THC screening tests: how do they work?

The saliva test looks for THC itself, present only in the case of recent consumption, which makes it the tool of choice for law enforcement. The urine test looks for THC-COOH, an inactive metabolite that remains detectable well after the effects have worn off, and which is used mainly in an occupational context. The blood test, more precise, serves as legal confirmation after an initial positive result. Lastly, the hair test offers the longest detection window and is used mainly in forensic assessment or occupational medicine.

At the roadside, the check takes place in two stages: a saliva strip detects THC above a threshold, generally around 25 ng/mL, then a blood sample confirms the result by chromatography. THC remains detectable in saliva for roughly 6 to 8 hours after occasional consumption, and for up to 24 hours with regular use. Article L.235-1 of the Highway Code provides for no tolerance threshold: detection alone constitutes the offence.

The urine test looks for THC-COOH, a metabolite that persists in the body well after the psychoactive effects have disappeared. Its detection window ranges from three days for one-off use to several weeks for prolonged daily use, because THC continues to be released from fatty tissue. This is the format used in certain occupational contexts, notably safety-critical posts, governed by the Labour Code.

The blood sample serves to confirm an initial positive saliva result, with a regulatory threshold set at around 1 ng/mL of active THC. It is this result, and not the first saliva test, that constitutes the legal proof relied upon in the event of prosecution. Its detection window is shorter than the urine test's, from a few hours to about a week depending on how often the person consumes.

Analysing a lock of hair makes it possible to trace consumption over several months, up to 90 days depending on the length of the sample, because THC is gradually incorporated into the structure of the hair as it grows. This test, more costly and slower to carry out, is reserved for specific contexts: forensic assessments, certain medical follow-ups or occupational medicine.

A positive test indicates exposure to THC, not necessarily recent use nor impaired faculties at the time of the check. This is particularly true of urine and hair tests, which detect metabolites present long after the effects have ended. On the road, this nuance changes nothing in legal terms: the Cour de cassation, in its ruling of 21 June 2023, confirmed that detection alone is enough to establish the offence, including for a lawful CBD product containing traces of THC.

Can CBD trigger one of these tests?

No, no approved test looks for CBD. But a full-spectrum CBD product contains lawful traces of THC, up to 0.30 % of the finished product, which can be enough for a positive result.

Which test is the most reliable?

The blood test is the most precise and serves as legal confirmation. The saliva test, which is quicker, remains the initial screening tool on the road.

Can a single joint make a urine test positive several weeks later?

In an occasional consumer, the window rarely exceeds a week. In a regular consumer, THC accumulated in fat can remain detectable for several weeks after stopping.

Is there a guaranteed safe interval before driving?

No, no official interval is guaranteed. The prudent course is never to get behind the wheel after consuming, whatever the substance or product concerned.