Explanation

THC carboxylic acid (THC-COOH, urine): what is measured?

THC is the active substance in cannabis (weed and hash). The liver converts THC into THC carboxylic acid (THC-COOH), a breakdown product without effect that leaves the body through the urine; because THC builds up in fatty tissue and is slowly released from it, THC-COOH stays detectable much longer than the effect lasts. The laboratory first runs an immunochemical screening. Only if it is positive does it measure the concentration of THC-COOH with gas chromatography-mass spectrometry (GC-MS): the substance is first separated from other substances and then recognized by its mass. The result is then a number in micrograms per liter (µg/l); if the screening is negative, the result is negative. The result does not say whether someone is under the influence at this moment.

What can it detect?

Use of cannabis in the past days to weeks, by smoking, vaporizing or eating
With daily or heavy use: use up to several weeks or longer before the urine was collected

Result

What does the result mean?

THC carboxylic acid (THC-COOH, urine) too high

A detectable amount of THC-COOH above the laboratory's limit fits cannabis use in the past days to weeks. 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. A higher value often fits more frequent or heavier use, but the number cannot be converted into the amount, the number of times or the time of use: it also depends on how concentrated the urine is, on fatty tissue and on the time since last use. According to the Dutch medical specialists' guideline (FMS) on fitness to drive, a single urine measurement cannot distinguish heavy use from light use. A detectable value does not say whether someone was under the influence at the time of the test and says nothing about dependence.

THC carboxylic acid (THC-COOH, urine) too low

A negative screening or a value below the laboratory's limit means that there was no detectable amount of THC-COOH in the urine. This fits no use in the past days to weeks. It does not fully rule out use: right after use the breakdown product may not yet be in the urine, and after little or one-time use longer ago the value may already be below the limit. Very diluted urine, for example after drinking a lot, lowers the concentration.

After an abnormal result

A detectable value does not need medical follow-up on its own. The GP looks at the context: amount, frequency, symptoms and any wish to cut down or stop, and refers to addiction care if needed. To see after stopping whether cannabis has been used again, repeated measurements are needed in which THC-COOH is divided by creatinine in the same urine.

Reliability

How reliable is the test?

GC-MS is a specific method: a false-positive result due to other substances is unlikely, and the method itself counts as the second, different analysis method that the Dutch Society for Clinical Chemistry (NVKC) requires to confirm a positive screening test. The meaning of the number is limited, though: it shows use in the past days to weeks, but not how much or when, and not whether someone was under the influence. According to the FMS guideline, only a series of urine measurements, in which THC-COOH is divided by creatinine, can distinguish whether a value comes from new use or from earlier use that is still wearing off.

What can affect the result?

How concentrated the urine is partly determines how high the number is; without creatinine in the same urine, two measurements are hard to compare.
How long THC-COOH stays detectable differs greatly from person to person and depends on how much and how often cannabis was used and on the amount of fatty tissue.
Medicinal cannabis, dronabinol and CBD products that contain too much THC rightly give a detectable value.
The test measures the breakdown product of THC and not that of synthetic cannabinoids.

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 stop after daily use, you can still have a detectable value for weeks; a series of measurements then says more than a single result. 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

After previous daily use, the value can stay detectable for weeks after stopping; this can be confusing if the result is meant to show that cannabis is no longer used.
A result can wrongly be used as evidence, while it says nothing about the time of use, the amount or fitness to drive.

Get tested

THC carboxylic acid (THC-COOH, urine): test panels

THC carboxylic acid (THC-COOH, urine) is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.

Urine
THC
A urine test for cannabis. If the screening is positive, the lab measures THC-COOH, the breakdown product, as a number with an accurate GC-MS measurement.
€ 87,00

Evidence

Sources

This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.

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