The hair drug test is a method of drug detection that's more reliable and better able to detect historical drug use than urine, saliva and blood samples. Drug tests which sample bodily fluids are really only able to detect drugs that are still lingering in the system from recent use. Hair follicles, however, tend to hold traces of drugs for long periods of time, up to years depending on the length of the sample. This makes it possible to detect a history of drug use even when drugs aren't present in the system. Another advantage is that the test isn't limited to hair from the scalp; it can detect drugs in any follicle on the body. Hair drug tests are capable of detecting a wide array of drugs, including cocaine, opiates and narcotics.
The hair drug test works so well because of traces of drugs that the bloodstream deposits in hair follicles. As the hair grows out, drug traces remain present in the strand. The only way to completely ensure that a drug trace is removed from a strand of hair is to completely remove the strand itself. The shortest follicle is able to measure a period of about 90 days, meaning that a person could have stopped using drugs three months prior to a test, and still come back positive for a history of drug use. The longer the hair samples, the longer the period of drug history able to be measured. Some lengths of hair can take years to grow, making it possible in certain cases to detect drug use over a period of years.
A hair drug test typically requires multiple strands of hair for the most accurate results. The standard test measures at least 50 strands of hair. The best samples come from the scalp, where hair stays rooted for longer periods of time. If strands are absent from the head, body hair samples are also able to return relatively accurate test results.
The hair drug test is one of the better drug screening tests for employers, due to its difficulty to manipulate and ability to detect a history of drugs even when none are present in the system. Whereas there are methods of ridding the system of drugs in order to greatly enhance the chances of passing a blood, saliva or urine test, it is far more difficult to rid hair strands of any drug traces. The simplest way to avoid drug detection when facing a hair drug test would be to completely shave all body hair. This, however, is not a technique lost on most employers, who have reasonable cause not to hire prospective employees with no hair samples.