The only way to make sure you pass a drug test is the old fashioned way.You have to totally quit and flush your system out with plenty of water along with cutting out fatty food(s),especially fried. That along with exercise and activities will ensure you a clean drug test.Then when your system is clean, studies that were conducted at the university of Missouri showed that infrequent(and a small amount) pot smoking will be out of your system in 5 days.
I tried this method out a few years ago and my test came up clean.