Fentanyl is in the same class as Morphine, however, it is synthetic and does not metabolize like other opiates, ie Vicodin,
Morphine, Codeine. The urine test is testing for
metabolites, ie norhydrocodone, hydromorphone. It depends on the test that is being used and that the assay cutoff is for that particular test. The test for Fentanyl is specific to look for that drug and a normalized value can be detected usually 3-4 days after.