Although they are sensitive and function well as screening tests, they are susceptible to cross-reactions, resulting in false-positive results, which limit their specificity. They are easy to perform, relatively inexpensive, and use immunoassays, such as enzyme-linked immunoassay or radioimmunoassay, that give instant positive or negative results. Qualitative tests are point-of-care tests and home drug test kits. Two types of drug testing assays are available: qualitative tests usually used for screening and quantitative tests used for confirmation.
17, 18 No point-of-care tests are available laboratory testing is costly, and it takes a long time to obtain results.
Drug concentrations in hair can be altered by shampoos, bleaches, or dyes, and false-positive results may be obtained with volatile drugs such as marijuana, which may adhere to hair. In addition, hair structure, growth rate, melanin content, hygiene, and cosmetic treatment can affect the results. However, hair testing is not useful clinically, because it has a long window of detection. Hair collection can be directly observed and is noninvasive, hair is easily stored and transported, and adulteration and substitution is difficult. However, hair grows at a slower rate on the body, and a time frame cannot be used in this circumstance. If the scalp is shaved, hair from other body areas may be sent instead. If longer hair is sent, the laboratory will cut it into segments before testing, and 4 tests will allow for screening over a period of a year. The hair needs to be cut as close to the scalp as possible, and usually the first 3 cm is used for testing, which covers a 90-day period (hair grows 1 cm a month).
It is most reliable for heavy, frequent past use and not for detection of recent or occasional drug use. Hair cannot be used for detection of use in the previous 7 to 10 days. Hair testing allows for detection of past use that has occurred over an extended time because drugs and metabolites are incorporated into the hair matrix over time.