![]() “This is the gold standard for identification,” he said. ![]() Modern definitive tests use gas or liquid chromatography to separate analytes, followed by mass spectrometry to identify them based on their unique mass. “Definitive assays rule out false-positive results and identify true-positive drugs or drug metabolites,” Dr Gudin said. “False positives cannot be distinguished from true positive results in the presumptive test,” Dr Gudin noted, and the test cannot provide definitive identification of which drugs or drug metabolites are present.Ī definitive test uses different methodology and identifies the specific molecules in the sample. Ibuprofen and marijuana cross-react, for instance. This is because the antibody binding that is at the heart of the test depends on shape and charge, which may be shared by several different molecules in the sample. “Unfortunately, immunoassays will detect substances with similar characteristics, resulting in cross-reactivity leading to false-positive results,” Dr Gudin said. The benefit of a presumptive test is that it can screen a large number of analytes at once, potentially finding the proverbial needle in a haystack, and can do so quickly. Presumptive tests typically use an immunoassay, in which an antibody binds to the substance of interest and suggests drug positivity. “A presumptive test is often used as an initial drug screen and can be done either in the lab or on-site with a point-of-care test,” Dr Gudin said. They differ in the methodology, but even more important, in what information they can, and can’t, provide. The most important distinction in testing is between presumptive and definitive testing, according to Dr Gudin. In a recent podcast, Drs Jeff Gudin and Jack Kain outlined the key differences among different types of tests and provided advice on how to choose the one best suited to your needs. Understanding the terminology is critical for making the right testing choice for your patient. Presumptive, definitive, qualitative, quantitative, immunoassay, mass spectrometry, point-of-care: there is a lot of terminology to get right when it comes to drug testing.
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