Thank you so very much for your responses clinilab. You have helped to set my mind at ease and it has enabled me to retreat and rethink my attitude towards this whole thing.
I am proud of myself, my accomplishments, my recovery. But, I agree, that not everything is for everybody and I do not want my employer to know that I am on MMT. They do know I am in recovery but, it has been an advantage to me in this situation. I work at an agency where many of the clients are in recovery. Upon hire, I was very staightforward as to why I decided to go into social services, my life experiences and recovery being some of the main reasons. They were thrilled that I had overcome addiction and felt that I would be a good example and would be able to give hope and inspiration to the clientele.
But, no client is admitted if they are on MMT, since the agency considers it "maintenance" and not "abstinence" (that is their policy). So, you can see why I am reluctant to have my personal information come to light... I detest the stigma attached to MMT. I am the same person who has been so highly praised by my employer, whether I am on MMT or not. I am still responsible, dependable and qualified.
I called my counselor this morning to make sure she has a letter ready for me tomorrow, as that is when our monthly appt. is scheduled for anyway - and that was before I read your response to my post!

I was thinking that same thing - that if I give the testing center my letter, stating that I am prescribed the drug, then how can they say that I came back positive for an illicit substance? And yes, they would be remiss to violate any HIPPA practices, and I highly doubt they will tell my employer. THANK YOU SO MUCH for clearing that up for me.
I will certainly walk into that center with my head held high and with the confidence that I have earned. I cannot thank you enough for your words....