Genetic Results Negative

The genetic test for Tuberous Sclerosis only detects the defect on the chromosome 80% of the time, meaning 20% of the time one can get a negative result and still have the disorder. Lily's genetic test came back negative, but she has enough symptoms now to have a clinical diagnosis. What does this mean? I'm not sure. I'm still researching...some articles have said that this could mean that she doesn't have it, but by coincidence has two symptoms of it, or it could mean that she has a milder case. I hope that one of these conclusions is correct, but we have more research to do, and on some things only time will tell.