--were originally developed at Johns Hopkins, where a lot of the original research into using skin biopsy to determine small-fiber neuropathy was done; the primary researchers (the lead researcher was Dr. McArthur, hence the "McArthur protocols") tested the intraepidermal nerve fiber density on hundreds of subjects of varied ages, some with neuropathy symptoms, most without,and came up with the numbers, and then (to me, rather arbitrarily) decided that figures below the 5th and above the 95th percentile would be considered evidence of definite neuropathy.

That doesn't mean that numbers between those might not represent neuropathy in people with symptoms though. I've written here and elsewhere on numerous occasions how since skin biopsy isn't normally done on asymptomatic people, there's generally no way to know what a symptomatic person's starting point for nerve density was. One can get a skin biopsy showing one is the twenty-fifth percentile and be considered "normal", but there's no way to know if that person was always around there or maybe started at a higher percentile and has been losing fibers due to a neuropathic process.

Fortunately skin biopsy is repeatable and one can have them done over the years--the comparative densities from the biopsy sites, the condition of the nerves as well as the enumeration, and the numerical trends of the time may all be significant clues.