Hair Testing of Doping Agents: Potential and Limitations

Abstract

Potential detection of endogenous substances or xenobiotic in hair represents a quantitative rather than qualitative challenge. Almost any small molecule is theoretically incorporated and can be detected in hair. The choice of the most suitable matrix for forensic testing is determined by:

• Chemical properties and stability

• Dosage

• Metabolism

• Method sensitivity

• Renal excretion and stability

• Washout from hair

• Potential risk of external hair contamination

Traditional targets of hair testing were compounds showing good hair incorporation but poor (and short) renal excretion. Cocaine abuse can be detected in hair at ng/mg levels for months, while urinary concentrations drop to zero within hours or few days. The absolute amount of cocaine in a typical hair sample (50 mg) is approximately 1 microgram and therefore higher than the substance amount in a corresponding urine test sample (1 mL). Many pharmaceuticals or drugs of abuse exhibit chemical structures which are well suitable for hair testing. The lower polarity, acidity or molecular mass of xenobiotic substances, the better is its hair incorporation. Related to anti-doping testing, most stimulants, certain anabolic agents (clenbuterol, tulobuterol) or anti-estrogens are very good targets for hair testing, whereas neutral (anabolic steroids, selective androgen receptor modulators SARMs) or acidic drugs (e.g., diuretics or most phase 2 metabolites) cause more difficulties, due to their physicochemical structure in combination with lower concentrations and complex biotransformation. However, even substances that are poorly incorporated into hair and rapidly removed by washout can be rewarding targets of hair testing if dosages and blood concentrations are sufficiently high, e.g., ethyl glucuronide, which became one of the most frequently tested compounds in hair due to its high blood levels.