If ca19-9 within normal levels or negative, are you cancer free? is there a test to show that you are in the % group that can't produce the antigen?

Misconception: Only a handful of cancers produce ca19-9, and even then it's won't be detectable at a significant level unless there's enough mass of the cancer to produce it. This is not a screening test. If levels drop to undetectable after surgery, it's no guarantee of a cure. It's most useful to follow and watch for recurrences of a known ca19-9 producing cancer.