Blood, urine and hair are the three commonly accepted methods drug testing. Quite often law enforcement is limited to a urinalysis to detect the presence of an impairing substance during a DUI investigation. There are multiple reasons that law enforcement elects to utilize a urine test for the detection of drugs during a DUI investigation. In many states, such as in Florida, blood testing is limited to cases involving death or serious bodily injury. The exposure range for hair follicle testing is approximately 90 days and would, more than likely, be excluded as irrelevant in any DUI prosecution. Similar to a hair follicle test, a urinalysis is incapable of determining when substances were ingested. It is an established fact that a urinalysis will yield a positive test result for impairing substances long after the cessation of impairing effects.

A basic understanding of human physiology is required to understand why a urine test cannot prove impairment. In addition to human physiology, it is necessary to have a general understanding of the scientific principals behind the testing methods. With regard to physiology, the issue comes down to the difference between the renal system and the circulatory system. With regard to the scientific principals, it is imperative to know what constitutes a positive test result.

Regarding the systems, there are three main points to remember. Point 1: The renal system produces and eliminates urine. Point 2: The circulatory system can be looked at as a blood distribution network. Point 3: For the purposes of a DUI urinalysis, there is no correlation between the renal and circulatory systems.

For purposes of determining the impairment of an individual, the only thing that matters is what substances are in the blood. As such a urinalysis testing for impairing substances is an indirect test. In order for a urinalysis to be useful as an indirect test, there must be a correlation between what is found in the urine and what is contained in the blood. As stated above, there is no correlation between the two. Urinalysis in a DUI case will almost always be a qualitative analysis meaning that the lab only tested for the presence of a given substance. A qualitative analysis of urine gives no information as to whether a suspect is impaired. It does not tell how much or when a drug was ingested. The results are simply a "marker" indicating prior exposure to a substance. Performing a quantitative analysis is possible, but it is also pointless. Determining how much of a given substance is present in a suspects urine is irrelevant because of the lack of correlation between urine and blood.

The amount of time that it takes a drug to be undetectable in urine varies by drug. A classic example of this is marijuana. Marijuana's effects typically last for two to three hours. However, marijuana's metabolites are highly fat-soluble. Therefore, the THC and metabolites are stored in body fat. Individuals can show up positive on a urinalysis for a DUI up to four weeks after smoking marijuana. The actual length of time depends on the frequency of use and amount used.

While marijuana is an extreme example detection times, other drugs render the results of a urinalysis in a DUI irrelevant. For example, the metabolites of cocaine can be detected in urine for two to five days after use. Likewise, amphetamines have a short detection window, but can still be found in urine up to five days after use.

Drug metabolism is the breakdown of the parent compound into metabolites. A parent compound is the actual substance that is ingested. A metabolite is the byproduct of the parent compound when the parent compound is "broken down" through the biochemical process after ingested.

Metabolites can be divided into two categories for purposes of a DUI. A metabolite can either be active or inactive. In simple terms, an active metabolite of an impairing substance will have an impairing effect. An inactive metabolite is simply a marker of prior use and has no impairing effect on the individual.

A problem arises in DUI urine testing when an inactive metabolite yields a positive result on a urinalysis. For example, the main metabolite of marijuana is THC-COOH. THC-COOH is a nonpsychoactive substance and highly fat soluble. As such, THC-COOH has no impairing effect on an individual. However, it is detectable in urine for 3 to 4 weeks after using marijuana. The vast majority of tests currently used to determine if an individual has ingested marijuana do not test for THC and do not test for active metabolites of THC. The preliminary and confirmatory tests do not test for any substance that would cause impairment. Accordingly, if an individual tests positive for marijuana, he or she has only tested positive for the inactive metabolite of marijuana. The only thing that this would prove is that the individual smoked marijuana sometime within the last 3 to 4 weeks.

There is a four point analysis to defending a DUI urinalyses case. Issue 1: It is crucial to know exactly what substance is being tested for in the urinalysis. Issue 2: Research what exactly constitutes a positive result. Issue 3: Assuming the test is accurate, identify the time range in which the substance could have been ingested. Issue 4: Compare the known effect of the drug on an individual's behavior. Compare the known effects against the DUI video, the officer's report and a DRE report if any.

In order for the state to prove a DUI, the state must prove that an individual was impaired. Urinalysis alone can not prove impairment. The results of a urinalysis should be considered by an attorney defending a DUI case. However, with an effective cross examination of the State's expert, the results of a urinalysis usually do not carry much weight.