Having a blood test to determine drug use is not very common, but it is very costly. This test is usually reserved for insurance policies, or if a person is seeking employment where safety issues are mandatory.
To administer a blood test, a sample is taken and sent to a lab to test for drug use.
Blood tests, unlike urinalysis, detect the presence of illicit drugs, not inactive drug metabolites. In general, THC only remains detectable in the blood of cannabis consumers for a few hours (though low, residual levels may be detected in chronic smokers for up to 12-24+ hours if more sensitive technology is used). Because of this narrow detection window, blood tests are typically only administered in the workplace post-accident in order to estimate recent cannabis consumption. Therefore, most after-hours consumers have little to fear from a blood screen.