Hello Racknstack,
Blood serum tests are more sensitive, thus more accurate than urine testing.
I have not experienced the false positive issue as a patient, but as an RN, I'd suggest that a patient's history, length of time the MD is familiar with the patient's situation, if the test in question is the first positive test, would all be factors that may be considered.
Another factor is if the patient gave an honest verbal profile-when the tech or medical assistant asks what meds and at what time the patient has taken his/ her prescribed and OTC meds.
It is imperative to inform pain management MDs or their RNs of any new Rx that one is taking: some antibiotics, cold medicines, and other substances can alter urine drug screens. If this situation occurs, it's a good idea to bring the Rx bottles in to the MD appt, although in most states, but not all, all of the patient's prescription meds can be seen by all of his/ her treating physicians.
Some pain management contracts do not allow for a second chance, false positive or not. In these cases, it would be standard that the specimen is tested with a more sensitive and less ambiguous type of testing.
The ultimate determination is individualized to the MD, contract, history, and whether the specimen is sent for more sensitive testing.
There is a person who posts her upon occasion who is very knowledgeable about drug screen evaluation.
Good luck.
CTB