In drug testing, “Cut-off” is a value which the results obtained in the analysis are compared against. When results are below the cutoff, they are considered as “Not Detected “or” Negative “, and when above they are considered” Detected “or” Positive “
When testing drugs in urine, saliva or hair samples, results are evaluated against “cut-offs” for each of these type of samples for interpretation before the issue of certificate of analysis.
A positive result in drug testing can confirm if a person has used or was exposed to a drug. A negative result, however, does not necessarily mean that someone has not used or been exposed to the drug at all.
Cut-offs are used for several reasons
- to reduce the chance of reporting drug use when the drugs were taken involuntarily
- as is the case with drugs that are smoked.
- when people tested don’t use drugs but they are in an environment where drugs are being handled. This explains why the cut-off for cocaine is currently 0.5 ng/mg.
- urinalysis: to minimise the detection of drugs used over the weekend outside the workplace. This is very important in the workplace, where it is crucial to know whether an individual is under the influence of drugs or not, so as to minimise risk of injury to colleagues and the general public or when recruiting new employees.
- hair analysis: to minimise detection of drugs used in previous periods and increase detection of current use. Hair drug testing is the ideal for pre-employment and in-employment test for those industries where zero tolerance is required.
In practice, it means that results are reported as “Not detected” when levels are below the established cut-off even though the presence of drugs is clear – in other words even though the individual has taken or been exposed to a low level of drugs.
Cut-offs may change according to type of sample analysed. In contrast with the cut-offs employed in urinalysis, the cut-offs for hair samples are much lower and are usually at around the limit of the detection of the methods (all methods have a minimum level below which they cannot detect drugs).
This is in contrast with the cut-offs employed in urinalysis, which are much higher than the limitation of the methods, which means that results of urinalysis are reported as “Not detected”even though the presence of drugs is clear.
The cut-offs employed in hair analysis are usually analytical cut-offs established at the limit of quantification of the methods used.
Thus, you need to be aware that:
- Cut-offs may vary between different substances (e.g. cocaine and cannabis)
- Cutoffs are different for different types of samples (e.g. urine and hair)
- Cut-offs vary depending of the method of analysis (e.g. ELISA and LC/MS)
- Cut-offs may vary between testing laboratories (e.g. different procedures)
- Cut-offs may vary with the purpose of the test
The Society of Hair Testing (SoHT) and the European Workplace Drug Testing Society (EWDTS) have recommended cut-offs for substances and metabolites in hair. There are different cut-offs that are applicable to verify chronic use of drugs or to test for single use in cases of drug facilitated crime cases or for the workplace.
Before engaging in testing a sample, you need to be provided by the laboratory with the cut-off list for the drugs that you want tested for and with adequate information to make sure that the testing is relevant to your needs.