
   • Widespread screening (not cost-effective or clinically useful) 1
   • Using non-validated tests (validate against gold standard) 1

Special Considerations

• For central nervous system infections (e.g., encephalitis), NAAT/PCR of cerebrospinal fluid is the diagnostic standard 5, 6
• Immunocompromised patients may have atypical presentations requiring more extensive testing 2
• Quality control with internal positive controls and negative controls is essential for reliable results 1

By following this evidence-based approach to HSV testing, clinicians can ensure accurate diagnosis, appropriate treatment, and improved patient outcomes.