Let us provide a rational answer to a nonsensical question. It is a nonsensical question because blood is never impaired by THC. Never. Alcohol doesn’t impair blood either. These drugs only impair the brain, not the blood.
Links
[1] http://www.duidvictimvoices.org/what-level-of-thc-in-blood-causes-driving-impairment/
[2] http://rethinkpot.org/tags/driving
[3] http://rethinkpot.org/tags/impaired
[4] http://rethinkpot.org/tags/drug-testing