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How long do drug tests detect marijuana?
There is no simple answer to this question. Detection time depends strongly on the kind and sensitivity of the test employed; the frequency, dosage, and last time of use; the individual subject's genetic makeup, the state of one's metabolism, digestive and excretory systems; and other random, unknown factors.
The basic drug types and their approximate detection times are shown in the table below.
The most popular kind of drug test is the urine test, which can detect marijuana for days or weeks after use. Note that urine tests do not detect the psychoactive component in marijuana, THC (delta-9-tetrahydrocannabinol), and therefore in no way measure impairment; rather, they detect the non-psychoactive marijuana metabolite THC-COOH, which can linger in the body for days and weeks with no impairing effects. Because of THC-COOH's unusually long elimination time, urine tests are more sensitive to marijuana than other commonly used drugs. According to a survey by Quest Diagnostics, 50% of all drug test positives are for marijuana.
Blood tests are a better detector of recent use, since they measure the actual presence of THC in the system. Because they are invasive and difficult to administer, blood tests are used less frequently. They are typically used in investigations of accidents, injuries and DUIs, where they can give a useful indication of whether the subject was actually under the influence.
Hair tests are the most objectionable form of drug testing, since they do not measure current use, but rather non-psychoactive residues that remain in the hair for months afterwards. These residues are absorbed internally and do not appear in the hair until 7-10 days after first use. Afterwards, they cannot be washed out by shampoos (though smampoos may help remove external smoke particles that get stuck in the hair). Hair tests are more likely to detect regular than occasional marijuana use. One study found that 85% of daily users tested positive for marijuana, versus 52% of occasional smokers (1-5 times per week). Ingested cannabis was less likely to be detected than smoked marijuana . It is doubtful whether hair tests are sensitive to one-time use of marijuana.
Saliva testing is a newer, less proven technology. The sensitivity of saliva tests is not well established in the case of marijuana. In theory, they are supposed to detect recent use, but this may range from several hours to over a day. They are supposed to detect secretions from inside the oral tissues that cannot be washed out with mouthwash. Because they are less intrusive than blood or urine tests, the industry has been eager to develop saliva tests. Due to reliability problems, they have yet to gain acceptance in the U.S., but they have come into use in some other countries, such as Australia.
Urine Testing Detection Times for Marijuana
How long marijuana is detected in the urine depends strongly on frequency of use. In regular users (more than once a week), the marijuana metabolite THC-COOH builds up to a high level, often on the order of hundreds of nanograms per milliliter (ng/ml), from which it may take weeks to decline back below the threshold of detection. The situation is different for occasional users (no more than once a week), who often clear up in a couple of days.
Figure 1 illustrates typical urine testing profiles for "one-time" users - that is, subjects who were clean before going into the test. This would typically be the case for the occasional user who indulges a couple of times per month.
Subjects B and G illustrate typical "one-time" responses in a group of study subjects. All subjects received a dose of one standard NIDA cigarette . They were monitored for urine levels of marijuana metabolite (THC-COOH) at regular intervals after smoking. Both B and G were positive at the standard cutoff threshold of 50 ng/ml (the standard in most government-required testing programs) for up to two days after smoking.
Note that urine levels of metabolite fluctuate throughout the day. For example, Subject G fell beneath the 50 ng threshold around 24 hours after smoking, but rebounded above it a short time later. Thus it is possible to flunk a drug test despite having passed one a while earlier.
Subject "E" was an exceptional subject who never exceeded 50 ng/ml despite having smoked the same dose as other study subjects. This illustrates the high degree of individual variation in urine testing. While Subjects G and B would likely have flunked a drug test the day after smoking, Subject E wouldn't have.
Occasional use can sometimes be detected much longer, as shown in Fig. 1 by the oral dose from a different controlled study . This represents a subject who was given a strong oral dose of 20 mg of THC, equivalent to a strong brownie or two high-dose Marinol pills. The subject tested above the 50 ng/ml cutoff for up to six days after dosing. California NORML has heard unconfirmed reports of occasional users testing positive for as long as two weeks in exceptional cases. In most cases, however, one-time users are likely to pass after a couple of days.
