What is the difference between mild, moderate and severe UC?

I suppose it's the amount of inflammation/ulceration. The amount of intestine affected might be included but you can have "mild pancolitis" or "severe proctitis" so I don't know. It was always the biopsies that came back and told the doctor whether I had moderate or severe disease. Here is what I found in regards to the criteria:

Mild = less than 4 stools a day with/without blood, normal sed rate, no signs of toxicity
Moderate = greater than or equal to 4 stools a day, minimal signs of toxicity
Severe = greater than 6 bloody stools/diarrhea, with fever, tachycardia, anemia, or elevated sed rate
Fulminant = more than 10 stools/diarrhea, continuous bleeding, toxicity, abdominal tenderness/distension, transfusion requirement, colonic dilation on x-ray

There's likely a finer line between mild and moderate compared to either and severe, I think it goes by level of inflammation when measured more so than symptoms in the eyes of doctors at least.

:)

I actually asked my doctor this question and he told me it all depends on what they see in the colonoscopy. My doctor showed me the pictures of my colon, and it was not pretty. It was very very red, you could see the bleeding and the ulcers. Because everyone reacts different to the actual colitis it can only be determined how bad your colitis is by the photos. For example some people may have minimal bleeding and redness, but may go to the bathroom 15X a day, while others could have severe UC and their symptoms may not be so extreme.
I hope that helps. It helped me understand it a little more :)

enjoyingnow