Antibody Structure & Immunoglobulins — Practice

A 40-year-old man bitten on the hand by a stray dog 6 weeks ago now has hydrophobia, aerophobia and hypersalivation. A skin biopsy from the nape of the neck is sent for direct fluorescent antibody testing, and brain histology in fatal cases would show eosinophilic cytoplasmic inclusions in hippocampal neurons. What is the most reliable antemortem confirmatory test in this patient?