The link you give is for colon cancer blood tests. Colonoscopies themself are basically definitive, because if they see something suspicious they will snip it and send it to the lab for biopsy. I had one such "suspicious polyp" and it turned out it wasn't a polyp at all, just normal intestinal tissue they mistook. There's your false positive. But the risks and long-term consequences are basically nil, the intestine heals itself in a week or two.

It's a very unpleasant procedure (the prep, not the procedure itself), but it's also very low risk and effective at catching colon cancer cases early when they can still be treated.