I don't have anti-saccharomyces cerevisiae antibodies (asca's). my doctor still suspects that i might have crohn's disease. can one have crohn's and no asca ab's?

Yes: Most of the pathologists i know think the "cerevisiae antibodies" are of little value in making the call, though the information may be useful down the road.

ASCA: Absolutely you would need to undergo a colonoscopy, endosocpy, and perhaps even a small bowel capsule endoscopy nothing is 100% in terms of diagnosis, especially the asca test.

Yes you can: The antibody tests are not 100% definitive. There are several other ways to look for crohn's disease such as colonoscopy, GI x-rays, etc. A careful physical exam is also sometimes revealing.