Читать книгу An Introduction to Testing for Drugs of Abuse - William E. Schreiber - Страница 33

Drugs in blood diffuse into the hair follicle, where the hair shaft grows. As the keratin matrix of hair is formed, drugs are incorporated and remain there, moving outward as the length of the hair shaft increases. A human hair is therefore a biological record of drug exposure.

The unique advantage of hair testing is the extended time period over which drugs can be detected. Hair samples can test positive as early as 1 week following drug use, and the window of detection is months or even years, depending on hair length.

Hair is usually collected from the crown of the head. The average growth rate for hair at this site is about 0.5 in (1.3 cm) per month. Sample collection is fairly simple – multiple strands of hair are cut at scalp level and sent to a testing facility for analysis. Collection can be witnessed or performed by trained personnel to prevent substitution or adulteration of the hair sample.

The laboratory may test a defined length of hair, which corresponds to an approximate period of time (i.e., hair trimmed to 1.5 in will detect drugs taken within the past 3 months). It is also possible to cut hair into short segments, which are analyzed separately. By comparing the test results on adjoining hair segments, an approximate timeline of drug exposure can be determined.

Analysis of drugs in hair is technically more demanding than other specimens. Hair samples must be washed to eliminate any surface contamination, and drugs must then be extracted from the hair shaft into a liquid medium that can be analyzed. This is a specialized procedure that is not available in most routine toxicology laboratories.