Sorry Dude, but you're busted.
This is true.
Most people are surprised to hear that the lab can tell the difference between hydrocodone and oxycodone, but they can.
The metabolism of each opioid leaves traces of metabolites and other chemical byproducts. By assaying the levels of these substances in the blood or urine, the lab can determine, with some degree of certainty, which particular medication you're using -- codeine, oxycodone, morphine, hydromorphone, etc.
However, these tests can be wrong because of differences in our individual ability to make specific enzymes for the metabolism of these medications. As a result of individual genetic differences in our ability to make specific enzymes, especially those liver enzymes known as CYP450, these lab results can be skewed. Sometimes a particular metabolite can be missing entirely.
This is why I hate to see people dismissed from a pain practice because they fail a particular urine test. There's too much faith placed in reliability of these tests and too often, ignorance in their application hurts chronic pain patients.
It's a tough time to have chronic pain, yet there are 116,000 million of us in America. We'd all benefit from getting rid of these tests (and controlled substance agreements) altogether.
The type of drug test I get would show the two drugs separately. It tests for quite a few different medications in my system, and those two would show different from one another.
While it is possible to test for the two different drugs independantly, in general the test is simply for opiates, and you would test "positive" regardless of which drug you were taking.