Oxycodone Metabolism
- Oxycodone is metabolized to oxymorphone by conversion of a methyl group (-CH3) to a hydroxyl group (-OH) mediated by CYP2D6.
- Oxymorphone is further metabolized by glucuronidation to oxymorphone -3-glucuronide and noroxymorphone by CYP3A4.
- Oxycodone is also metabolized to noroxycodone by demethylation by CYP3A4.
- Noroxycodone is further metabolized to noroxymorphone by CYP2D6.
The half-life of oxycodone is 2 to 6 hours and can be detected in the urine for 2 to 4 days after ingestion.
Interpretation:
- Both oxycodone and oxymorphone can be detected in patients taking oxycodone.
- The ratio of oxycodone to oxymorphone is generally greater than 5 to 1.
- Oxymorphone can be detected at higher concentrations than oxycodone when oxycodone is taken alone if it has been several days since ingestion.
- Oxymorphone has a significantly longer half-life than oxycodone. Therefore, as time increases from ingestion of the drug, the ratio of parent to metabolite will get lower and lower until eventually none of the parent remains in the urine and only a small concentration of metabolite remains. For this, reason, oxymorphone alone may be detected at low concentrations if it has been several days since ingestion.
Since the cross-reactivity of oxycodone and its metabolites in opiate immunoassays is very low, laboratories must employ the use of a separate oxycodone screen in addition to the opiate screen.
