It is not uncommon for people to mistakenly believe that because their blood or urine test showed the presence of marijuana, they are guilty of DUI. This is wrong. In Georgia, it is not unlawful/illegal to have marijuana in your blood or urine and drive a vehicle. It is only illegal to drive with marijuana in your body if your ability to drive is impaired as a result of marijuana.
The National Highway Traffic Safety Administration has acknowledged that the presence of marijuana in a person's body does not necessarily show that the person's ability to drive has been affected:
The reader is cautioned that drug presence does not necessarily imply impairment. For many drug substances, drug presence can be detected after impairment that might affect driving has passed. For example, traces of marijuana use can be detected in blood samples several weeks after heavy chronic users stop ingestion. In this study, for marijuana, we tested only for THC (delta 9 tetrahydrocannabinol), the psychoactive substance in marijuana, and 11-OH-THC, its active metabolite. When marijuana is smoked or ingested, THC is absorbed into the blood stream and is distributed into areas of the body, including the brain. There are over 100 marijuana metabolites detectable in blood that research has not associated with the psychoactive effects of marijuana use.
Source: NHTSA - https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/812118-roadside_survey_2014.pdf