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The Problem with Oxycodone and Oxymorphone (Oxys) In Immunoassay Methods

A typical drugs-of-abuse (DOA) screen contains the following tests:
  • Amphetamines
  • THC
  • Barbiturates
  • Benzodiazepines
  • Methadone
  • Cocaine
  • Opiates
  • Oxycodone/Oxymorphone (oxys)
Notice that methadone, opiates, and oxycodone are all individually tested. This may seem strange since these are all opioids/opiates. But the fact is, there are modifications to the chemical structures of opiates that will make them undetectable to immunoassay methods that recognize the general prototype structure of morphine. As a result, it takes three immunoassays to detect the three common opiate drugs/classes of methadone, opiates, and oxcodone/oxymorphone (oxys). In general we can think of opiates and opioid screens in this way:

Drug Screen
Morphine, codeine, hydrocodone, hydromorphone
Oxycodone, oxymorphone

If a clinician fails to recognize that different immunoassays are needed to screen for different opioids, confusion will result. It is not uncommon for clinicians to misinterpret screens and accuse patients of not taking their medications when in fact the patient is positive for the medication but the wrong screen was used. Clinicians may assume that any drug, which moderately resembles an opiate in its action, will be detectable using an opiate drug screen. This is not true. For example, one of the most commonly prescribed drugs in the US, oxycodone, will not typically be detected on an opiate screen but instead requires a specific "OXY screen."
It may be the case that a regular opiate immunoassay screen will pick up oxycodone, yet the OXY screen will usually not detect regular opiates. For example, a patient taking morphine should be positive for opiates, but will likely be negative for oxys. Yet a patient taking oxymorphone may be positive for oxys and positive on the opiates immunoassay screen as well. Note that oxycodone and oxymorphone can produce a positive opiate screen as well as a positive OXY screen. However, codeine, morphine, hydrocodone, and hydromorphone will typically not produce a positive OXY screen on most immunoassay instruments. For these reasons it's critical that you know the performance of your laboratory's assays. The toxicology technologist must be able to reference the laboratory vendor's cross-reactivities information to know what to expect.