A DOA screen can be done quickly using an immunoassay method. Immunoassays use antibodies directed against specific prototype chemical structures associated with specific drugs. They are ideal for screening since they can often pick up several different drugs within the same class. For example, an immunoassay screen for benzodiazepines will likely pick up diazepam, oxazepam, lorazapem, etc. All of these are benzodiazepines and so it is expected that the immunoassay will be positive in the presence of any of them.
In general, screening tests like DOA immunoassays have adequate sensitivity but are not usually highly specific for a given drug. The low specificity of DOA immunoassays, which is helpful for detecting the presence of any drug within the same class, is not helpful when the screen is being used to detect drugs used for pain management. An immunoassay can tell you that an opiate is present but it cannot tell you which opiate is present. In pain management, it is not enough to know simply that a class of drugs was detected. Rather, we need to know specifically which pain drugs are present (ie, is it morphine, hydrocodone, etc.?)