I’m not sure if this will answer your inquiry re testing for viability of nerves, presumably for PN in the feet.

I had a ‘perfusion study’ where neurologist uses an electric pin pricking device across the bottom of the feet.
While it’s not comfortable it’s tolerable. I would do it again without reservation.

I would not guess that there’s a systemic biopsy because of the branching and localization issues, e.g. nerve problems in feet may not affect the hands, head, eyes, et al.🤷🏼