George Roland | Denton Criminal Defense Lawyer

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Marijuana DWI’s in Texas: Blood Tests

Increasingly, driving while intoxicated charges are being filed against people who drive—not under the influence of alcohol—but under the alleged influence of marijuana. Often enough, a warrant is sought for the person’s blood so that it can be tested for the presence of marijuana, drugs, and prescription medication. A blood test showing marijuana metabolites can look like damning evidence for the State, but what the test really shows is little, if anything, other than that a person used marijuana, at some point.



Who Tests the Blood and What Is the Blood Tested For


First, the blood is typically sent the regional lab (in Garland for Denton County cases) of the Department of Public Safety. Here, the blood is only tested for alcohol. If the blood returns no alcohol, or only a very low level of alcohol, the blood is sent to a second lab for toxicology testing for the presence of marijuana, prescription medications, and drugs.


The Department of Public Safety crime lab in Austin will then will test the blood for marijuana, prescription medication, and drugs using immunoassay tests. “Immunoassay” simply refers the idea that the lab is using biochemistry to measure the presence of a specific substance—in the case of a marijuana DWI, the lab is testing for the presence of Delta-9 THC and 9 Carboxy-THC. Delta-9 THC is the primary metabolite of marijuana, whereas 9 Carboxy-THC is a secondary metabolite of marijuana.


The Results


Where either metabolite is present, the results are typically measure in nanograms per milliliter (ng/ml). So, for example, a lab might show that there were 6.6 nanograms per milliliter of Delta-9 THC in a person’s blood. It is important to note that it would be very odd to see a result where there was Delta-9 THC in a blood sample, but not 9 Carboxy-THC; as they are both metabolites of marijuana.


What The Results Actually Mean


What the results of the lab test showing Delta-9 THC and 9 Carboxy-THC actually mean—versus what the lab technician might attempt to testify at trial they mean—is, to put it generously, not a lot.


No per se Intoxication Level of Delta-9 THC or 9 Carboxy-THC


For starters, there is no “per se” level of intoxication for marijuana in Texas. For alcohol, we all know that .08 is the “legally intoxicated” (or “per se” intoxicated) level; however, no such level exists for marijuana. So regardless of what the results of the test are, it is impossible that they equate to a per se level of intoxication.


The lab technician CANNOT testify that a person is “legally” intoxicated (per se intoxicated) based on amount of Delta-9 THC or 9 Carboxy-THC in a person’s blood.


Unable to Equate Level to Any Specific Degree of Impairment


For alcohol, there are well-established studies that show different blood alcohol levels and the degree of impairment typical at such levels (slurred speech, loss of motor skills, etc.) However, for marijuana it is not possible to equate a given level of Delta-9 THC or 9 Carboxy-THC to any specific degree of impairment. Even NHTSA (the folks who teach the cops the roadside tests for DWI stops) admits this much: “Peak THC levels can occur when low impairment is measured, and high impairment can be measured when THC levels are low. Thus, in contrast to a situation with alcohol, someone can show little or no impairment at a THC level at which someone else may show a greater degree of impairment.” (July 2017 Report.) Put simply, regardless of THC level, a person could show little, or no impairment.


The lab technician CANNOT equate the level of marijuana to any general or specific degree of impairment.


Timing of Use


Unlike alcohol, which generally is metabolized quickly because it is water soluble, marijuana is fat soluble and metabolizes much slower. THC is stored in fatty tissue, where it can remain detectable for up to 30 days after smoking. Conversely, the actual psychoactive effect (“high”) of marijuana lasts only hours. This is a problem for DWI’s where the relevant period is “while driving;” the State must show that a person was intoxicated (“high”) while driving—not that the person smoked pot two days ago or thirty days ago but still has Delta-9 THC in her blood. So, where police do not smell burnt marijuana, or find drug paraphernalia, or have an admission of recent use from the accused person, the best the lab technician could testify to with regard to the timing of use was that the accused used sometime within the past 30 days.


Additionally, the level of Delta-9 THC depends on a wide variety of factors: weight, manner of ingestion (smoking or edible), type of pot (wax or regular “dirt weed”), how often the person smokes marijuana, etc. The lab technicians almost never have this much information about the subject whose blood they tested.


Lab technicians CANNOT accurately testify about how recent the use of marijuana was based solely on the blood.


The information you obtain at this site is not, nor is it intended to be, legal advice. There are no two cases that are the same. You should consult an attorney for advice regarding your individual situation. I welcome the opportunity to serve you and invite your calls, letters and electronic mail. Simply contacting an attorney does not create an attorney-client relationship.