My patient is prescribed an opioid drug (hydromorphone) but it was not detected by urine drug screen for opiates. Does this mean he/she is not taking the medication?

This could be one of the answers, but there are other possibilities.  For example the opiate drug screen test has poor cross-reactivity with this drug and so may not detect it.  Pre-analytical error such as sample misidentification is another possibility.  Contact one our Medical/Scientific staff for further assistance.