There are no tests to date that are available to diagnose rabies infection in humans before the onset of clinical disease. However, rabies should be included in the differential diagnosis of all patients who present with unexplained, acute, progressive viral encephalitis, even in areas where the disease is rare.
The WHO defines a clinical case of rabies as a subject presenting with an acute neurological syndrome (i.e. encephalitis) dominated by forms of hyperactivity (i.e. furious rabies) or paralytic syndromes (i.e. dumb rabies) progressing towards coma and death, usually by cardiac or respiratory failure, typically within 7-10 days after the first sign, if no intensive care in instituted.
The diagnosis of rabies on clinical ground alone is difficult and often unreliable; therefore, it is recommended to confirm a clinical case of rabies through laboratory techniques (i.e. isolation of virus in cell culture).
For post mortem diagnosis, the gold-standard diagnostic technique is to detect rabies virus antigen in brain tissue by fluorescent antibody test.
More information on laboratory techniques in the diagnosis of rabies: