A THC test is a drug screening that checks your body for evidence of marijuana use. Most tests don’t actually look for THC itself. Instead, they detect a byproduct your liver creates after breaking down THC, a compound called THC-COOH. This inactive metabolite lingers in your system far longer than the high does, which is why you can test positive days or even weeks after last using cannabis.
How Your Body Processes THC
When you consume marijuana, the active ingredient (delta-9-THC) enters your bloodstream and quickly gets converted by your liver into an active metabolite and then into THC-COOH, an inactive waste product. Your body stores THC and its byproducts in fat cells, then slowly releases them over time. That slow release from fat tissue is the reason detection windows vary so widely from person to person.
Because THC-COOH sticks around much longer than active THC, most standard drug tests target it rather than the THC that actually produces a high. This is an important distinction: a positive result means you used cannabis at some point in the recent past, not that you’re currently impaired.
Types of THC Tests
Urine Testing
Urine tests are the most common format, used by most employers and courts. They screen for THC-COOH above a set threshold, typically 50 nanograms per milliliter (ng/mL). At that standard cutoff, a single use can be detectable for roughly one to three days. Higher doses push that window to about three to six days. Chronic daily users can test positive for considerably longer, sometimes several weeks, because THC-COOH accumulates in the body with repeated use.
If an initial screening comes back positive, a second, more precise test (called a confirmation test) is usually run at a lower cutoff of 15 ng/mL. This confirmation step uses more advanced lab technology and tends to detect use for roughly twice as long as the initial screen at 50 ng/mL.
Blood Testing
Blood tests measure active delta-9-THC rather than the inactive metabolite, so they reflect much more recent use. THC in the blood typically becomes undetectable within 12 to 24 hours, and after smoking specifically, it may only be measurable for 4 to 10 hours. Blood tests are primarily used in medical emergencies and accident investigations where the goal is to determine whether someone was under the influence at a specific moment.
Saliva (Oral Fluid) Testing
Saliva tests are gaining popularity for roadside testing and some workplace programs because they’re fast and non-invasive. A cotton swab collects a sample from inside your cheek, and point-of-collection kits can deliver results within minutes. The detection window for marijuana in saliva is generally up to 24 hours, making it the best option for identifying very recent use.
Hair Follicle Testing
Hair tests have the longest detection window by far. A technician collects roughly 1.5 inches of hair closest to the scalp, which corresponds to approximately three months of growth. This means a hair test can reveal cannabis use from up to 90 days prior. Hair testing is less common due to cost, but some employers use it when they want a broader picture of substance use patterns rather than a snapshot of the past few days.
Why Detection Times Vary So Much
Two people who smoke the same amount of marijuana on the same day can get very different test results a week later. Several biological factors explain this.
Body fat percentage plays a major role. THC and its metabolites are fat-soluble, meaning they get stored in your body’s fat cells and released gradually. People with more body fat tend to retain THC-COOH longer. Researchers have hypothesized that conditions that break down fat, like intense exercise, fasting, or rapid weight loss, could trigger a temporary release of stored cannabinoids back into the bloodstream, though this effect has been difficult to confirm in studies of people with lower body fat.
Frequency of use matters just as much. A single session produces far less metabolite buildup than daily use over weeks or months. Chronic users accumulate THC-COOH in their fat stores over time, and clearing that backlog takes much longer. Hydration, metabolism speed, and even the potency of the cannabis all shift the timeline as well.
Standard Cutoff Levels
Drug tests aren’t simply positive or negative. They use a numerical threshold, and anything below it counts as a negative result. For urine, the most common workplace cutoff is 50 ng/mL for the initial screen and 15 ng/mL for the confirmation test. Lower cutoffs detect use over a longer window. One study found that switching from a 50 ng/mL cutoff to a 20 ng/mL cutoff roughly doubled the number of days a single use remained detectable.
Federal workplace testing, including all Department of Transportation programs covering truck drivers, pilots, and transit operators, follows standardized cutoff levels under 49 CFR Part 40. Marijuana remains a Schedule I substance under federal law, and DOT regulations continue to prohibit cannabis use for safety-sensitive employees regardless of state legalization or medical marijuana cards.
False Positives
Initial immunoassay screens can sometimes flag a sample as positive when the person hasn’t used marijuana. Several common medications have been documented to cause false positives for cannabinoids, including ibuprofen, naproxen, and certain prescription drugs like the antidepressant sertraline, the sleep aid doxylamine, and the antipsychotic quetiapine. This is one reason confirmation testing exists. The more precise lab method used in the second step can distinguish THC-COOH from other compounds, so a true false positive that survives both rounds of testing is rare.
If you’re taking any medication and facing a drug test, letting the medical review officer know beforehand can help contextualize the results.
CBD Products and THC Tests
Legal hemp-derived CBD products are allowed to contain up to 0.3% THC by weight. That small amount can add up. An open-label study published in JAMA Psychiatry found that regular use of full-spectrum CBD products, which contain trace amounts of THC alongside other hemp compounds, can cause users to test positive on a standard THC screening. Quest Diagnostics, one of the largest testing laboratories in the U.S., has specifically flagged this risk, noting potential consequences including job loss and legal complications despite the products being federally legal.
CBD isolate and broad-spectrum products are less likely to trigger a positive result because they’re processed to remove THC. But labeling accuracy in the CBD industry is inconsistent, and some products contain more THC than advertised. If passing a drug test is important to you, the safest approach is to avoid all CBD products that aren’t verified by third-party lab testing showing non-detectable THC levels.
What Happens During a THC Test
For a urine test, you’ll be directed to a collection site, given a specimen cup, and asked to provide a sample in a private restroom. The sample is sealed, labeled, and sent to a lab. Some rapid tests give preliminary results within minutes, but a lab confirmation typically takes two to five business days. If the result is positive, a medical review officer may contact you to ask about prescriptions or other explanations before reporting the result to your employer.
Saliva tests are even simpler. A swab sits between your cheek and gum for a few minutes, collecting enough oral fluid for analysis. Blood draws require a needle stick and trained personnel, which is why they’re reserved for situations where timing of use matters, like post-accident testing. Hair collection is painless and typically taken from the back of the head, close to the scalp.
Regardless of the sample type, the process follows a chain-of-custody protocol to ensure the sample isn’t tampered with or mislabeled between collection and analysis.