Drug Metabolism and its Importance in the Interpretation of Drug Tests in the Clinical Toxicology Laboratory

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Drug Metabolism and its Importance in the Interpretation of Drug Tests in the Clinical Toxicology Laboratory

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 due to the fact that 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 drug is reduced to about 5%, the concentration is considered to be negligible. The half-life can be influenced by many factors, 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 depends on many factors. Many physicians would like to be able to correlate drug concentrations with the amount consumed; however, due to a multitude of factors we have discussed, this is impossible.