In general, the concentration of the parent drug in urine exceeds that of its metabolite(s). For this reason, it is ideal to target the parent drug rather than metabolites in drug testing. However, this is not always possible because some drugs are rapidly metabolized. For example, heroin and cocaine can only be detected in urine for a few hours after consumption, but their metabolites can be detected for several days.
It takes approximately six half-lives to eliminate 99% of any drug. The half-life of a drug is the time it takes for 50% of the amount of drug consumed to be metabolized and eliminated. After two half-lives, the amount of drug remaining in the body is 25%. After three half-lives, 12.5% of the drug remains; after four half-lives, it is 6.25%, etc. When the concentration of a drug is reduced to about 5%, the concentration is considered negligible. Many factors can influence the half-life, as we will discuss.
Some drugs have metabolites that can also be prescription drug formulations. Examples are:
- Methamphetamine --> Amphetamine metabolite - Amphetamine (Adderall)
- Codeine --> Morphine metabolite - Morphine (MS Contin)
- Hydrocodone --> Hydromorphone metabolite - Hydromorphone (Dilaudid)
- Codeine --> Hydrocodone metabolite - Hydrocodone (Vicodin)
- Oxycodone --> Oxymorphone metabolite - Oxymorphone (Numorphan)
Drug concentrations seen in confirmation testing depend on many factors. Many physicians would like to correlate urine drug concentrations with the amount consumed; however, this is impossible due to the many factors we have discussed.
The remaining content in this course will concern the interpretation of urine drug testing, with a view toward how drugs of abuse are metabolized.
