Why are POCT and lab cutoff levels different?

By Work Place | Last updated: January 16, 2019

The short answer is that they are not different. The difference comes in when we look at the different testing methods used, specifically immunoassay screening testing (both in-lab screening test and POCT are examples of this) and confirmation testing (GC/MS or LC/MS/MS).

A screening test is a test done via immunoassay method. There are two ways to do this. First, with a POCT test, which uses a lateral flow colloidal gold method. Secondly, at a lab using a chemistry analyzer that uses a liquid immunoassay process. The major differences between the two processes, with respect to the results, are that POCT only gives a positive/negative, where a chemistry analyzer can give a quantitative result. Either result requires confirmation using a different method. That is where GC/MS (Gas Chromatography Mass Spectrometry) comes in.

Because the screening and confirmation tests are completely different scientific methods, the cutoff concentrations are different. Immunoassay testing is using a chemical reaction looking for common binding sites of a drug, where the GC/MS is looking for a specific molecule.

To illustrate and explain this better, lets look at marijuana. The parent drug of marijuana is THC, the psychoactive constituent is trans-delta9-tetrahydrocannabinol.

Why are POCT and lab cutoff levels different?

When it is broken down in the body, it breaks down into more than 100 different metabolites. Of those 100+ metabolites, more than 5 metabolites (varied from “more than 5” to 30 metabolites in the literature) are present in the urine. The most common of those metabolites in urine is carboxy-THC (THCC); this metabolite accounts for ~30% of all of the metabolites in urine.

During the immunoassay test, the common binding (antigen/antibody) site is the same in the 5+ different metabolites that THC breaks down into in urine. Because the immunoassay test is sensitive to all of the THC metabolites in the urine it has a higher cutoff level (50 ng/mL).

During the confirmation test that is performed on either a GC/MS, or a LC/MS/MS (for us with Gamma, it is a GC/MS), the lab is looking for one specific metabolite THCC. Because it is only the one metabolite, the cutoff concentration is 15 ng/mL. Essentially meaning 15 ng/mL of THCC is equivalent to 50ng/mL of all THC metabolites in urine. The same holds true for oral fluid testing.

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