The different types of drug tests are tested in very similar ways. Before testing the sample, the tamper-evident seal is checked for integrity. If it appears to have been tampered with or was damaged in transit, the lab rejects the sample and does not test it.

One of the first steps for all drug tests is to make the sample testable. Urine and oral fluid can be used "as is" for some tests, but other tests require the drugs to be extracted from urine beforehand. Strands of hair, patches, and blood must be prepared before testing. Hair is washed in order to eliminate second-hand sources of drugs on the surface of the hair, then the keratin is broken down using enzymes. Blood plasma may need to be separated by centrifuge from blood cells prior to testing. Sweat patches are opened up and the sweat collection component is soaked in a solvent to dissolve any drugs present.

Laboratory-based drug testing is done in a two-tiered fashion using two different types of detection methods. The first is known as the screening test, and this is applied to all samples that go through the lab. The second, known as the confirmation test, is only applied to samples that test positive during the screening test. Screening tests are usually done by immunoassay (EMIT, ELISA, and RIA are the most common). A "dipstick" drug testing method which could at some future time provide screening test capabilities to field investigators has been developed at the University of Illinois.^[3] Screening tests are typically less sensitive and more prone to false positives and false negatives than the confirmation test.

Once a suspected positive sample is detected during screening, the sample is flagged and tested using the confirmation test. Samples that are negative on the screening test are discarded and reported as negative. The confirmation test in most labs (and all SAMHSA certified labs) is performed using mass spectrometry, and is extremely precise but also fairly expensive to run. False positive samples from the screening test will be negative on the confirmation test. Samples testing positive during both screening and confirmation tests are reported as positive to the entity that ordered the test. Most labs save positive samples for some period of months or years in the event of a disputed result or lawsuit.

D.O.T. mandated tests require a Medical Review Officer, an M.D. or a D.O., to contact the donor to determine if an alternative medical explanation justifies the presence of the drug identified by the lab. Doctors need not be licensed in the state where the donor resides nor where the test was administered. She must be a licensed physician in one state within the U.S. to participate as an MRO. This substantially limits the degree of influence from out of state medical review boards. In addition, many courts do not allow a private cause of action against MROs, labs, collectors, etc. If an MRO and a D.O.T. participating employer conspire to falsify drug test results, the innocent donor has little if any recourse. The perception that American citizens are "innocent until proven guilty" is just that. Specific mandated drug testing regulations which entitle access to drug testing chain of custody and procedural evidence from the commercial sector may be discounted or ignored by the ODAPC and government agencies. In other words, the government may chose not to enforce their own regulations. To address negligence and abuse, agencies may utilize a PIE (Public Interest Exclusion) to restrict doctors, labs, hospitals and others from participating in D.O.T. testing. Since the implementation of this kind of testing over a decade ago with tens of millions of tests performed, a PIE not has been issued. [needs citation as to accuracy of tests]

 *source Wikipedia