A percentage of THC in the blood is a lot more complicated to figure out than your blood alcohol concentration. It all depends on how long cannabinoids stay in your system (a whole lot longer than alcohol does) and how stoned your local lawmakers decide is too stoned.
In many states, as well as several European countries, there is a legal limit set for blood cannabinoid concentration. Just like alcohol, if your concentration level exceeds legal limits, then you can be convicted. However, what happens if it has been a few days or months since you last smoked?
Metabolites, which are small molecules that are produced by the metabolism of marijuana’s chemical compounds, are what drug screens detect. When you consume pot, your liver metabolizes THC by taking HydroxyTHC and converting it into THC Carboxylic Acid. THC Carboxylic Acid is a fat-soluble metabolite that slowly leaves the body.
After a hit or a dab, it takes only minutes for it to enter your bloodstream and most times, it will leave within 24 hours. It can still be detected in your urine, but urine screens are not accurate on how long it has been since you have consumed. Once your body converts the THC-to-THC Carboxylic Acid, it can be detected in your urine for roughly 13 days.
Frequency of use can also be a factor in how quickly cannabinoids leave your system. In a study performed on a Norwegian woman back in 2009, it was shown that chronic smoking of pot causes the length of detection time to be extended. When 25 people were analyzed who were frequent users after they were forced to go without marijuana for a week. The tests showed that the whole blood THC levels were varied between participants, including nine of them not having any trace. However, six participants still showed to have high THC levels in their blood after the seven-day period.