In some cases, people may suffer from the symptoms of celiac disease yet have all tests come back negative.
Since blood testing only works if you are actively consuming gluten at the time, it's possible that some people can have celiac disease and test negative if they already stopped eating gluten for a long enough period prior to testing.
In any case, NCWS is the term used to describe people who experience celiac symptoms, see improvement when they go on a gluten-free diet, and yet have tested negative for the disease. Intestinal damage for these individuals is still a danger, according to the Celiac Disease Foundation.
It's important to be aware that other factors could explain why cutting out gluten may result in feeling better. You may be eating more fruits and vegetables where before you filled up on processed carbs, for example.
For some people at risk of eating disorders, eating gluten-free, like any restrictive eating pattern, can conceal what's really an unhealthy behavior.