OK - Then I will give you general information about how most companies perform drug testing based on the industry standard - the drug testing done under the Department of Transportation rules.
The company collects (or has someone collect) the urine - it is then sent to a certified laboratory. Two different tests are done of the specimen. A screening test (sensitive, but may have some false positives) and if that is positive, a confirmation with GC/MS (a specific test - more expensive to run, but basically a "chemical fingerprint")
If both tests are positive - the "certified result" is then sent to a Medical Review Officer (MRO) - a physician trained and certified in the interpretation of drug test results.
The MRO contact the donor to review the results to determine if there is valid medical explanation of the results. Some legal prescription drugs
will be detected in drug tests (examples might include cough medicine with codeine, morphine given to someone in a hospital emergency room, Adderal for attention deficit disorder). If the doctor confirms appropriate medical use (based on the telephone interview
and any necessary verification from doctors, pharmacies etc) - then the drug test is reported to the employer as "negative"
If the positive test result is NOT adequately explained by a prescription product - then the test is reported to the employer as positive.