Interpretation of Drug Tests in the Clinical Laboratory

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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Interpretation of Drug Tests in the Clinical 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 use, 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%, after four half-lives 6.25%, etc. When the concentration of the 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 in the following pages.
Some drugs have metabolites that can also be prescription drug formulations. Examples are:
  • Methamphetamine --> Amphetamine
  • Codeine --> Morphine metabolite
  • Hydrocodone --> Hydromorphone
  • Codeine --> Hydrocodone
  • Oxycodone --> Oxymorphone
Drug concentrations seen in confirmation testing depend on many factors. Many physicians would like to be able to correlate drug concentrations with the amount consumed. But due to a multitude of factors, this is impossible. While serum levels of drugs are related to dose, urine concentrations are not. This is because of the concentrating effect in urine and the fact that urine is not a hemodynamically-controlled fluid (its concentration depends on how much one drinks, which fluctuates greatly).