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Is there a test to detect marijuana in a driver's blood?

Iowa's operating while intoxicated law prohibits driving under the influence of alcohol or illegal drugs, including marijuana. In drunk driving cases, police use breath, blood or urine tests to determine a person's blood alcohol concentration. But is there a test to detect the presence of marijuana in a person's blood?

Tests for alcohol and drug ingestion don't directly measure the amount of alcohol or drugs in a person's bloodstream. Instead, they measure the concentration of byproducts, known as metabolites, that are left behind when alcohol or drugs metabolize in the blood.

Alcohol metabolizes fairly quickly in the bloodstream, and so breath and other tests for alcohol impairment are considered accurate measures of impairment. But marijuana takes far longer to metabolize. Marijuana metabolites may be present in the bloodstream for hours or even days after the user was actually impaired by the drug. This makes no difference under Iowa's OWI law, however.

Iowa has a zero tolerance standard for driving under the influence of illegal drugs. If a person has any detectable amount of marijuana metabolites in their bloodstream, they can theoretically be charged with OWI. Proof of actual impairment is not required.

However, Iowa's OWI statute also requires the state Department of Public Safety to apply nationally recognized standards for determining when a driver has detectable levels of an illegal drug in their bloodstream. Currently, the only such standard is for a particular metabolite of marijuana, found in urine but not in blood. As a practical matter, a driver is unlikely to be prosecuted in Iowa unless they have more than 50 nanograms per milliliter of this particular metabolite in their urine.

When a person is charged with OWI based on marijuana use, there are a number of defenses that can be raised. These include many of the defenses that can be raised in alcohol-related OWI cases, such as the accuracy of the testing equipment, the legality of the initial traffic stop and others.

Source:, "Metabolite DUI: Driving with a Measurable Controlled Substance," accessed Feb. 20, 2017

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