Apologies for the recent sparcity of posts as I continue my futile efforts at finding a suitable apartment in Paris…
A recent paper in the Journal of Clinical Microbiology caught my interest regarding the use of rejection parameters for deciding whether or not to test CSF for Herpes Simplex Virus (HSV) by PCR, according to the white cell count and other clinical parameters.
Click here for the abstract. (Unfortunately the full article needs subscription to JCM)
More and more laboratories are implementing rejection criteria for testing HSV in CSF. The test is often requested before the cell counts and protein are known (ie on the initial request form), and often the CSF parameters are completely normal.
The researchers put forward a case for only testing HSV PCR if CSF samples had >10 cells/mm3 or if the sample was from an immunocompromised patient or a child aged <2 years.
The researcher’s paper looks convincing. However before we accept such criteria we need to reflect on how serious HSV infection of the CNS is, particularly HSV encephalitis. This is not a diagnosis where as a laboratory we can afford to miss a positive diagnosis. The consequences could potentially be catastrophic. See this article for some sort of an explanation.
Very very occasionally, as previous similar papers have demonstrated (a pseudo meta-analysis if you like..), HSV encephalitis does occur where the child is older than 2 years, doe not have “classical” immunocompromise and CSF parameters are plum normal.
So while we strive for laboratory efficiency (which I strongly support), we need to be very careful in our interpretation of such papers, maintain close co-operation between the clinician and the laboratory, and be prepared to be flexible in our testing protocols for individual patients.