Explanation

Cannabis screening (THC): what is measured?

The analysis looks in urine for breakdown products of THC, the main active substance in cannabis (weed and hash), mainly for THC carboxylic acid (THC-COOH). The laboratory uses an immunoassay: a quick measurement with antibodies that react to the breakdown product. The cut-off is 20 µg/l: if the concentration is above it, the result is positive. Because the antibodies can also react to related substances, a positive result is provisional until it has been confirmed with a different method.

What can it detect?

Use of cannabis in the past days to weeks, by smoking, vaporizing or eating

Result

What does the result mean?

Positive result

A positive result means that there are breakdown products of THC in the urine above the cut-off. According to the Trimbos Institute, cannabis is detectable in urine 5 to 30 days after use: most people test positive for about 5 days after one joint, and with daily use this can be longer than 30 days. The result does not say when, how much or how often cannabis was used, nor whether someone was under the influence at the time of the test. Medicinal cannabis and CBD products that contain too much THC also give a positive result, and according to the Trimbos Institute the stomach medicine pantoprazole can give a false-positive result. A positive screening is a provisional result; only confirmation with a different method, such as GC-MS, makes it certain.

Negative result

A negative result means that no breakdown products of THC were found above the cut-off. This does not fully rule out use: right after use the result can still be negative, and after little or one-time use longer ago or in diluted urine the value can stay below the limit. Drinking a lot lowers the concentration in the urine. Synthetic cannabinoids are not found with this test.

After an abnormal result

With a positive result that can have consequences, the laboratory has the result confirmed with a second analysis method, such as the GC-MS measurement of THC-COOH in urine. The GP then looks at the context: amount, frequency, symptoms and any wish to cut down or stop, and refers to addiction care if needed.

Reliability

How reliable is the test?

The test says something about use in the past days to weeks, not about health or the degree of use, and not whether someone is under the influence at this moment. The Dutch Society for Clinical Chemistry (NVKC) states that a positive result of a screening test must be confirmed with a second, different analysis method, because a positive result can have far-reaching consequences; for legal purposes, this is confirmation with mass spectrometry (GC-MS or LC-MS).

What can affect the result?

A screening test can give a false-positive result; confirmation with a different method is needed before a result can carry much weight.
The detection time differs greatly from person to person and depends on how much, how often and in what way cannabis was used.
Fluid intake determines how concentrated the urine is; diluted urine can give a negative result.
The test measures breakdown products and not the effect: someone can test positive without being under the influence.

Who it is for

Who is the test useful for?

Useful for

Useful for a medical question, for example in addiction care or with unexplained symptoms in which cannabis use may play a role, or if you want to monitor your own use. If you are worried about your own use, help with cutting down or stopping is more useful than a test.

How common is it?

In 2025, 7.1 percent of adults in the Netherlands had used cannabis in the past year. Almost 1.5 percent used cannabis (almost) every day.

Trimbos Institute, cannabis figures and trends 2025

Drawbacks and risks

A positive result can still occur long after the last use and can therefore give a wrong picture of recent use.
A result without a confirmation test can wrongly be used as evidence.

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