This is a question that comes up quite often in our our criminal defense law practice, particularly when someone is facing a charge of driving while impaired. We have found it best to break the above question into two parts: “Can a urine test reveal the presence of marijuana?” and “Can a urine test determine the degree of impairment?”
Can a urine test reveal the presence of marijuana?
There are a number of chemical tests that can detect the presence of marijuana in a urine sample, but none of these are simple to administer, or are sufficiently reliable, to be used at the roadside or within a police station / state police barracks. There are a number of factors that contribute to such tests that can make them unsuitable for routine use. First among these shortcomings is the realization that there a number of substances that may trigger a “false positive” reaction.
Among the more common self-administered medications that are known to case a sample to “test positive” for THC, the compound that is responsible for the “high” that accompanies marijuana use, are ibuprofen (such as Advil, Motrin, and Midol), naproxen (Aleve and Naprosyn) and “proton blocker” antacids (such as Pepcid, Nexium, and Prevacid). Since many of these medications are available without a physician’s order, many people (including police officers) would have no reason to suspect that these “OTC” medications could suggest recent marijuana use when, in fact, no such use had taken place.
While it is true that marijuana can be detected in urine, the testing techniques required to detect THC and related compounds require sophisticated laboratory facilities and highly trained laboratory technologists. In fact, many state crime labs do not routinely perform such testing but will send such samples to one of a few testing centers nationwide with the capability to perform such analysis.
Can a urine test determine the degree of impairment?
In a word, “no.” Unlike the case with alcohol, where a certain amount of alcohol in a urine sample can be used to presume that the person submitting that sample is intoxicated or impaired, there isn’t a “presumptive” level of THC in the urine beyond which a person is considered to be impaired. There are a number of reasons why this is the case.
First of all, as is the case with alcohol use, a frequent marijuana user will have a higher tolerance for THC than will an infrequent user. In other words, the overall effect caused by a given amount of THC will vary depending on how frequently a person uses the substance: a relatively small amount of THC in the urine could have little effect on a regular user but that same amount could render an infrequent user to be all but helpless.
The second problem with using a urine test to determine the degree of impairment attributed to marijuana use is related to three separate, but related, characteristics that determine the effects of any substance: 1) intake, the manner in which a substance enters the body, 2) metabolism, how that substance is processed by the body, and 3) excretion, how and by what route that substance leaves the body.
In general, intake and metabolism take place at relatively constant rates while excretion is the most variable. Depending on factors such as body weight, the percentage of body fat, and the amount of fluid intake, the presence of marijuana and its byproducts can be detected up to 10 days after its most recent use. Since there is so much variability in the rate in which marijuana is excreted, and the fact that the amount excreted in the urine cannot be used to determine how much (and certainly not when) marijuana was ingested by an individual, such testing has very little value as evidence that could be used to support of a charge of driving while impaired.
In summary, there are far too many variables that must be taken into consideration if the amount of marijuana and its related compounds detected in a urine sample is to be used as evidence of impairment at the time such a sample was collected.
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