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Cannabis is one of the most widely used drugs in the world. Marijuana is a type of cannabis with high amount of Δ9-tetrahydrocannabinol (Δ9-THC) which is psychoactive and intoxicating. Δ9-THC affects alertness and concentration, reducing driving skills when one is intoxicated. Law enforcement of driving under the influence of drugs (DUID) is difficult for police because there is no rapid way to test for Δ9- THC intoxication at the scene of an accident or police stop. Currently, police have to take suspects to a facility to acquire blood samples for testing and the average time before testing is more than 2 hours. We developed an analysis using a microplasma collection card for Δ9-THC and Δ8-THC blood testing. With this method, a drop of blood can be sampled from a suspect by law enforcement at the scene of an incident, and analyzed later with liquid chromatography tandem mass spectrometry (LC-MS/MS). The range of the method is 2 to 200 ng/mL of plasma for THC and metabolites with the method which is adequate for the recommended per se level of 5 ng/mL for Δ9-THC. A batch of samples could be analyzed in about one hour with the method.