Although there are circumstances in which the mere presence of a drug on the surface of the hair may provide significant information, the vast majority of situations require that external drug contamination be distinguished from presence of drug due to use. These environmental issues involve passive exposure of the hair surface to dust, aerosols, smoke, or powders. Although drugs in a subject's sweat may also deposit onto the hair, only a subject who uses drugs is likely to have drug-containing sweat, and thus sweat may affect quantitation but is less likely to cause an erroneous qualitative (i.e., false-positive) conclusions. This is not to imply that the need for removing sweat by washing is unimportant—in fact, its importance will be discussed further under Subheading 6. Removal of contamination has been attempted by various means, including multiple washes with organic and aqueous phases for various periods of time. For example, Koren (26) used extended washes with ethanol and was able to remove from hair all contamination due to cocaine free-base vapors. Wang et al. (27) were unable to remove cocaine contamination from hair using three 1-min methanol washes; their work was re-investigated and compared to extensive aqueous washing by Schaffer et al. (28). Marsh et al. (29) compared three methods for removal of contaminating methadone on hair: four 15-min methanol washes at 37°C, two 15-min acetone washes followed by two water washes, and three 15-min washes with 1% dodecyl sulphate at 37°C followed by two water washes. The latter authors found, in most cases, greater than 90% removal of contaminating drug. Although adequate washing procedures can usually remove most contaminating drug (2,30,31), it is critical also to identify those samples that may still exceed the cut-off as a result of the small percentage of contaminating drug that may not have been removed by the wash procedures. In the authors' laboratory, a very extensive wash schema includes 3.75 h of washing, and subsequent testing and evaluation of the last wash to better assess the effectiveness of the process. In this process the hair is washed at 37°C for 15 min in dry isopropanol to remove greasy contamination and loosely adhering drugs from the surface of the hair. This is then followed by three subsequent 30-min and two 60-min washes with phosphate buffer containing 0.1% bovine serum albumin (BSA), with all the washing done at 37°C and with vigorous shaking. From analysis of the last wash for cocaine, opiates, amphetamines, or PCP, the amount of drug in the last wash relative to the amount of drug remaining in the hair is used to calculate a wash criterion to distinguish between drug use and contamination (31).
The necessity of an effective wash procedure, validated by testing on contaminated specimens and known positives, is generally recognized as a necessary precaution in determining drug use. Some laboratories, however, may attempt to skirt the requirement by reporting results indicating only the presence of drug, without interpretation regarding evidence of use, or relying on the presence of metabolites to indicate use. The presence of metabolites in the sample can sometimes constitute evidence of use in spite of contamination issues; however, without effective washing, this is true only if the metabolite is formed exclusively in vivo and is not found in the source or parent drug. For example, ben-zoylecgonine (BE), cocaethylene (CE), and norcocaine are all metabolites of cocaine, but only cocaethylene is a definitive marker of use, because it is formed only by simultaneous ingestion of cocaine and ethanol. An effective wash coupled with a metabolite profile and a judicious cutoff level are the requirements of an appropriately conservative approach to drug testing. Studies using extensive wash procedures have demonstrated that even in extreme exposure scenarios such as undercover narcotic officer's hair exposed directly to cocaine vapor and persons exposed in the room with users, contamination can still be distinguished from ingestion (26,32).
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