Diagnosis of medical illness often starts with a thorough history, and then a physical examination to suggest possible diagnoses. This is often followed by testing to help sort through the possibilities. No test is 100% accurate, but test results can provide independent objective evidence about whether a particular diagnosis is or is not true.
Three tests are recommended by the American Academy of Neurology or the European Academy of Neurology: skin biopsy, autonomic function testing, and nerve ultrasound. In addition, a diagnostic workup may include blood tests or electromyography and nerve conduction studies.
Reference:
Practice Parameter: Evaluation of distal symmetric polyneuropathy: role of autonomic testing, nerve biopsy, and skin biopsy (an evidence-based review).
England, JD, Gronseth, GS, Franklin, G, et al.
Neurology. 2009;72:177-184. Abstract.