The impact of non-indicated CSF testing on the true detection rate of viral encephalitis: A single-centre retrospective study

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Introduction: Timely and accurate diagnosis is required for viral encephalitis, which can result in devastating consequences. However, inappropriate ordering of cerebrospinal fluid (CSF) tests risks artificially lowering the documented positivity rates of viral encephalitis. Our study aimed to evaluate the ordering pattern of CSF viral testing and estimate the true incidence of herpes simplex virus (HSV) and varicella-zoster virus (VZV) infection in adult patients with clinical features of encephalitis.Methods: This study retrospectively reviewed adult CSF specimens collected at National Hospital, Galle, from December 16, 2023, to December 15, 2024. Based on diagnostic thresholds outlined by the British Infection Association (BIA) and the Association of British Neurologists, medical request profiles were split into two cohorts: clinically justified encephalitis and non-encephalitic cases. We compared HSV and VZV incidence between the two groups to assess the appropriateness of viral testing and its diagnostic yield.Results: Of 224 CSF specimens, 89 met the criteria for clinical encephalitis, while 135 were grouped as non-encephalitic. The incidence of HSV and VZV in the encephalitis group was 6.74%, whereas in the non-encephalitis group, it was 0%. Statistical analysis showed a higher diagnostic yield of viral infections in the clinical encephalitis group (p-value = 0.0035). Of the six positive samples, one was positive for HSV-1, and the remaining five were positive for VZV.Conclusion: This study highlights that CSF viral testing is indicated and yields higher diagnostic value in patients with encephalitis. Non-indicated testing may underestimate the true incidence of HSV and VZV viral encephalitis. Optimising CSF testing by establishing diagnostic criteria is essential for improving diagnostic stewardship.