THC can only be detected in a blood test for a max of 3-5 days for a chronic user. Usually just 24 hours for a non chronic user. THC can be detected 30-60+ days for a chronic user and usually 3-7 days for a one time user. Many variables are in the formula with the most important being body metabolism and height and weight and history of usage. Immigration usually checks with a background investigation, physical examination which does include a urine test and blood test. After 911 I'm sure they check everything. Good Luck.