Urine tests for tetrahydrocannabinol, or THC, a chemical found in marijuana, can detect use of marijuana within the last 13 days, according to About.com. However, heavy users of marijuana may have detectable THC in their urine for up to 90 days after their last use of marijuana.
As of 2015, urine tests for THC are almost never false-positive due to today's sophisticated tests, says About.com. The initial test uses an immunoassay to detect metabolites. If the initial screening test is positive, then it is tested again with a gas chromatograph mass spectrometer to confirm it is a true positive. No known substances are capable of causing a false-positive on modern urine THC tests.