Situation: A patient being treated with high dose morphine who has hydromorphone in the urine as well as the expected morphine.
urine ratio of hydromorphone to morphine =
= (urine hydromorphone in ng/mL) / (urine morphine in ng/mL)
Interpretation:
• A ratio < 3% suggests endogenous metabolism of morphine to hydromorphone by a minor metabolic pathway.
• A ratio from 3 to 12% is harder to interpret. Reisfield et al felt that endogenous metabolism should have a ratio < 6%. On the other hand, McDonough encountered ratios up to 12% from apparent endogenous metabolism.
• A ratio above 12% cannot be explained by a minor metabolic pathway and suggests ingestion from an exogenous opiate source.
Additional issues:
(1) The situation of the patient needs to be taken into consideration. Some patients have no access to outside opiates. Other patients do.
(2) As pointed out in the previous section, an opiate formulation may be contaminated with other opiates. Analysis of the drug that the patient is receiving should be considered.
(3) Carefully review the patient's medications for another prescription for pain medications.
(4) If there are still unanswered questions, then diversion by a heathcare worker or family member should be considered.
