Also known as urinalysis, this procedure requires that one provide a sample of urine. Either a test card is used on site for immediate results (see "General" section), or the sample is sent away to a lab to undergo gas chromatography/mass spectrometry (also known as GCMS), high performance liquid chromatography or immunoassay analysis. Sample substitution or adulteration have become a significant issue in the United States due to the prevalence of synthetic and/or drug-free urine and a wide range of adulterants on the internet.
Some people attempt to defeat a urine drug test by drinking copious amounts of water, however, a sufficiently diluted sample may be rejected due to its clear color. Samples that are too clear may be flagged and tested for specific gravity. If the sample fails the specific gravity test, the sample is rejected and the dilution is reported to the entity that ordered the test. Some diuretics and herbal extracts, such as goldenseal, are marketed as a quick "detox" from controlled substances, but their effectiveness is questionable.
Some types of urinalysis can even detect the use of these "detox" products. One of the methods to test for adulterants is to add some amount of an actual drug to a small portion of the sample and then retest that portion. If a masking agent is present in the urine, the resulting drug test will have a negative result despite the fact that a drug was added. This situation is also usually reported to whomever ordered the test.