The gist is that they found a significantly higher false positive rate - probably between 2.4x higher and 4.8x higher - and in absolute terms, a meaningful FP rate (15% without or 25% with other abnormalities):
> Among 3946 participants, 704 (17.8%) showed positive FIT results and 1303 (33.0%) had hemorrhoids. Of the 704 participants with positive FIT results, 165 had advanced colorectal neoplasia (ACRN) and 539 had no ACRN (FP results). Of the 1303 participants with hemorrhoids, 291 showed FP results, of whom 81 showed FP results because of hemorrhoids only. Participants with hemorrhoids had a higher rate of FP results than those without hemorrhoids (291/1176, 24.7% vs. 248/2361, 10.5%; p<0.001). Additionally, the participants with hemorrhoids as the only abnormality had a higher rate of FP results than those experiencing no such abnormalities (81/531, 15.3% vs. 38/1173, 3.2%; p<0.001).
Also, everything is different for a patient than a population. If a patient has reasonable expectations (ie, that a positive result could easily be false, and is nowhere near conclusive), and they would otherwise obtain a colonoscopy rather than putting it off, then by far the bigger concern is false negatives. That is, if you can take a FIT (or two a few months apart) and skip a colonoscopy based on negative results, the false positive rate doesn't matter much - you're replacing something annoying with something not at all annoying. I couldn't find any good studies of false negatives.
After reading this stuff, the one thing I wouldn't do is completely replace colonoscopies in favor of FIT.
Just as I thought: A test that bases its effectiveness on occult fecal blood will obviously stumble around the very common phenomenon of hemorrhoids, and only a colonoscopy can clarify better. On the other hand, if no blood is detected, there still exists the possibility of polyps or tumor growth which have not recently or yet started to release blood, making this test more than a bit unreliable as a measure of something that could be life or death.
