Glentaj is the person to ask about the skin biopsy. There are certain high and low percentiles that are considered abnormal. He knows much more than me about it, and would also better be able to tell you when you should do it again.
I do know on tests you can be trending towards abnormal but still be in normal range for that test. For example i was usually at 115% of expected for my age and height and weight, on a pulmonary function test in July 2001. I retook the test in October of 2001 and was at 80% of expected. 79% would be failing the test. The doctor told me that the test came back normal and when i looked at the numbers i said to him yeah but i dropped 35% in 3 months and he said yes but you are still normal.

For neurological tests for peripheral neuropathy, emg/ncs can come back negative even if you have large fiber nerve damage because a certain amount of damage must have occured for it to show up on the test.
The skin biopsy you took also of course has percentages.
I just wanted to let you know about the percentages thing so that you can look for yourself and better understand where you stand at the present time and be better able to compare it to where you are in the future.