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Definition - What does Cross-Reactivity mean?

Cross-reactivity refers to a situation in which an antibody reacts to a substance other than its corresponding antigen. In drug testing, cross-reactivity occurs when the antibody used in an immunoassay test reacts to an antigen other than the targeted drug. Because the reaction of the antibody to the prohibited drug, or its analyte, is used to detect the presence of that drug in the test specimen, a cross reaction may result in a false positive finding.

SureHire explains Cross-Reactivity

Antibodies attached to other substances called antigens similarly to the way two puzzle pieces connect, or like a key fitting into a lock. Sometimes, an antibody can also connect to a substance that is not its exact match. This is called a cross reaction.

Cross-reactivity occurs during drug testing when a substance is present in a specimen that has a chemical structure similar to one that the antibody is designed to detect, its antigen. Because the immunoassay test measures the connection of antibodies to a specific corresponding antigen, any connection by the antibody to a similar compound will corrupt the results. Antihistamines, antidepressants, and several other prescription or over-the-counter drugs have the potential to react with the antibodies targeted in a urine drug test.

Under federal Department of Transportation guidelines, whenever a drug screening test results in a positive result, secondary testing is conducted using a different method. This confirmatory drug test does not use antibodies and is not susceptible to the problems caused cross-reactivity.

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