Good question.
You can't code microscopic colitis as a dx for this patient unless you see it on the path report, or if the doctor states the patient has been diagnosed with microscopic colitis. I don't get that from reading what you said about the op report. The "due to evaluation of microscopic colitis", that's a little bit confusing. It could mean they want to evaluate for microscopic colitis. The way it reads now, I wouldn't code microscopic colitis from this sentence assuming it is being done for evaluation purposes. You don't have that definitive diagnosis yet.
I read something like this frequently in op reports and it usually reads "to evaluate FOR microscopic colitis". And since that is a rule - out scenario, I code the symptoms that are given. If diarrhea is given , then I would just code the diarrhea. Or whatever signs/symptoms/indications you have on the op note. History of polyps and diarrhea, for example, if that is what is on your op note.
IF you have a pathology report and IT states microscopic colitis, then you can code that.
Hope this helps.