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Drug Testing Cut-Off Levels

One of the key concepts within drug testing is the application of a cut-off level. This is the point, which determines a test result as being either positive or negative.

Reported in nanogrammes per millilitre (ng/mL), a cut-off level is the minimum concentration set to determine whether a sample is positive or negative for the presence of a drug or its metabolite. The cut-off levels used differ for each testing matrix (urine, oral fluid or hair) and must be in line with the European Workplace Drug Testing Society (EWDTS) guidelines with each testing matrix having its own set cut-off level for each drug type.

There are two stages to drug testing in the workplace, immunoassay and confirmation analysis. Immunoassay is an initial screen to determine if a drug type is present above the set cut-off level and confirmation analysis using gas chromatography mass spectrometry (GC/MS) or liquid chromatography mass spectrometry (LC/MS) to confirm the presence of a specific drug or its metabolite.

Immunoassay can be carried out using an onsite test kit or laboratory screening, but the confirmation analysis can only be carried out by a United Kingdom Accreditation Service (UKAS) approved laboratory using the cut-off levels set by the EWDTS. Simply put, immunoassay is a qualitative test; laboratory confirmation analysis is a quantitative test.

As these two stages of testing differ in their testing methodology, they each have their own cut-off levels set. Similarly, as each drug is different, they each have their own cut-off level set. For example, in urine, the cut-off level for cannabis is 50ng/mL for immunoassay and 15ng/mL for confirmation analysis, whereas in oral fluid the cut-off level for cannabis is 2ng/mL for immunoassay and 15ng/mL for confirmation analysis.

Medical Review

A confirmed positive laboratory result may be due to medication (prescribed or over-the-counter) or due to dietary causes (poppy seeds). Therefore, all positive laboratory confirmation results must be reviewed by a Medical Review Officer (MRO) who is a medical physician with the responsibility for interpreting the result and can issue a negative result if the confirmation analysis result is due to the use of medication or due to another valid reason such as dietary.

Further Information

For more information or clarification on workplace testing, legally defensible testing programmes or if you have any queries regarding a testing result, please contact Hampton Knight on 01827 65999.

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