What diagnostic test confirms herpes simplex encephalitis?

• A. PCR for HSV DNA in CSF
• B. EEG with periodic activity
• C. Serum HSV IgM
• D. CT head without contrast

Answer: (B)

Detecting HSV DNA in cerebrospinal fluid with PCR is the test that confirms herpes simplex encephalitis. This method directly identifies the viral genetic material in the CNS and is both highly sensitive and specific, making it the gold standard for confirmation.

Electrical activity studies like EEG can support the diagnosis and often show characteristic patterns such as periodic lateralized epileptiform discharges in the temporal regions, but these findings are not specific to HSV encephalitis and do not prove infection. Imaging with CT without contrast is not diagnostic for HSV encephalitis, especially early on, and may be normal. Serum HSV IgM is not reliably useful for confirming CNS HSV infection due to limited sensitivity and specificity.

In practice, if suspicion is high, start empiric IV acyclovir promptly while awaiting CSF PCR results, since treatment is urgent and PCR provides definitive confirmation.