I have had idiopathic PN for about 10 years. I am 81. In the beginning I had the entire gamut of tests, including a positive skin biopsy. A few years later, my University neurologist contacted me to suggest a battery of new tests only offered by select institutions. They were all negative.
Personally, I don't really see the need for follow up. Unless there is a new treatment (unlikely in my lifetime) anything done will just be a confirmation of what I already know, perhaps with a little more information. Standard advice is to be seen periodically if the condition is progressing (mine is), particularly if a purely sensory condition is developing autonomic symptoms (I think mine is). Unfortunately, even with these new symptoms, there's still no treatment other than supportive (PT, devices like walkers, etc.). If these become necessary, then follow up is needed.
For me, it doesn't feel worth the time and energy, but opinions certainly vary. This is a particularly weird disease.