Metals, toxins and medication
THC
Measures the breakdown product of cannabis (THC-COOH) in urine; shows whether cannabis has been used in the past days to weeks.
Everything about this testWhat it measures
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 measures 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 a number in micrograms per liter (µg/l) and does not say whether someone is under the influence at this moment.
- 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
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.
Low
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.
If your result is abnormal
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
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.
Limitations
- 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.
How common is this
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.
Who is this 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.
Downsides 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.
Sources
- Trimbos-instituut, drugsinfo.nl: Aantoonbaarheid van cannabis in speeksel, bloed en urine
- FMS Richtlijnendatabase: Drugsmisbruik in het kader van rijgeschiktheidskeuringen, module Biomarkers cannabisgebruik
- NVKC, allesovertesten.nl: Drugstest
- Thuisarts.nl: Ik gebruik vaak wiet of hasj
- Trimbos-instituut: Cannabis, cijfers en trends
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results