An ‘alternative,” “complementary’, and/ “unconventional” matrix for forensic drug testing, hair is being commonly used as it can provide an extended history of drug use and/or exposure. While conformist matrices like blood and urine document an individual’s drug exposure for a period ranging from minutes to days, hair can extend the detection period for months to years depending on the hair sampled and the collection process. Furthermore, conformist matrices can be both physically and socially invasive to gather can need preservation through refrigeration or freezing, and as in the case of urine, may be vulnerable to adulteration/substitution. In contrast, hair is easier to collect relatively stable, and complicated to adulterate or substitute. Figure 6.1 compares the biological matrices currently utilized in workplace drug testing (WPDT) including blood, urine, oral fluid, and hair.
Even though hair analysis for drugs of abuse has loads of advantages, it has a number of issues that continue to challenge its utility for forensic testing. These issues include the absence of consistent techniques between laboratories, consistent proficiency testing materials, a laboratory certification program, consistent results within and between laboratories, and easily identifiable drug analytes that will discriminate between environmental contamination and drug use, as well as the presence of a potential bias of drug incorporation into hair (i.e., color or ethnic differences).
The extent to which a drug analyte is incorporated into hair is affected by hair growth patterns and biological and ecological factors influencing its growth. For instance, the drug content in plucked hair is different from that found in shed hair because shed hair undergoes a resting period (no growths) before falling out. Factors influencing intra-individual growth rate of hair include age, gender, ethnicity, heredity, climate, health, injury, and physical stress as well as anatomical site of hair growth.
Head hair develops an average rate of 1.0 to 1.3 cm/month (0.35 mm/day} with the growth rate depending on its location on the head. Hair from the vertex Crown region of the head grows fastest (1.3 cm/month), followed by temporal and occipital (1. 1 cm/month), and frontal (1.0 cm/month). It takes 10 to 15 days for newly incorporated matrix cells of the hair to move from the root to the surface of the scalp. Therefore, acute drug use would not be captured by hair analysis. Average growth rates at anatomical sites other than the scalp are as follows: Axillary- 0.9 cm/month; facial- 0.8 cm/month pubic- 0.9 cm/month; and chest- 1.3 cm/month. Although hair from many of these anatomical sites has been collected for workplace drug testing, it accounts for only 5% of the collected hair specimens.
Hair undergoes stages of growth (anagen stage) rest (telogen), and degradation (catagen). For scalp hair, these stages last up to 2 to 5 years, 100 to 150 days, and 35 days, respectively. The length of each cycle and the ratio of growth to rest vary depending on the body region, age, and gender. The growth-to-rest ratio of chest hair is less than or equal to 1.0 in comparison to the growth-to-rest ratio of scalp hair of 9:1 (89. Individual hair shafts do not have synchronous growth cycles. On a healthy head at any given time 80 to 96% of the hair follicles are in the anagen phase, 5 to 18% in the telogen phase, and 0 to 2% in the catagen phase A, 12).
In general, drugs may enter the hair through multiple routes and more than one may contribute to the amount incorporated. Processes proposed for the incorporation of drug into the hair matrix include (1) passage of the drug from the blood supply to the hair by simple diffusion via cell membranes; (2) diffusion of drug into glandular secretions such as sweat or sebum and the absorption of the drug as the secretions bathe the hair follicles; (3) exposure to an external environment containing drug residues (i.e., contaminated surfaces, or vaporized drug); (4) exposure of the hair fibers to transdermal diffusion of drug from the slain. Figure 6-2 shows the anatomical configuration of a hair follicle and the surrounding skin morphology.
Many of these routes require a free, non-ionized and lipid-soluble drug for entry into the hair shaft. Given that the normal hair pH range is between 4.5 and 5.5, it is more acidic than blood pH 7.4. Since the pH of hair is lower than blood, weakly basic drugs are detected in higher concentrations in the hair than in blood.
The integration of drugs into hair is influenced by many physiochemical factors. Firstly, the functional groups of intracellular components such as melanin and melanin-associated proteins enhance drug binding. Secondly, the occurrence of acidic and basic functional groups promotes binding of some drugs while repelling others. Lastly, cellular permeability preferentially allows the passage of basic drugs such as cocaine and codeine (high pKa) over neutral or acidic drugs into the developing hair. The preferential incorporation of analytes by their lipophilic properties is further illustrated by the observation that cocaine is found at much higher concentrations than its more polar metabolizes, benzoylecgonine and ecgonine methyl ester.