Dose–concentration relationship
A critical question about hair analysis that remains controversial is the relationship between intake dose and concentration in hair. In cases of chronic abuse, daily intake varies significantly from day to day and the establish ment of a dose–response relationship requires a large amount of data to take individual differences into account. Weak dose–concentration relationships may be explained by the following points:
• drug dose of the abuser is uncertain
• purity of the illicit compound is unknown
• uptake of the drug from blood to hair varies between individuals.
Conversely, some articles report a significant dose–concentration relationship for digoxin, cocaine, PCP, cannabinoids, morphine, meprobamate, haloperidol and amitriptyline. These results strongly suggest that a dose–response relationship exists between drug concentrations in hair and the administered dose, which seems particularly true in controlled studies in which a drug is taken for the first time or under close supervision (Kintz 1996). The possible effects of genetic differences in melanin concentrations or in hair porosity are still under discussion. Melanins are responsible for the colour of hair. Two types of melanin are present, eumelanin (with a low sulfur content) and pheomelanin (with a high sulfur content). Both black and brown hair contain more eumelanin than red and blond hair. It appears that it is not simply the concentration of drugs in blood that determines the concentration in hair. Numerous factors may influence the incorporation of drugs into hair, such as the nature of the compounds (pKa , lipid solubility, metabolism pattern) and variation in hair-growth cycles. Until these mechanisms are elucidated, the quantitative results of such a hair analysis should be considered with extreme caution, as should extrapolation to the amount of drug taken (Harkey et al. 1991; Nakahara et al. 1992b).
