What is the role of Polymerase Chain Reaction (PCR) in diagnosing neuroviral infections?

Role of PCR in Diagnosing Neuroviral Infections

PCR testing of cerebrospinal fluid (CSF) is the gold standard for diagnosing viral infections of the central nervous system, with sensitivities and specificities of 95-99% for common neurotropic viruses, and should be performed promptly in all suspected cases to reduce morbidity and mortality. 1

Diagnostic Value of PCR for Neuroviral Infections

PCR has revolutionized the diagnosis of central nervous system viral infections by:

• Providing rapid detection with high sensitivity (96-98%) and specificity (95-99%) for herpes simplex virus encephalitis 1
• Replacing brain biopsy as the diagnostic standard for herpes encephalitis 2
• Allowing detection of minute quantities of viral nucleic acids in CSF and tissues 2
• Enabling early diagnosis and prompt initiation of appropriate antiviral therapy 1

Optimal Specimen Collection

For suspected neuroviral infections, multiple specimens should be collected:

• CSF samples are essential (unless contraindicated) 1

  • Collect adequate volume (at least 20cc when possible) 3
  • Timing matters: optimal collection between days 2-10 of illness for HSV 1
  • A second LP should be performed 24-48h later if initial results are non-diagnostic 1

• Additional samples to collect:

  • Respiratory specimens and stool samples for enteroviruses (especially for EV-D68) 1
  • Blood samples for serology (acute and convalescent) 1

Recommended PCR Testing Algorithm

First-line PCR testing for all suspected viral encephalitis:

• HSV-1 and HSV-2
• Varicella zoster virus (VZV)
• Enteroviruses
• These three tests will identify 90% of cases due to known viral pathogens 1

Second-line testing based on clinical features:

• For immunocompromised patients:

  • Cytomegalovirus (CMV) - sensitivity 82-100%, specificity 86-100% 3
  • Epstein-Barr virus (EBV)
  • HHV-6 and HHV-7

• Based on epidemiology and exposure:

  • West Nile virus
  • Tick-borne encephalitis virus
  • Other arboviruses 1

Interpretation of PCR Results

Positive PCR results:

• Positive HSV, VZV, or enterovirus PCR is highly reliable and indicates active CNS infection 1
• Positive EBV PCR requires cautious interpretation as it may represent latently infected mononuclear cells rather than active CNS infection 1, 4

Negative PCR results:

• A negative PCR result does not definitively exclude viral infection 1
• False negatives may occur due to:
  • Testing too early or late in disease course
  • Low CSF viral load
  • Presence of PCR inhibitors like hemoglobin 1
  • Low CSF white cell count (<10 WBCs/mm³) 1

Important Caveats and Pitfalls

1. Timing matters: PCR sensitivity is highest between days 3-14 after symptom onset 4

2. Consider repeat testing: If clinical suspicion remains high despite initial negative results, repeat CSF PCR 3-7 days later 1

3. Normal CSF parameters don't exclude infection: Viral CNS infections can occur with normal cell counts and protein levels 4

4. Antibody testing as adjunct: For suspected cases where PCR was not performed acutely, CSF and serum antibody testing (10-14 days after onset) can be diagnostic 1

5. Virus-specific considerations:

   • EV-D68 is rarely detectable in CSF (respiratory samples preferred) 1
   • West Nile virus PCR is positive in only >60% of serologically confirmed cases 1
   • For flaviviruses, CSF IgM antibody testing is often more reliable than PCR 1

6. Multiplex PCR platforms can detect multiple viruses simultaneously, improving diagnostic yield but requiring careful interpretation of coinfections 5

By implementing appropriate PCR testing strategies for neuroviral infections, clinicians can achieve rapid diagnosis, initiate appropriate treatment promptly, and significantly improve patient outcomes.