A 28-year-old man from rural Maharashtra presents with fever, anxiety, and excessive salivation 10 days after a dog bite on his leg. He is now showing signs of hydrophobia and autonomic instability. Which investigation would be most appropriate to confirm rabies virus infection at this stage of illness?

Explanation

## Investigation of Choice for Rabies Confirmation ### Clinical Context The patient presents with classic rabies encephalitis (hydrophobia, autonomic instability, fever) 10 days post-exposure. At this symptomatic stage, rapid and sensitive diagnosis is critical for contact prophylaxis and supportive care decisions. ### Why RT-PCR on CSF is the Best Choice **Key Point:** RT-PCR on cerebrospinal fluid is the gold standard for antemortem diagnosis of rabies during the clinical phase of illness. It detects viral RNA with high sensitivity (80–95%) and specificity (99%). **High-Yield:** RT-PCR becomes positive early in the symptomatic phase and remains positive throughout the illness, making it ideal for confirmation when clinical signs are evident. ### Comparison of Diagnostic Methods | Investigation | Timing | Sensitivity | Specificity | Clinical Use | |---|---|---|---|---| | **RT-PCR (CSF)** | Symptomatic phase | 80–95% | 99% | **Antemortem confirmation** | | Direct fluorescent antibody (skin biopsy) | Symptomatic phase | 50–80% | 99% | Adjunctive; lower sensitivity | | Viral culture (saliva) | Symptomatic phase | 50–60% | 100% | Slow; requires 2–3 weeks | | Serum neutralizing antibodies | Late/post-exposure | 70–90% | 95% | Useful for post-exposure prophylaxis assessment, not acute diagnosis | **Clinical Pearl:** In a symptomatic patient, CSF is the preferred specimen for RT-PCR because viral load is highest in the CNS during encephalitis. Saliva culture is slow and insensitive in the acute phase. **Warning:** Serum antibodies may not be detectable in the first 7–10 days of illness, especially in unvaccinated individuals, making them unreliable for early diagnosis. ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion of rabies<br/>Post-exposure + neurological signs]:::outcome A --> B{Symptomatic phase?}:::decision B -->|Yes| C[RT-PCR on CSF]:::action B -->|No| D[Serum neutralizing antibodies<br/>+ skin biopsy DFA]:::action C --> E[Rapid confirmation<br/>High sensitivity & specificity]:::outcome D --> F[Supportive evidence<br/>Lower sensitivity early]:::outcome ``` [cite:Harrison 21e Ch 196]