Elaine,
I am not at therapeutic level according to the Prometheus Test either and I take 50 mg also. Since I take 50 mg, my blood work is okay (blood count just slightly down, liver is okay) and one would think that 6MP is not working, BUT ... it is. I just had a flare that lasted 2 weeks and the only symptoms were colon cramps, more BMs, a bit loose BMS and nausea and that is gone. My flares usually last at least 3 months. So there is hope. Curious what your new doctor will say, but I ain't quit taking 6MP. It is doing something - probably just "cushioning" the extent of flares, but I am happy with that - so be it.
lenagirl is right. 6MP is basically the cleaner version of Imuran meaning less by-products (for a lack of better description). My doc also said that she will put me on Imuran should I ever have problems with 6MP. People who cannot tolerate 6MP or need higher doses usually fare better on Imuran.
Good luck!