For the most accurate test for detecting THC in the system the blood test is the one to use.  High levels of THC in the blood indicate recent use and low levels may linger on for up to 3 months in some individuals.

Because everyone has a different metabolism the time it takes for THC to leave your system varies.  For people with a larger body mass it generally takes longer to detoxify from marijuana because the THC is stored in the fat cells.  The body needs to break down the fat cells in order to release the THC metabolites into the urine and then released out of the body.  As we age our metabolism slows down so it does take a little longer for older people to detox.  Also, the strength of the marijuana may affect the time it takes for one to be clean as stronger marijuana, like medical grade marijuana contains higher concentrations of the active ingredient in THC, known by its scientific name delta-9-tetrahydrocannabinol.  The frequency of usage will also affect the detox time for the THC to leave the blood because the longer that a person has smoked the more THC metabolites build up in the system.  Blood tests are more difficult to administer so they are usually not used in the case of pre employment testing and are generally reserved for more sensitive employment situations and accident investigations, DUI cases, and job related injury investigations. (To Be Continued)