A Drug Test is commonly a technical examination of urine, semen, blood, sweat, or oral fluid samples to determine the presence or absence of specified drugs or their metabolized traces. 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 laboratory. 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.