Choice of specimen for drug testing in impaired driving cases
As noted above, oral fluid may be used in some jurisdictions as a matrix in roadside tests to detect whether a driver has drugs in their system. These tests are generally based on immunoassay type kits and in many jurisdictions are not considered acceptable for evidential purposes. In some jurisdictions, testing using oral fluid is not accepted, even for screening purposes, and road side tests rely on impairment tests as described above. Thus the norm would be for a driver suspected of driving under the influence of drugs to be screened using a roadside test or assessed using an impairment test and, if the test indicated drug(s) to be present, the suspect would be taken in so that samples could be taken for evidential testing. Specimens of choice for evidential testing are urine and blood. As seen in Chapter 2, drugs and metabolites are typically excreted in urine. Depending on the drug, it or its metabolites may be detected well after the effect of the drug has disappeared. Thus, while urine can be considered an excellent specimen for answering the question, ‘Did the donor of the specimen at some time prior to provision of the specimen use or ingest this drug?’, it is of little use in determining, in isolation, whether the subject was impaired or intoxicated at the time the sample was collected. Instead, the investigator must rely on objective evidence of impairment (slurred speech, staggering, inappropriate conduct or response, poor coordination, glassy stare, agitation, restlessness, etc.) to provide the necessary connection between the observed bad driving and the urine drug test result. The toxicologist can then use his or her specialised knowledge to assess whether the observed effects could be consistent with the known properties of the drugs detected. It should also be borne in mind when using urine as a test matrix that, after administration of a drug, it may take some time before the drug is metabolised and excreted. Therefore, if a drug was ingested not long before the driver was stopped and tested, and urine was taken as the test specimen the test result may be negative, even though the drug is present in the body and exerting an effect in the subject.
In spite of these limitations, urine has become a popular specimen for drug testing in impaired driving cases because of the ease with which it may be collected (although there are privacy and associated chain-of-custody issues to be addressed), and its ease of screening in the laboratory by immunoassay on automated instruments. Blood is a preferred specimen for collection and analysis in an impaired driving investigation, since blood drug concentrations can usually be related to some degree to published concentrations associated with known doses or patterns of recreational use. The blood concentration inevitably, however, is also a function of both the dose and the time since last dose, and whether the use is acute or chronic. Typically, this information is unavailable or unreliable. Relationships between blood concentration and any specific degree of impairment or effect on driving have been established for very few drugs. Apart from alcohol, the exception to this may be other members of the depressant drug class, which have been studied to a greater degree. As a result, the interpretation of blood drug concentrations still needs to be treated with caution, and the full range of possible ingestion patterns, and likely effects, should be considered. In the laboratory, blood drug analysis is more challenging, and many clinical laboratories are not properly equipped for this. Blood is more difficult to collect, and requires a qualified person to collect the sample who may not be available at the roadside or at the time the suspect is apprehended.
Oral fluid is being explored as a possible alternative to blood, because of its ease of collection at the roadside. Oral fluid may also prove useful in countries with laws that prevent the collection of blood. Recent studies, however, have shown that the relationship between blood and oral fluid drug concentrations is not always predictable and, again, it suffers from the same lack of ability to correlate a concentration to a specific degree of effect. At present, oral fluid drug testing to demonstrate effect is not widely accepted in the field. Hair drug testing is well established, but has little application in impaired driving toxicology because the time frame in which it can demonstrate drug use (weeks to months) is usually not relevant to the time of driving.
