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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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Some drugs are metabolized primarily to glucuronic acid conjugates and very little of the parent drug will be detected in the urine. 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 to glucuronic acid conjugates:
  • Morphine
  • Hydromorphone
  • Oxymorphone
The concentration of drug detected in urine depends on many factors, including the amount and frequency of use of a drug, the affinity of the drug for the enzyme, the rate of the enzyme reaction, the concentration of the enzyme, the susceptibility of the enzyme to induction or inhibition, body mass, age, liver and kidney health, overall health, genetics, age, diet, fluid intake, urine pH … and the list goes on. Due to the many factors involved, it is impossible to draw a correlation between the results of a urine drug test and the time the drug was taken, the dose, or the frequency of use.