Cross-reactivity describes when an antibody interacts with a compound different from its intended antigen. In drug screening, cross-reactivity happens when an immunoassay test’s antibody binds to an antigen different from the drug it’s meant to detect. Antibodies bind to substances known as antigens in a specific manner, similar to how matching puzzle pieces fit together or a key fitting into a lock.
Occasionally, an antibody may bind to a molecule that isn’t its exact target molecule. This phenomenon is termed cross reaction. Cross-reactivity emerges in drug testing when a sample contains a compound whose molecular structure resembles that of the antibody’s intended target antigen. Since immunoassay tests measure antibody-antigen binding interactions, any binding between the antibody and a similar molecule will distort the test results.
Common medications like antihistamines, antidepressants, and various prescription or non-prescription drugs can potentially interact with antibodies used in urine/oral fluid drug screening tests. Put simply, drug tests are designed to detect specific chemical markers within particular drug categories. Sometimes, this detection process reacts with different drugs containing similar chemical markers. This typically happens when an individual takes multiple medications or substances, hence the term cross reactivity between different drug categories.
Following AS/NZS 4760:2019 and AS/NZS 4308:2023 guidelines, when initial drug screening yields an unconfirmed result, secondary confirmation testing is conducted. This confirmation test does not use antibodies and is not susceptible to those caused by cross-reactivity. Amphetamines (AMP) and on occasions Tetrahydrocannabinol (THC) drug screening tests that can be influenced by and through cross reactivity.
Employers and individuals need to be aware that cross reactivity exists, as does the possibility that prescription medications &/or over the counter (OTC) could be the cause of an unconfirmed drug screening result. All unconfirmed drug screening results need to be followed by a Laboratory confirmation test, utilising the same testing matrix as the screening test via a NATA Accredited laboratory.
Looking for Confirmation Testing? Our Oral Fluid confirmation kit is available online as well as our urine confirmation kit.
Looking for Advice or Assistance? We can provide information and direct support to ensure all relevant Australian & New Zealand Standards are complied with throughout your entire testing process. Our team can be contacted on 1300 789 908 or register your contact details online and one of our team can get back to you.
