“Currently, THC-blood tests are the norm in most states that have laws regulating marijuana use while driving. They generally dictate that drivers caught with THC in their bloodstream at a level of five nanograms of THC per milliliter of blood are considered ‘under the influence’ and may be subject to penalty. But unlike blood-alcohol level, which have a direct correlation with a driver’s intoxication, blood-THC levels don’t necessarily indicate a level of impairment. In one study, six of 25 participants who had consumed marijuana still tested positive for active levels of THC after a full seven days of abstaining.”
http://www.huffingtonpost.com/2013/10/22/marijuana-breath-test-pot-dui_n_4143264.html