Hi, sunandocean. T-cells are a subtype of lymphocytes and when they are high in number it can be useful to determine *why* they are high. One reason is that they are benign (not-cancer) and the cells are 'reactive'. They are multiplying because there is some infection to fight, or there is even some autoimmunity going on. That would be the most common cause. The second cause is less likely, and that is a cancer of the T-cells which makes them endlessly multiply -- each one being the same as all the others.
The T-cell Gene Rearrangement test looks to see whether your excess T-cells are mostly all the same. If they are not mostly all the same, the terms are polyclonal or heterogeneous -- that is good. But if instead they are mostly all the same, that could be cancer, or else it could still be from an infection or some autoimmunity -- the terms then are clonal or homogeneous.
So what you want is for the test to say 'heterogeneous' T-cells. If not, it might still be not-cancer. Going from the rest of your post, e.g. that your numbers have been high since 2015 & your flow cytometry was normal, it is very probably not-cancer. I'd look at your upcoming test as a way to set your mind at ease.
After the normal flow cytometry, I suppose we could say that the doc is further being doubly sure that it is not-cancer.