Hair testing is a type of drug testing considered highly accurate and can go back normally 3 months (6 months or longer possible with specialist tests), showing any drugs of abuse used in the detection window. As hair grows out, any drugs used are encased in the hair shaft, so the longer the hair, the longer back in the individual's drug history the laboratory can detect. Accredited hair drug testing laboratories, however, only use hair within about 2.5–4 cm of the scalp, and discard the rest. With head hair each 0.5 inch (1.27 cm) corresponds to about 30 days. This limits the detection history to about 90 days, depending upon the rate at which the individual's hair grows. Some people attempt to circumvent this through shaving their heads. This does not usually work.In the absence of the required amount of hair on the scalp, body hair can be used as an acceptable substitute. If all hair is shaven, the follicles of the hair may be used in place of the hair. Removing the hair follicles is more painful.
Additionally, for pre-employment hair testing, the inability to obtain a sample may be grounds for not hiring the individual. There is a growing trend in major companies and law enforcement agencies to utilize hair analysis on account of its efficiency and reputation as the gold standard when considering test accuracy. This technology makes use of radioimmunoassay technology with subsequent confirmation by mass spectrometry.
In recent years, hair testing has been the subject of a number of law suits. Studies have shown that different ethnic groups have different hair structure, potentially leading to false-positive