Understanding ‘cut-offs’ work in drug testing

The presence of drugs in an individual’s hair sample may not equate to a positive result. Even if drugs are found, the result may still be reported as “Not Detected”. Why?

In cases where the custody of a child or an individual’s employment could be affected by drug test results, we need to be sure that any positive result we report is 100% accurate.

That’s why we use “cut-offs”: test result levels that indicate when substance use within the period covered by the sample is significant, and therefore likely to be deliberate.

Here’s everything you need to know about cut-offs in hair drug testing: their purpose, how they vary, and what they mean for test results.

How are cut-off levels determined?

It’s vital that the cut-off level used in any hair test is high enough to rule out any external contamination, yet still low enough to be completely certain that a negative result shows the donor has abstained from drug use. As a result, cut-offs must be determined carefully. Should a test reveal a level of a drug above the cut-off, we take that as a positive indicator of deliberate drug use.

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These cut-offs mean we can improve the detection of current drug use. They also mean we can minimise the detection of drugs used in earlier periods, as well as minimising the risk of results affected by external contamination. What it’s vital to understand, though, is that a cut-off isn’t a single figure that applies across the board - it’s a range of values that vary depending on different factors.

Cut-offs may vary between different analytes

There is not a single cut-off level that applies to all drug types: when we do a drug test with hair, we use the cut-off levels recommended by the Society of Hair Testing. Recommended cut-off levels vary from drug to drug, and also differ in screening vs. confirmation tests.

The cut-off for cocaine in hair samples, for example, is 0.5 nanograms per milligram (ng/mg) of hair in both screening and confirmation tests, while the cut-off for THC metabolite THC COOH is 0.002 ng/mg of hair. The reason for this substantial difference is that the two substances are incorporated into the hair at different rates [1].

Studies of drug levels in donor samples explain the variation. For example, in a survey of over 7,000 hair samples, levels of cocaine ranged from 0.2 to 159.9 ng/mg of hair in 99% of the samples [2]. In the same study, levels of THC metabolite THC COOH ranged from 0.001 to 0.052 ng/mg of hair. The ranges displayed by the two drugs reveal just how differently various drug types can be incorporated into the hair. 

Cut-offs may vary depending on sample type

Cut-offs also vary according to the type of sample tested. Hair samples, for example, will have different cut-off levels for different drugs compared with urine or oral fluid (saliva) samples.

While the cut-off for cocaine in urine is 300 ng/mL, the figure is 0.5 ng/mg for hair, when analysed using mass spectrometry and liquid chromatography.

Hair sample cut-offs are lower than other types, and are generally set around the minimum levels at which drugs can be detected. In urinalysis, however, the cut-offs are higher than the limitation of the methods, which means that urine testing results may be reported as “Not detected” even though the presence of drugs is clear.

While a positive result in a urine or oral fluid test may confirm if an individual has used or was exposed to a drug, this means a negative result does not refute use of or exposure to the substance. 

Cut-offs vary depending on analysis method

We analyse samples using various methods, according to whether we are conducting a test for screening, or for confirmation. Screening tests are conducted using immunoassays, while we use LC-MS/MS testing for confirmation. As an example, when testing for cannabis use, Society of Hair Testing guidelines recommend a cut-off of 0.1 ng/mg for THC using immunoassays, and a cut-off of 0.002 ng/mg of hair for THC COOH using chromatographic testing.

Cut-offs vary between laboratories

Not every drug testing laboratory is the same: from lab to lab, equipment and testing procedures vary. Combined with the experience of the laboratory, this affects both cut-off levels and results. It’s therefore important to choose a testing lab that’s fully accredited to ensure the reliability of the results.

Uncertainty of measurement at cut-off

Your hair testing report from Cansford Labs will feature both the test results, and our toxicologist’s interpretation of these results. This interpretation will factor in circumstances that may have affected the results, including pre-analytical and analytical variations that contribute to the total uncertainty of drug detection in the hair sample.

Any uncertainties associated with the measurements and/or interpretations, and any assumptions made during the process should be explained by your laboratory, following testing.

In a drug test with hair, cut-offs are of vital importance. When a false positive could lead to an innocent worker losing their livelihood, or a drug-free parent losing custody of their children, it’s imperative that test results are accurate, with any risk of false positives minimised.  

[1] Nakahara, Y, Takahashi, K, Kikura, R. (1995) Hair analysis for drugs of abuse, X: effect of physicochemical properties of drugs on the incorporation rates into hair. Biol.Pharm. Bull. 18, 1223.

[2] Tsanaclis, L, Wicks, JFC. (2007) Patterns in drug use in the United Kingdom as revealed through analysis of hair in a large population sample. Forensic Sci. Int. 170 (2-3): 121-128.

[3]  Nielsen, MKK., Johansen, SS, Linnet, K. (2014) Pre-analytical and analytical variation of drug determination in segmented hair using ultra-performance liquid chromatography–tandem mass spectrometry. Forensic Sci. Int. 234, 16-21.