Drug testing is done routinely before new employment is begun and randomly at times when testing seems warranted. Most drug tests, also known as toxicology tests, evaluate for amphetamines, cocaine, marijuana, opiates such as heroin and PCP. False positives occur rarely, around 1 to 2.5 percent of the time, according to NetWellness, so a positive first screen is followed up with a more sensitive test. Some over-the-counter medications or prescribed medications can result in false positives. False negatives can occur with fentanyl, oxycodone and methadone. Several testing methods can be used.
Urine Tests
Urine tests are the most common type of toxicology screening test. A clean-catch specimen, meaning that a thorough cleaning of the urethra is done and the first few drops of urine are voided into the toilet rather than into the cup to avoid contamination. In some cases, a person may be required to produce the specimen in the presence of another person, to avoid substitution of another person's urine sample. The urine sample can also be assessed to make sure it's been produced within the past four minutes by checking the temperature, which should fall between 90 and 100 degrees Fahrenheit, the pH, which should fall between 4.5 to 8, and the creatinine levels, which should measure 20 mg/dL or greater, according to Family Practice Notebook.
Blood Tests
Blood tests are often done to assess blood alcohol levels, but they also serve as toxicology screens. Blood toxicology screens can report not only the presence or absence of drugs but also the amount of drug found in the blood, making it more sensitive than urine screening. A blood alcohol reading remains positive for between three and 10 hours, while opioids remain in the blood or urine to one to two days and cocaine remains for only around five hours. Barbiturates remain in the blood or urine for up to two to six weeks, Medline Plus says. Daily use of marijuana may also show up in toxicology screens for as long as three to six weeks.
Saliva Tests
Saliva screening eliminates the possibility of sample substitution or the need to observe sample production. Saliva testing is 95 to 98 percent accurate, according to LabFocus.com, and can detect drug use within the last one to two days.
Hair Sample Testing
Hair sample testing is non-invasive and tests for drug use within the past three months with 100 percent accuracy, LabFocus.com says.
