What is the role of Gram staining and polymerase chain reaction (PCR) in the diagnosis of encephalitis?

Updated: Aug 07, 2018
  • Author: David S Howes, MD; Chief Editor: Barry E Brenner, MD, PhD, FACEP  more...
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The most important diagnostic test in the emergency department (ED) to rule out bacterial meningitis is prompt Gram staining and, if available, polymerase chain reaction (PCR) of the CSF in patients with suspected HSV encephalitis. PCR for HSV DNA is 100% specific and 75-98% sensitive within the first 25-45 hours. Types 1 and 2 cross-react, but no cross-reactivity with other herpes viruses occurs. Arguably, a series of quantitative PCRs documenting the decline of viral load with acyclovir treatment is strongly supportive of the diagnosis of HSV, and selected patients my avoid need for brain biopsy.

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