--is much less invasive than nearly any other kind of biopsy--it is a skin punch biopsy.

Usually, 3mm wide, 1 mm deep circular skin samples are taken from the skin just above the ankle, at the thigh, and sometimes just above the elbow--these are the sites that the most work has been done it terms of standardization to "normals". Technically, though, this type of biopsy can be done anyplace there is hairy skin.

The samples are then stained and examined under an electron microscope to determine the intraepidermal nerve fiber density and the condition of the fibers themselves.

Many places can remove the samples--it's very simple and something almost any doctor can do--a small shot of lydocaine or other local pain killer, a quick sampling and collection, and you walk out in ten minutes with some band-aids. The analysis, though, tends only to be done at very specialized centers (Jack Miller, Columbia Presbyterian, Massachusetts General, etc.), so often the samples are sent there.

The procedure has the advantage of being repeatable at the same sites over time for comparison purposes--I've had it done three times, and it has documented slow but definite re-enervation.

See:

http://www.medscape.org/viewarticle/563262

This is an excellent overview, and the reference list has about as many other papers on the procedure as you'll likely ever need.