Immunoassays can be better utilized if the user knows the assay’s limitations while conducting the procedures. Every assay has its respective strengths and weaknesses that should be assessed by the user while choosing the best type of assay for a specific program. As far as workplace drug-of-abuse testing programs are concerned, the biggest strength is the effectivity of the assay as a basic screen – being simple, easily automated, inexpensive and having cross reactivities that are detectable for the basic compounds. However, the assays can have limited dynamic range and can be prone to interference. The results, therefore, should be interpreted in the context of limitations.
Any factor that causes a bias in the assay’s result except the presence of other target compounds is known as ‘Interference’. The interference caused in assays can be dependent of the analyte or even independent on it. This can increase or decrease the measurement of the result. These interferences can be caused by similarly structured or dissimilar, unrelated compounds or adulterants which are found in the specimen and are able to change the effects or the performance of the test system. Since different assays are available, they are all used for different purposes. They have varying formulations, specific antibodies, different affinities and use different labels to detect compounds using unique mechanisms. These interferences should be kept specific to the assay type or the dealer. It is also quite important to make sure that the reagent changes, formulation changes and variation in the specificity of the antibody can change the cross reactivities and the interferences that an assay causes. Many manufacturers’ give updated information, dealer communication methods and cross reactivity guidelines while giving out the drug test kits. This facilitates the user and helps them to better understand the requirements of the procedure.
The reagent manufacturers are able to perform widespread cross reaction studies of the structurally unrelated and related compounds while product is being developed and also include cross reactivity tables in their package kits and dealer’s websites. This, however, doesn’t include the latest update on new compounds and chemical, so some metabolites can have unidentified reactions in the assay. It is even essential that the concentrations given in the manufacturer’s studies are evaluated and reviewed, as in few cases, urine concentrations which are achieved from therapeutic or supra therpeutic drugs may exceed the tested concentrations. (Eg: Amphetamine and ephedrine assays).