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Every vendor will disclose a list of drugs that can cross-react with their immunoassay. This information is essential for proper interpretation of immunoassay results. In the immunoassay example on the right, morphine is defined as the standard (100%). Notice that some drugs, such as codeine, are detected better than morphine with this assay. Yet some drugs, like oxycodone, oxymorphone, and meperidine, cross-react less than15%. That is, these drugs will not be detected with sufficient sensitivity using this opiate screen.
Laboratory personnel need to educate clinicians in how to use the laboratory's drug screen. Laboratories may note in the patient results which drugs of the same class are NOT detected using their particular immunoassay, or may supply an interpretation table so that clinicians are aware that one assay cannot detect all drugs of a given class.