CD40LG and Flavivirus Infections: JEV-specific MAC-ELISA is more sensitive than JEV RNA detection, due to the low-level, transient viremia that is generally cleared by the onset of illness, and the rapid elicitation of JEV-specific IgM early in acutely ill patients.4–6 CSF is the preferred specimen for JE diagnosis, as IgM in serum elicited against other flavivirus infections does not cross into the CSF.4,6 Also, in cases of neuroinvasive disease, antibodies in serum may not be the cause of the neurological disease.