THC saliva test: how it works, interpretation, and limitations
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How does the test work?
Most rapid tests use a competitive immunochromatographic method. After sampling, the saliva migrates on a membrane containing reagents capable of recognizing the target substance.
On many competitive devices, the absence of a test line may correspond to a presumptive positive result, while its presence corresponds to a negative result. This logic may seem reversed compared to other rapid tests.
What does the threshold in ng/mL mean?
The threshold corresponds to the concentration around which the test is designed to change its result. A 15 ng/mL test is intended to detect lower concentrations than a 50 ng/mL test, all other things being equal.
In France, 15 ng/mL is the minimum threshold set by the decree of December 13, 2016, for roadside saliva THC screening. This technical proximity does not give a self-test the value of an official control.
How long does THC remain detectable?
There is no single duration valid for everyone. The result depends notably on:
- the dose and THC content;
- the frequency of use;
- the mode of administration;
- the elapsed time;
- the test threshold;
- the quality of the sample;
- individual characteristics.
Tables promising an exact number of hours after occasional or regular use give a false impression of precision.
How to read the three possible results?
Negative
THC was not detected at the test threshold under the sampling conditions. This does not prove a total absence of THC nor fitness to drive.
Presumptive positive
The test reacted. In a medical, professional, or legal context, confirmation by a laboratory method may be necessary.
Invalid
The control zone is not valid. The result should not be used and a new test must be performed.
CBD and positive result
CBD is not the target of the THC test. However, a CBD product may contain traces of THC. Depending on the amount consumed, the product, and the delay, these traces can lead to a positive result.
Self-test and official control
Roadside checks combine screening by law enforcement with a verification procedure. The home self-test can help prevention, but it does not have the same procedure, traceability, or probative value.
Best practices for use
- check the expiration date and packaging integrity;
- respect storage conditions;
- follow pre-sampling restrictions indicated in the instructions;
- collect a sufficient amount of saliva;
- strictly respect the reading time;
- consider any invalid or doubtful result as uninterpretable.
