In most cases, a DNA test will give you an accurate answer as long as the test was conducted with thoroughness and accuracy.
However, it is also important to look at the symptoms someone is experiencing and pay attention to those as well. There is a small number of cases where there are symptoms and issues that constitute a "clinical diagnosis" of VHL, but we can't see an alteration in the gene that looks like it is the culprit. These take special handling in a center where there is a tight relationship with the research teams.
Please see vhl.org/dna for a list of the centers with higher "hit rates" and better integration with the research teams.
If you can please tell me more about the situation you are concerned about, I would be glad to help with it.
Joyce, wellness@vhl.org