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Suspected herpes simplex encephalitis with an early negative CSF PCR — MRCEM SBA MCQ

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HardInfection and sepsisSuspected herpes simplex encephalitis with an early negative CSF PCRMRCEM SBA

A 46-year-old man is admitted after 18 hours of fever, new disorientation and a focal seizure. The seizure has stopped, but he remains confused. Lumbar puncture performed 18 hours after symptom onset shows 14 white cells/µL, predominantly lymphocytes, and a negative Gram stain. Intravenous aciclovir was started after the lumbar puncture. The initial CSF herpes simplex virus (HSV) PCR is negative. MRI obtained the following morning shows left temporal-lobe signal change without mass effect. He has no further seizures or new neurological signs. What is the most appropriate antiviral and diagnostic plan?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: D — Continue intravenous aciclovir and repeat CSF HSV PCR in 48 hours.

Fever with acute behavioural change and a focal seizure indicates encephalitis, while temporal-lobe MRI changes make HSV a particular concern. The first lumbar puncture was performed only 18 hours after symptoms began: a negative HSV PCR at this stage does not reliably exclude HSV encephalitis. Continue intravenous aciclovir and arrange repeat lumbar puncture for CSF HSV PCR after 48 hours, with inpatient specialist review. Current UK clinical guidance supports maintaining suspicion after an early negative result; a 2026 UK review specifies repeat lumbar puncture after 48 hours while aciclovir continues. ([dbth.nhs.uk](https://www.dbth.nhs.uk/wp-content/uploads/2025/11/CNS-infections-2025_FINAL.pdf)) A offers useful imaging reassessment if the diagnosis or clinical course warrants it, but MRI does not resolve the early negative PCR. B arranges the right repeat test but leaves suspected HSV encephalitis untreated meanwhile. C confuses this immediate diagnostic problem with reassessment near the end of treatment for *confirmed* HSV encephalitis. E is premature: intrathecal antibody testing may assist later in persistently PCR-negative cases, rather than replace repeat PCR this early. ([pn.bmj.com](https://pn.bmj.com/content/early/2026/04/29/pn-2024-004299?utm_source=openai))

Reference: Policy for Treatment of Central Nervous System Infections in Adults, section 3: Encephalitis (October 2025) — https://www.dbth.nhs.uk/wp-content/uploads/2025/11/CNS-infections-2025_FINAL.pdf Suspected encephalitis in adults (29 April 2026) — https://pn.bmj.com/content/early/2026/04/29/pn-2024-004299