Fistulas can certainly occur without Crohn's, particularly if they are associated with a surgical connection (e.g. your pouch anastamosis). OTOH, they are probably a better indicator of Crohn's than anything likely to be found on a biopsy. Rather than fussing over the diagnosis, which is likely to remain uncertain, you might want to focus on things that *might* clear up the fistula. If I developed a fistula I'd try the biologic, because the surgical treatment of fistulas isn't very good at all. Good luck!

Yes, if the fistula comes out of the anastomosis, likely from the surgical connection. If not, and you have a history of IBD, highly likely it's an IBD fistula, though there are no guarantees and can just be bad luck. 

I'm copying and pasting a previous reply of mine:

"So here's my two cents, if worth anything, again. 

I was the poster child for J pouch surgery for 20 years. I had, at the time, a clear cut case of UC pancolitis that eventually, at age 19, did not respond to the modalities of the day (ie. high dose IV steroids, bowel rest, and TPN). That was why surgery became a necessity. 

Flash forward to 2012. I began to feel poorly in some weird "UC light" way. Hard to explain, but I just in a GI sense began to feel "off."  Then 12/12/12 (the day of infamy) I found a lump right near my anus. Long story short, I developed a perianal fistula after an abscess. I continued to feel blah and poorly. 

My CRS recommended, after my seton placement, a GI who specializes only in IBD and who has a lot of J pouch patients. He has told me that "old" pouches are coming to him with "new" IBD type issues. Our pouch's mucosa slowly changes over time to more "colonic" tissue, and he said if he doesn't say biopsies are from J pouches, the pathologist will even read them as "colon."  Cellular mutations happen in there; it's all very weird. BUT. This conversion HE thinks is triggering our immune systems to refire, like you can take the colon out of the girl, you can't take away her genetics. Hence: new IBD issues are cropping up. 

Is it Crohn's?  Not necessarily. IBD is a HUGE umbrella of diseases, and as CT said, the "Crohn's vs UC" argument exists mainly only for the ease of insurance purposes these days.  Think about Crohn's, anyway. There is perianal Crohn's, ileal Crohn's, Crohn's colitis, Crohn's affecting other places: Crohn's patients rarely present the same way!  Yet we call it all "Crohn's disease."  Not necessarily the best way to classify such a varying type of illness, but we still do. 

Does he think *I* have Crohn's?  He's non-committal on that, but for what was necessary to get me approved for Humira. I did get + inflammation markers, and Humira has stopped the worst of the overall blahs, overall GI "discomfort," and low-grade fevers I was getting (didn't heal the fistula), so I'm on it til he scopes me in October, then we will make another plan...my pouch, apart from a few apthous ulcers scattered around it and in the neo-ileum has always looked pretty good. I did start to have some stricturing/narrowing issues at my outlet (another "Crohn's" thing, but also an issue of our surgery (though I never had that issue in two decades/until the fistula popped up). I now self dilate and use Canasa suppositories 3x/week. I've also had to resort to some steroid tapers, too, and things are great on them (except that they're steroids, and simply systemically terrible for you!)

All this from a girl who ate anything she wanted, and took zero meds, for over 20 years!  I'm now on Humira, periodic steroids and Cipro, Canasa suppositories, vitamin D, B12 injections, plus I've developed dry eyes (another IBD issue), and take meds for that. Argh!

My daughter also developed IBD. She presented as I did, we really were set to call it UC, but her scope showed "skip lesions" which = a more indeterminate dx, so her docs call her Indeterminate colitis. No other GI involvement but in her colon. She also has entropathic arthritis that started when she was five, six years before her first flare. She is on Humira, methotrexate, vitamin D, and folic acid. 

Remicade does indeed have a track record of closing IBD fistulas. We tried Cipro to see if the fistula would do better on a low-dose abx (I'm strongly intolerant of Flagyl), but it really didn't (though my GI often puts patients who respond/feel better with an abx on Flagyl or Cipro low-dose. The abx won't heal the fistula, but some find their drainage and pain decreases on it. However Remicade and Humira some of the time actually heal the fistula. That's partly why I tried Humira. I'm not even opposed to Remicade "down the line," but we aren't there yet. I'd say it's kind of your choice here. If the fistula is tolerable on the abx, and you're ok with things, that might be enough. But if you want the chance to try to cure the fistula, Remicade may be more tolerable to you. It's kind of just that: what you find acceptable in your life. And you can likely also take both meds, with monitoring, at first, then taper off the Flagyl and see what happens. It's kind of a crap shoot, I suppose."