I was told by my GI that approximately 5-10% of IBD cases cannot be classified as either Crohn's or UC. Apparently there is a blood test (last I knew done by a lab in California) that analyzes the DNA but even it can be inconclusive. The best determination is your symptoms although some people have components of both Crohn's and UC. Anyway, this is what my GI told me.
Dx UC 1979 at age 14 - (I know, some kind of record)
Was Mildly Chronic (flares every couple of years) until last 2 years now Chronically Acute
Can only get about a month symptom free between flares now
Asacol, Canasa, Rowasa, Cortifoam (on Sulfasalazine for 28 years previously also Pred)
GI Doc recently told me I have "lead pipe syndrome" which is advanced scarring of colon
No surgery - multiple colonoscopies reveal uc "from rectum to appendix"
My Best Friend is My Shih Tzu/My worst enemy is uc