There is a test that shows how much theraputic benefit you're getting from 6mp/imuran. If your liver enzymes are in the normal range and you're below the theraputic range, they will try raising the dose to get you into the theraputic range. The theraputic range is the range where most people respond to the medication. Are you in a flare up or stable? 50mg is a VERY low dose of imuran. I can't see that low of a dose proving any results.
when you get in touch with your GI ask what tests were run and why he wants to increase the dose. My GI is very open with these test resuts, she shows them to me & explains the meaning, you may want to askf or a copy if you're interested.