--on this board and others--so just to summarize:

Skin biopsy is repeatable, as it is relatively simple and non-invasive; it's the analysis of the nerve fibers, which involves special staining and electron microscopy, that is complicated (which is why it is done generally only at specialized medical centers). But serial skin biopsies from the same anatomical regions done a year or two apart can provide valuable information about worsening, or improving, small-fiber neuropathy.

The numbers for normal intraepidermal nerve fiber densities were established a few decades ago primarily at Johns Hopkins (the McArthur protocols) on samples of both "normal" and symptomatic" people, but there was wide variation in the numbers among both types and from across anatomical sites (primarily thigh above the knee, calf above the ankle, and upper arm above the elbow). At the time, it was (to me rather arbitrarily) decided to designate those scoring below the fifth percentile and those above the ninety-fifth percentile as having "definite evidence" of small fiber neuropathy. The problem with this in clinical practice is that since most people don't get a skin biopsy until symptomatic, it's hard to know how their results would compare with those they might have had pre-symptoms. For example, someone who might have been in the fourtieth percentile pre-symptoms might show up in the twentieth percentile upon symptom generated biopsy, but would still not technically be diagnosed with definite small fiber neuropathy (though, had this person had a pre-symptomatic baseline biopsy done, the comparative reduction would be suspicious). This is why the condition of the fibers--frayed, swollen, excessively branching--and the trends over time of the density numbers, might make serial biopsies valuable. But, yes, there are cost and insurance considerations.