--with skin biopsies is to take 3mm diameter punches from the outside of the thigh and from the lower outside leg just above the ankle. This is primarily to distinguish length dependency; in many (but not all) neuropathies, including those that are predominantly small fiber, the nerve fibers farther from the center of circulation show damage first (know as "die back") and seeing this, or not seeing it, may lead to the suspicion of various etiologies.

Sometimes especially forward-thinking facilities will take a sample from the arm right above the elbow as well.

The problem is that symptoms of small-fiber neuropathy can be almost exactly mimicked by those caused due to spinal nerve root compression or spinal cord compression itself. And even if the cause of symptoms is peripheral there can be many causes leading to similar symptoms. Investigation into neural symptom cause if often very long and very process-of-elimination (and very expensive), but it should also include nerve conduction studies and spinal MRI's along with blood tests and skin biopsies.

Take a look at the LizaJane spreadsheets just to get a sense of how extensive this testing can get--these were put together to try and encompass anything that could be thought of that would lead to neural symptoms:

www.lizajane.org

Certainly, rheumatoid arthritis or other autoimmune diseases can lead to neuropathy, and can be a presenting symptom--you can see from the spreadsheets there are a LOT of autoimmune conditions that have neuropathy as a component that can be tested for.

Glucose tolerance testing can also not be definitive if it is done for too short a time and without frequent draws. The best protocol for that, I think, is an elongated one with baseline glucose and insulin level draws, and then draws a half-hour, one hour, 1.5 hours, 2 hours, 2.5 hours, 3, 4 and 5 hours to measure the patterns of glucose and insulin rises/falls--specifically, this can help to flag insulin/resistance or impaired tolerance short of frank diabetes, which can itself be a cause of neuropathy, particularly burning small-fiber types. There's plenty of evidence now this can happen before one would be diagnosed with frank diabetes.