The language of "zero tolerance" per se laws is critical. Most state zero tolerance DUID laws contain the following language:

State DUID Laws

It is critical to comprehend the distinction between "parent drugs" and "drug metabolites." The term "parent drug" refers to the identifiable psychoactive compound of a controlled substance (i.e., for cannabis-based drugs, marijuana and hashish, the parent drug is delta-9-tetrahydrocannabinol aka THC). By contrast, the term "drug metabolite" refers to those substances produced by the metabolism after a drug is ingested. Though the presence of metabolites in blood or urine is indicative that a certain drug may have been consumed previously,15 not all metabolites are psychoactive (i.e., Marijuana's THC-COOH metabolite, which is readily detectable in urine, is not psychoactive.), nor does their detection prove per se that the parent drug is still present in the body. Consequently, the US Department of Justice affirms that a positive drug test result for the presence of a drug metabolite "does not indicate ... recency, frequency, or amount of use; or impairment."16 A recent US Department of Transportation report further states that while a positive test for drug metabolites is "solid proof of drug use within the last few days, it cannot be used by itself to prove behavioral impairment during a focal event."17

Understanding the various methods of drug detection is also critical. As stated above, most zero tolerance DUID legislation allows for police to mandate a defendant to have his or her "bodily fluids" screened for the presence of drugs or drug metabolites. In most cases, the "bodily fluids" in question are: blood, saliva, and urine. Whether or not a defendant tests positive for DUID will usually be a result of which fluid is analyzed for what compound (parent drug versus metabolites) and how sensitive the detection method is.

Urinalysis remains the most popular means of drug detection available in the United States, particularly in workplace drug testing programs. Courts have generally looked upon urine specimen collection, when compared to blood sampling, as a relatively non-invasive practice, and there are national standards for urine testing in place as well as national certification programs for laboratories performing forensic urine drug testing. However, standard urinalysis tests for marijuana, in their current form, are not suitable for detecting drug impairment or recent drug use because the procedure only looks for and detects drug metabolites, not the parent drug THC. Presently, no dose-concentration relationship exists correlating drug metabolite levels to drug impairment,18 and, as stated beforehand, the presence of a drug metabolite, even when confirmed, "does not indicate ... recency, frequency, or amount of use; or impairment."19 Nevertheless, because urinalysis is regarded as relative non-invasive and offers testers a multi-day window for the detection of drug metabolites, and because rapid response point-of-collection-testing (POCT) immunoassay devices are available on the commercial market, "a number of states with per se 'zero tolerance' laws are currently using urine tests to enforce their laws under which the prosecutor must only show that the driver of the car had prohibited metabolites in his/her system."20 Needless to say, "zero tolerance" DUID laws that rely solely on urine testing for THC metabolites will inappropriately target and define as "impaired" many otherwise sober marijuana consumers.

Because blood collection is generally viewed by the courts as invasive and requires the use of medically trained personnel, its use in DUID cases is often seen as impractical. However, many European DUID laws rely on blood specimen collection. This is because, unlike urinalysis, both drug metabolites and parent drugs are readily detectable in the blood. In general, peak THC serum levels typically exceed 100 ng/ml minutes after drug ingestion and then fall rapidly. As a result, detection times for marijuana and other parent drugs in the blood at levels above 1 ng/ml is typically only a few hours after past use.21 (Heavy cannabis users, however, may show residual THC serum levels of more than 2 ng/ml up to 48 hours after last use.22) Consequently, the Department of Transportation speculates, "In terms of attempting to link drug concentrations to behavioral impairment, blood is probably the specimen of choice."23

Nevertheless, scientists have not reached a consensus on the establishment of specific plasma concentrations that could be designated as evidence of driver impairment -- primarily because few adequate studies have been performed to date. Recently, a pair of scientific reviews of automobile crash culpability studies have indicated that THC levels in blood serum below 5 ng/ml are not associated with an elevated accident risk.24 (Levels below 5 ng/ml are attained in recreational marijuana users, on average, within 1 to 3 hours after cannabis consumption.25) Moreover, some studies suggest that "even a THC serum level of between 5 and 10 ng/ml may not be associated with an above normal accident risk."26 However, additional studies are necessary before reliable THC/blood threshold for impairment may be derived.

Saliva testing detects the presence of parent drugs only, and its detection times27 are similar to blood (1-24 hours on average) for drugs other than cannabis. However, unlike other drugs, cannabinoids appear to be more difficult to detect in oral fluids, as only a minute amount of the drug is excreted into the saliva.28 As a result, most current saliva testing technology appears to only detect the presence of cannabis for a period of approximately one to two hours following drug ingestion.29 (Note that newer, more sensitive oral screening technology utilizing lower cutoff levels have detected residual THC levels at 1-2 ng/ml some 4 to 8 hours after ingestion, long after any psychomotor impairment from the drug has subsided.30)

Because saliva testing is generally seen as non-invasive, and rapid response point-of-collection devices exist, it is viewed by some law enforcement organizations, in particular the European Police Traffic Network TISPOL31, as ideal for use by police on the side of the road. Yet, recent studies have shown considerable variation in results among test subjects. An ongoing pilot program in Victoria, Australia, utilizing road side oral screening technology has also yielded several false positives when used under roadside conditions.32 In addition, there is no consensus on appropriate cutoff levels for the confirmation of drugs in saliva, nor are there any nationally established standards for oral fluid testing in traffic settings. As a result, many experts resolve that saliva testing is "unsuitable for reliable, ultimate determination of impairment by THC. Yet it [may] offer a suitable roadside screening tool for impairment, possibly followed by a blood test."33

In sum, recreational marijuana consumers face their greatest risks of being falsely defined as "impaired" in states with "zero tolerance" per se DUID laws reliant on urinalysis because this process currently detects only drug metabolites, not THC. Sober drivers are less likely to be identified as impaired by cannabis in states that rely on blood and/or saliva collection because the window of detection for parent drugs in these fluids is, by comparison, relatively narrow. In cases when parent drugs are detected, there is no general consensus regarding what concentration levels are indicative of impairment (though general estimates regarding the recency of drug ingestion may be ascertained). In a limited number of cases regarding the detection of marijuana in the blood serum, studies have preliminarily associated culpability and/or impairment at levels above 5-10 ng/ml, but not below this threshold. However, the low number of available studies prevent scientists from deriving a reliable THC-blood threshold for impairment at this time.34

How Dangerous is "Drugged Driving" Anyway?>