Don't go there @Unicornkisses (unless you have to) The test, from memory, doesn't test for cancer cells as such, but looks at proteins that the body can produce when cancer is present. Or that some cancers can produce. Something like that. The catch is, the production of those proteins can be triggered by other conditions. Someone with better science may be able to give a better explanation. Better still, ask your oncologist.
Back in the day when I first became aware of it it was a very popular marker test for men who had advanced prostate cancer and were trialing novel therapies. From there it seemed to be used as an indication that someone was checking on your cancer status. Just waive a bit of science--appropriate or not--get a guy with a coat involved, that should be reassuring. No?
My first oncologist, the Arsehole, used to put this little pause in before they'd tell me what the readings were. Imagine a reality TV host milking the reveal for all they can get...."And the winner is.......We'll be back right after this message from our sponsors" Put me right off, it did.