Cross-reactivity in Workplace Testing

Cross-reactivity occurs when an antibody interacts with a compound other than its intended target antigen. In drug screening, this means an immunoassay test may detect a substance with a similar molecular structure to the drug it’s designed to identify—leading to potentially misleading results.

Antibodies are designed to bind to specific substances (antigens), much like a key fits into a lock. However, sometimes they bind to molecules that closely resemble their target. This phenomenon—cross-reaction—can distort test outcomes, especially in urine and oral fluid drug screening.

Why Cross-Reactivity Matters

Common medications such as antihistamines, antidepressants, and various prescription or over-the-counter drugs can interfere with immunoassay tests. These tests are built to detect chemical markers within specific drug categories, but similar markers in other substances can trigger a reaction—especially when multiple medications are involved.

How National Standards Help

According to AS/NZS 4760:2019 (oral fluid) and AS/NZS 4308:2023 (urine), any unconfirmed screening result must be followed by a confirmation test. These tests are conducted in NATA-accredited laboratories using advanced techniques like GC-MS or LC-MS, which are not susceptible to cross-reactivity.

Drugs such as amphetamines (AMP) and occasionally tetrahydrocannabinol (THC) are known to be affected by cross-reactivity in screening tests, making confirmation testing essential for accurate results.

What You Need to Know

• • Cross-reactivity is real and can be caused by legitimate medications.
  • Unconfirmed screening results should always be followed by a confirmation test using the same sample type.
  • Our Oral Fluid confirmation kit and urine confirmation kit are available online.
  • Support and guidance are available to ensure compliance with National standards.

Further Information

For personalised support, contact our team at 1300 789 908 or register your details online and we’ll get back to you.