Cross-Reactivity in Immunoassay Drug Screening

A urine drug test utilizes immunoassay technology to measure the amount of a particular drug or drug class in a sample. Special antibodies bind to a drug by recognizing its distinct 3 dimensional shape. In simplest terms, each drug is a key that only fits into a suitably matched antibody lock. Each test has a predetermined cut off point. Concentration above this amount is presumptive positive and below it negative. When a drug binds to the antibody, the intensity of the color change corresponds to the concentration of the drug. Drugs with similar structures may also bind to the same antibody. This reaction is called cross reactivity immunoassay. Antibodies will bind with a target drug and make cross react with more than one drug. Sometimes, by design, it may bind with other drugs, as with assays that screen for a wide variety of structurally similar compounds. Other times, it is unexpected and undesirable. Amira, a hospital lab technician, knows that unexpected cross reactivity is commonly called a false positive. And when a substance is present below the cut off point, it's called a false negative. But there's more to it than that. Amira knows cross reactivity can impact the interpretation of a test, so she wants to understand it better. Today she has samples from 2 patients, Jerome in the pain clinic and Lily in the ER Jerome was prescribed a low dose of hydromorphone and his provider ordered an opiate screen before refilling the prescription. The result is negative, but Amira understands this doesn't necessarily mean Jerome is not taking his prescription appropriately. The opiate screen had a cut off of 300 nanograms per milliliter and a slightly less cross reactive with hydromorphone than other opiates. Follow up confirmatory testing detects a low concentration of the specific drug in Jerome's system. Jerome explains that he ran out of medication two days before the sample was taken. This delay lowered the urine concentration below the assay threshold. In the ER, Lilly presented with overdose symptoms. She tested positive for benzodiazepine and negative for the opiate screen. However, confirmatory testing after her release was positive for fentanyl but not benzodiazepine. So what triggered the benzodiazepine essay? The initial screening CROSS reacted strongly with something, so Amira orders an expanded confirmatory test to include new psychoactive and benzodiazepine analogs. The test detects flubromazolam, a highly potent derivative that can be detected days after use. By understanding how antibody based immunoassays work, CROSS reactivity can help Amira better interpret results. By choosing the right assay and confirmatory method for the right scenario and understanding the intrinsic properties of different assays, she can get the most out of the Siemens Health and Ears Immunoassay portfolio, supporting clinical decision making and better patient care.

Siemens Healthineers proudly presents Cross-Reactivity in Immunoassay Drug Screening Cut-off High control Assay Response Drug-free control Drug concentration Cross-reactivity Morphine (MRP) Oxycodone (OXY) Hydromorphone (HDM) Levofloxacin (LFX) False-positive False-negative 300 ng/mL EMERGENCY BENZODIAZEPINE BENZODIAZEPINE OPIOIDS Fentanyl Benzodiazepine Flubromazolam SIEMENS Healthineers