I Take Methadone 60mg 3x A Day An Roxicodone 30

I take methadone 60mg 3x a day an roxicodone 30 to 60mg 3x a day i have RSD, diabtic neuropathy an i have had 4 negatives for the roxicodone. the urine screen shows the methadone but not the roxicodone. so he said im not taking the meds an have to be selling them. i told him i take them the way i should escept on the days i have to drive to his office an then i dont till i get home. i asked him what else would cause a fals postive an he said nothing. however, i have been told the body size, hydration can cause the levels be so low it doesnt show up. I have a dual filtering system in my right kidney if i dont drink alot of liquid my kidney hurts. so i drink about 6 to 7 liter of water, 2 cups coffee an about 3 cans of diet pepsi a day. I weigh about 195 an 60 years old
I believe my tests is negative cause they arent looking for it. my research says the normal test they do is for morphine an if they dont look for roxicodone excatly it wont show up. Also cause i drink alot during the day always have. he said he wasnt going to drop me as a patient yet cause ive seen him since 1996 an i will have random drug screens. I said that was fine cause i am taking my meds. But i am confused why would it show for methadone an not the other. I alsoread it depends on the labs values. 1 lab level may show anything above 1000ng/ml where another lab may go down to 100ng/ml. im confused