Understanding Lab Results – Part 1

Dec 21, 2017

Part I: Definitions and Uses

What is a cut-off?

Cut-offs are used in drug testing as a measure to determine a positive or negative result. If the drug being tested is present in amounts above the predetermined cut-off level, it is considered positive, and if below, the test will be considered negative. For example, when testing for methamphetamines in a fluid sample, the screening or assumptive cut-off test amount is 500 nanograms of amphetamine per millilitre. If the amount of amphetamine is higher than 500 ng/ml, then a second, more substance-specific confirmation test is required, with a cut-off of 250 nanograms of methamphetamine per millilitre. If the tested sample procures results higher than 250 ng/ml, then that test is considered positive.

In a screening or assumptive test, the purpose is to detect the presence of general drug classifications like amphetamines, barbiturates, and THC. Once detected, the sample is then tested for specific drugs with gas chromatography mass spectrometry (GC/MS), and the results are based according to the established cut-off level for each metabolite. For example, the cut-off level for cocaine in a hair sample is 0.5ng/mg, while the cut-off for THC, the main psychoactive substance found in cannabis, is .002ng/mg.1

Why is a precise cut-off point important?

A cut-off point’s purpose is to optimize drug detection while minimizing false positive or false negative results. If the cut-off is set too low, the test will report small levels of drugs in the client’s system, resulting, for example, from secondhand smoke. As follows, a higher than recommended cut-off point will report false negatives and not indicate that the client has significant amounts of the drug in his/her system.

Variances in cut-off levels

Not all drugs share the same cut-off levels. Factors such as sample material, testing method, and testing environment can all change the cut-off point for specific drugs.
Compared to urinalysis, cut-offs are lower in hair samples, and are usually set at the minimum detection limit. This means that results from urine samples, with cut-offs much higher than the minimum detection level, can sometimes report no presence of the drug in the sample even though it is evident that drugs have been ingested.

The purpose behind setting the hair sample cut-off so low is to minimize the detection of drugs that were used previously, as well as to increase the detection of drugs currently in use, and to eliminate a positive result from passive and external contamination (i,e., when the subject has spent time in an environment where drugs were being used or handled).

Another variance in cut-offs for specific drugs is determined by the methods being used to test the sample, as well as the laboratory in which the sample is tested. Individual labs use different procedures as well as different equipment to test samples; not only that, but the experience of the lab will also impact cut-off points and subsequent results.2


1. “Drug Test Cut-Off Levels | Cut-Off Levels for Popular Drugs of Abuse.” Confirm BioSciences, www.confirmbiosciences.com/knowledge/terminology/drug-test-cut-off-level/
2.“Understanding ‘Cut-Offs’ in Drug Testing.” Cansford Laboratories Ltd., www.cansfordlabs.co.uk/understanding-cut-offs-in-drug-testing/