In the first 72 hours of symptoms, cerebrospinal fluid HSV PCR can be negative in a patient who genuinely has herpes encephalitis — the viral load in CSF may not yet be above the assay's threshold, and a very early tap can also be bland on cell count and protein.
So when the clinical picture fits — fever, confusion, dysphasia, focal seizures, temporal changes on imaging or focal slowing on EEG — a negative PCR taken early is not a reason to stop aciclovir. Continue it and repeat the lumbar puncture at three to seven days. The second sample is the one that settles it.
The converse error is quieter and just as common: the patient improves, the repeat tap is never done, and aciclovir is stopped at an arbitrary point. Fix the treatment duration to the diagnosis you end up with, not to the day the ward round happened to review it.
- Do not stop aciclovir on a single negative PCR taken within 72 hours of symptom onset.
- Repeat the lumbar puncture at day 3 to 7 if the clinical suspicion persists.
- A normal early cell count does not exclude encephalitis either.
- Send the repeat sample to the same laboratory where you can — assay thresholds differ.
- Decide the aciclovir end date from the final diagnosis, not from clinical improvement alone.
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