I do know that I've read that their is a range that they want to see your blood counts for the immunosuppression to be "effective". I don't remember if it was eosinophils or lymphocytes, etc... I could be going crazy or maybe read it wrong a couple of months ago.
You are correct about them looking for it to be too low I see that everywhere I read about crohns and 6mp. I also know that there is a 6-TGN test that shows the level of the drug.