Various samples types (including blood, urine, oral fluid, sweat and hair) can be used to test for illicit and prescription drugs. Of all these, hair samples can give the longest window of detection.
The growth rate of hair varies from individual to individual, however a growth rate of 1.0cm – 1.5cm per month for head hair is generally used. Many laboratories request hair samples of a minimum length of 3.0 – 4.0 cm representing 2 to 3 months of hair growth. Testing a hair sample of this length would assist in determining if the donor had routinely used drugs during this period.
There are three main methods of drug incorporation into hair:
- The drugs are incorporated directly into the hair as it grows. The hair root (follicle) is a rich source of blood, and drugs that are present in the blood are incorporated into the hair as the hair grows.
- Drugs can also be found in sweat and sebum (an oily substance secreted by sebaceous glands Hair is continuously exposed to both sweat and sebum and as hair is porous, drugs can be gradually absorbed into the hair.
- Environmental drug contamination. For example, exposure to cannabis or methamphetamine smoke may lead to the surface of the hair being contaminated with the drug.Drugs that have been incorporated into the hair body generally remain there.
Hair samples are usually collected from the back of the head and the amount of sample required generally represents less than a pencil thickness of hair 3 – 4mm. Other body hair can be used, however as the growth rate of body hair can vary considerably it is generally not possible to associate a timeframe to the test result.
The analysis of a hair sample involves several steps including:
- Washing the hair sample. Washing removes surface contaminants from the hair, some contaminants can interfere with the testing methods and this step also removes drugs found on the surface of the hair as a consequence of environmental ‘passive’ exposure.
- Prior to analysis drugs need to be extracted from the hair, this generally involves mechanically and chemically breaking down the hair structure to assist in the release of the incorporated drugs and then extraction of the drugs into organic solvents such as methanol.
- Drug analysis includes immunoassay (for screening) followed by mass spectrometry (MS) confirmation of positive results (LC-MS/MS or GC-MS).
- Prior to reporting a toxicologist will interpret the results to determine if they are consistent with drug use. This process includes ascertaining the levels of each drug in the hair sample and the level of drug metabolites in the hair sample. The presence of the metabolite is indicative of the drug being ingested, for example, the use of cannabis would result in the presence of both delta-9-tetrahydrocannabinol (THC) (the principal psychoactive constituent of cannabis) and its metabolite 11-nor-9-carboxy- delta-9-tetrahydrocannabinol (THC-COOH).
Whilst other drugs testing methods provide drug use data relating to the previous days/weeks, hair testing extends the detection period to weeks/months. This provides a valuable tool in determining historic drug use or monitoring abstinence from drug use in rehabilitation scenarios.