Everyone reacts different to all meds
1) Opiates, opiates are naturally occurring opiate alkaloids found in the opium poppy, basically only morphine and codeine. Most other opiate alkaloids such as thebaine are not used medically and are not analgesics. However many, including thebaine are used to make other drugs like hydrocodone. A drug test for opiates will detect morphine and codeine.
2) Semi-synthetic opioid's- these drugs are NOT naturally occurring substances and are NOT opiates. Examples include hydrocodone, oxycodone, oxymorphone, hydromorphone, and diacetylmorphine (heroin). These drugs are "built" on opiates and have similar chemical structures. A drug test looking for "opiates" will NOT detect these drugs except diacetylmorphine because it almost instantly breaks down into morphine.
3) Synthetic opioids these drugs are completely man made and are not chemically related to opiates. Examples include methadone, fentanyl, dextropropoxyphene, tramadol, and pethidine (called meperidine in US). Synthetic opioids will NOT be detected in a test for opiates.
Edit:
In order to test for a synthetic or semi-synthetic opioid a specific test is required. For example to find oxycodone there would have to be a specific test for oxycodone and oxycodone alone. A basic drug test is a five panel test looking for PCP, THC (marijuana), amphetamines, cocaine, and OPIATES. Since oxycodone and fentanyl are not opiates they will not be detected and specific testing is required. That is why many people, especially health care workers, use synthetic drugs like Demerol (pethidine/meperidine) or fentanyl because they are less likely to be tested for.