Knowing you have the mutation doesn't help to tell if you will test positive for celiac disease. 98-99% of all people with celiac disease have a mutation in an associated gene. The thing is that a mutation in this genes is really commen. 30 to 35 % of the population has a mutation. That means that roughly one of three people has a mutation. Only 2 % of the 30 to 35% will become celiac. Which means the majority with the mutation will never have celiac. An example to make it clear: If you have a 1000 (non celiac) people, 300 to 350 will have a mutation. And only 6 - 7 will become celiac. That means testing for the gene doesn't mean a lot. It can only be used to rule out a celiac diagnosis, not to comfirm.