It's a gene change in the molecule MTHFR and it shouldn't be tested for in those who have had a clot because it's got nothing to do with clots. It doesn't increase the risk of thrombosis.

Removing MTHFR from standard thrombophilia testing will reduce the confusion, emotional upset, and misinformation that it currently causes in the patient group.

https://www.worldthrombosisday.org/news/post/wtd-publishes-new-resources-recommending-mthfr-should-be-removed-thrombophilia-testing-panels/