there are at least 30 genes affected under the heading of Charcot-Marie-Tooth disease. It is very complex. My best guess why more blood is needed is that it takes a lot of blood to test for many genes. People start with the more common ones that also best fit the clinical situation, but because the clinical situation can vary in people with Charcot-Marie-Tooth, additional genes might need to be tested. If the common ones are negative, then one looks a little further done the line.
This is a really good reference sites about the genetics of Charcot-Marie-Tooth “ghr.nlm.nih.gov/condition=charcotmarietoothdisease”.
Also, one wants to be really sure about a genetic test, so another reason to give more blood is to confirm something if there is any question about it. I would guess that the reason is the “still looking for the answer one”, but it also could be this.
Hang in there.
WithHope for a cure of these diseases