Time to first positive test
Drug metabolites take time to reach the urine. A user who is clean beforehand can test negative for over 1 - 4 hours before turning positive at 50 ng/ml, as shown in the following data from Huestis . This is similar to the period of impairment after smoking. Therefore, in the case of one-time users, a positive urine test is a sign that the user is most likely NOT currently impaired by marijuana!
Figure 2 - Drug Testing Profiles in First Hours Following Single Use
Urine Testing in Regular Users
In regular users, metabolite levels can build up to background levels above 1,000 ng/ml. Many days or weeks of abstinence are required to clear out, as illustrated in the following graph of metabolite levels in test subjects who were regular users .
Subject B took over a week to fall beneath the 50 ng/ml cutoff, while Subject C took a month. Detection times over three months have been reported in extreme cases.
Note that it is possible to test negative on one day, then positive on the next.
As shown in the following graph, detection times for chronic users typically range from one to three weeks but can extend much longer. In this study by Ellis , chronic users were tested at 20 ng/ml and 100 ng/ml, but not at the normal standard of 50 ng/ml.
Unlike urine tests, blood tests detect the active presence of THC in the bloodstream. In the case of smoked marijuana, THC peaks rapidly in the first few minutes after inhaling, often to levels above 100 ng/ml in blood plasma. It then declines quickly to single-digit levels within an hour. High THC levels are therefore a good indication that the subject has smoked marijuana recently. THC can remain at low but detectable levels of 1- 2 ng/ml for 8 hours or more without any measurable signs of impairment. Chronic users may show a background level of 1-2 nanograms in serum for 24 - 48 hours after ceasing use .
Note: THC blood levels above are recorded in terms of concentration in serum or plasma. Most U.S. labs report levels based on whole blood instead. Concentrations in whole blood are about half as high. Therefore 1 - 2 ng/ml in plasma is equal to 0.5 - 1.0 ng/ml in whole blood.
Unlike urine, blood test results can give a useful indicator of whether one is impaired by marijuana. Studies have shown that high THC blood levels are correlated with impaired driving. An expert panel review of scientific studies on driving under the influence of cannabis concluded that THC levels above 7 - 10 ng/ml in plasma or 3.5 - 5 ng/ml in blood indicate likely impairment [Grotenhermen]. The same review found no increased driving hazard at low levels of THC. Despite the fact that accident studies have repeatedly failed to find evidence of increased driving risk at low levels (1 or 2 ng in blood) of THC, numerous states and foreign countries have enacted "zero-tolerance" laws, treating any non-zero trace of THC as legal evidence for driving under the influence.
Although high blood THC is a good indicator of impairment, it is not infallible. Chronic users who develop tolerance to THC may in some cases drive safely with high blood levels of THC. In one study, a subject with severe attention deficit disorder could not pass a driving test while straight, but performed well with a blood level of 71 ng/ml . No similar phenomenon is known for alcohol.
Oral ingestion produces a much different THC blood profile than smoking. Instead of peaking sharply, THC rises gradually over a couple of hours to a plateau of around 5 - 10 ng/ml in plasma, then declines (see blue curve in Figure 5).
In addition to THC, blood tests can detect cannabinoid metabolites. Not uncommonly, labs report levels of THC-COOH, the same non-psychoactive metabolite found in urine. As shown in Fig. 5, THC-COOH levels for blood are similar to urine. They may be detectable for a couple of days after a single use or weeks in chronic users, and are therefore not a valid indicator of being under the influence. There is no scientific basis for treating drivers who have THC-COOH but not THC in their blood as being legally "under the influence."
Another blood metabolite not shown in Figure 5 is 11-hydroxy-THC, a psychoactive byproduct produced when THC is processed by the liver after oral ingestion. While not detectable at appreciable levels in smoked marijuana, 11-hydroxy-THC shows a similar blood profile to THC after oral consumption. The presence of 11-hydroxy-THC may therefore be used as an indicator of recent oral use. However, most blood tests don't bother to check for 11-hydroxy-THC.
Figure 5 - Blood plasma levels of THC & Metabolite
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