How far back in time can drug tests detect your marijuana use?
Here is a table that lists reliable detection periods for each type of drug test:
NOTE: These are just averages, and your experience may vary.
Detection time depends on the kind of test used and the sensitivity of the test process and equipment used (they aren’t all the same).
Other factors that affect the accuracy of a drug test for marijuana include:
the frequency of use
the dosage/concentration of THC
the last date/time of use
the user’s metabolism (but not as much as people think)
the user’s digestive system (if ingested).
How Fast Does THC Leaves Your Blood, Over Time?
The following chart illustrates the results of two studies that measured the amount of THC and THC metabolite (THC-COOH) in the blood streams of a large number of marijuana users, over an extended period of time.
It shows how blood levels of THC and the metabolite decay, or dissipate, over time depending on whether you smoked or ate your weed/hash.
Please note that this chart displays the average blood levels for a group of people. Each test subject used a different amount of marijuana, and your chart would look different depending on how much and how frequently you used, etc.
What is interesting about this chart is that it shows:
if you smoke weed, your THC blood level will spike quickly then drop below the typical drug test threshold of 50 ng/mL
if you eat hash brownies, THC will gradually build over time, peaking at about 4 hours
THC-COOH, the metabolite used to detect marijuana use in most drug tests, takes a LOT longer to dissipate.
Urine Tests For Marijuana: How Long Does a Pee Test Detect THC?
The most popular kind of THC drug test is the urine test.
Because THC-COOH takes such a long time to eliminate from your body, urine tests are more sensitive to marijuana than other commonly used drugs. This is why urine tests are the preferred test for detecting recent marijuana use.
Like hair tests, urine tests look for the metabolite THC-COOH, usually with a cutoff of 50 ng/mL. Urine tests can detect marijuana for days or weeks after use. They can detect marijuana use for days after the high is gone, even in 1-time users.
The following chart shows the results of a recent study of 1-time users of THC.
The most striking thing about this study is how variable the results are – from person to person, and from hour to hour for the same person.
Note that it took Subject B two days to get clean. Subject E was safe the entire time. Depending on the time of day, Subject G might have passed or failed the 50 ng/mL cutoff.
The reason for this variability is that the concentration of THC-COOH in your urine is highly subject to the amount of water in your body (and urine) and your body’s metabolism. If you are poorly hydrated or have a low metabolism, you are more at-risk of failing the urine test.
The bottom-line for the urine test is: to be safe, you should assume you will fail if you use any amount of marijuana within a few days of the test.
A blood test is the most accurate form of marijuana drug test, because it measures the direct presence of THC (delta-9-tetrahydrocannabinol) in your system.
But blood tests only detect drug use back a few days, so they are primarily used in the investigation of accidents and crimes committed under the influence and for spot-testing employees in jobs and professions where any amount of drug influence could be catastrophic.
Blood tests are also ordered in domestic dispute and parole situations, where the court has required abstinence.
In the testing industry, the accuracy of the hair drug test for detecting alcohol use or abstinence is hotly debated. In short, the problem is that many common cosmetic procedures can alter the amounts of alcohol-related chemicals that the hair test is looking for.
The hair drug test detects two byproducts of alcohol in the hair follicle, including:
Fatty acid ethyl esters (FAEEs) including ethyl myristate, ethyl palmitate, ethyl oleate and ethyl stearate
The hair test is not as reliable for alcohol detection as it is for other drugs, because cosmetic treatments can dramatically affect the levels of EtG and FAEEs in your hair. This includes hair straightening, dyeing, bleaching and perming.
In addition, hair products that contain ethanol, such as hairsprays or hair lotions, will affect FAEE levels, though not EtG.