Drug test vary on which metabolite they screen for. Only delta 9 THC produces impairment while 11-COOH-THC can be in your system for weeks to months. More accurate tests measure the ratio of 11-OH-THC to 11-COOH-THC to predict time since impairment with delta 9-THC.
CYP2C9 is a p450 cyctochrome gene which is thought to metabolize these compounds. DNA polymorphisms in this gene have are predictive of Warfarin sensitivity and are likely important to other drug metabolism.