Testing is a two-step process, the men said. First, urine is checked for broad categories of substances. If any show up, a second test determines concentration levels and identifies specific drugs in those categories, such as codeine, heroin, speed and Valium. For example, a positive amphetamine sample contains at least 500 nanograms per milliliter of urine. A positive marijuana sample contains at least 15 nanograms per milliliter.
Labs protect themselves against false-positive results by collecting two samples.
One is frozen and kept in case a student's family wants the test redone at another lab. For example, cough medicine with codeine would make a test positive for opiates. Each substance has its own molecular fingerprint. Poppy seeds and heroin wouldn't produce the same one. Many labs also test for substances that aren't drugs but that can mask their presence.