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

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Conclusion

Drugs that are metabolized primarily to glucuronic acid conjugates by UDP-glucuronyl transferase show negligible potential for drug-to-drug interactions caused by enzyme induction or inhibition.
Most laboratories do not test for the glucuronides. Instead, they convert them back to the free form of the drug by performing acid, alkaline, or enzyme hydrolysis on the urine specimen before extraction and analysis by GC/MS or LC/MS.
Drugs metabolized primarily by UDP-glucuronyl transferase:
  • THC-COOH
  • Morphine
  • Hydromorphone
  • Oxymorphone
Drugs that are metabolized primarily by CYP enzymes can produce unexpected and confusing results in drug tests. Patients taking medications or substances that interact with the drug that is being tested for by inhibiting the CYP enzymes involved, or patients that are poor metabolizers of the enzyme will have elevated levels of parent drug in their urine and lower levels of metabolites than normally seen. Conversely, patients taking medications or substances that interact with the drug that is being tested for by inducing the CYP enzymes involved, or patients that are rapid metabolizers of the enzyme will have elevated levels of metabolites in their urine and lower levels of parent drug than normally seen. Patients who have a genetic polymorphism in combination with taking an interacting drug can have urine drug test results that are very confusing. More importantly, these patients can experience very high toxicity.