I am med student, i had hep b vaccine as a child 20 years ago now my ab titer is below 10, do i have to take a booster or start a whole new series ?

Up to you & doctor: Years ago, the recommendation was 1 booster shot. Now it is 1 booster series (3 shots). The idea is to raise the antibody level, and then it will come down over the years (and one can retest in the future). One shot won't raise the level as high as 3 shots will, meaning one may retest sooner after 1 shot than after 3 shots. It's not an exact science. Each person's antibody responses are different.