Hello, sub-therapeutic means that you need more medication for the optimal result. The most potent drugs regarding its side effects (like imuran) are studied to define the lowest possible dose that provides acceptable results. When the drug concentration in your blood is lower than the desired one (sub-therapeutic) then the drug s efficacy is reduced. This doesn t mean that the drug has no effect but the effect is not the best for you. Even a lower level may provide benefit (you re also on prednisone which can sometimes be used as the sole regimen for the treatment of autoimmune hepatitis) but a higher dose would have provided better results. I hope I ve answered your question. If you have further questions on the subject, I ll be glad to answer. And if you found the answer helpful, please indicate so while rating it. Kind Regards!