There has been a great deal of controversy over the use of hair testing and its uses in federal courts and in employment situations. These controversies vary in topic from arguments about external contamination to the changes cosmetic treatment causes in hair to ethnic bias. Sample sizes are usually quite small due to aesthetic concerns, and concentrations of drugs in hair are small requiring a sensitive and specific protocol and detection methodology instrument. Typical hair matrix is complex and variable, and makes a standard ratio of the amount of cocaine detected to the amount of cocaine ingested hard to identify.[5] In addition, it is still not understood how cocaine is incorporated into hair, a process which is likely to have varying efficiencies for different individuals. It has been suggested that drugs can become incorporated into the hair matrix through blood, sweat, and external contamination. External contamination occurs when one is in the presence of cocaine smoke, but is not smoking cocaine himself. Within the hair matrix, it is still unknown whether the drug is bound to the matrix itself, the keratin fibers, or the melanin fibers in the hair. While the problems associated with analytical methodology necessary to detect reasonable amounts of drugs in hair with good specificity have been solved, hair testing for cocaine use will be problematic until a clearer understanding of incorporation is developed.[11]

Questions about ethnic bias arise from the different structures and porosity of hair from different races. Differences between gender and age also exist in hair. The standard of one value as a cut-off between positive and negative samples, as used in urine testing, does not hold in the case of hair testing. In addition, the amount of drug ingested does not correlate to the amount of drug found in the hair. In some studies, larger amounts of cocaine have been found in the hair of occasional users than in frequent users. Hair testing cannot determine the amount of cocaine ingested by an individual because of this restriction, and thus this technique remains a marker for some individuals who have used cocaine within a limited time frame (weeks to months).[7]

Cosmetically treating hair can lead to a decreased level of cocaine, as well as other drugs present, detected as compared to levels detected prior to treatment. Cosmetic hair treatments which negatively impact cocaine detection include coloring, bleaching, and perming due to the changes in porosity of the hair induced by these treatments.[21]

The Society of Forensic Toxicologists, The National Institute on Drug Abuse, and the National Institute of Justice held a conference in 1989 called the Conference on Hair Analysis for Drugs of Abuse.[22] They agreed that: (1) the use of hair testing for employment purposes is premature, and (2) hair testing must be accompanied by blood or urine testing in courts to be admissible. The second clause was later changed (1989) to allow hair testing results in the presence of "competent evidence."[23]

There are several unanswered questions regarding hair testing including: (1) What is the best way to prepare hair for testing? (2) To what extent is external contamination a problem and how can it be eliminated? and (3) What constitutes a positive or negative result? Therefore, while much can be achieved via testing of hair samples for cocaine use, additional research is needed to make the technique practical.[12]

The issue of external contamination warrants special consideration. It is not thought to be possible to remove all external contamination from hair by washing.[12] Higher concentrations of cocaine have been found in unwashed samples from non-users who were externally contaminated than in regular users.

The samples which are used in most of the controlled testing of procedures are, unfortunately, not authentic due to the inadequacy of self-reporting of illicit drug use. The "authentic" samples used in laboratories are prepared by soaking hair in cocaine solutions. The "contamination" so produced is not identical to purely external or internal contamination limiting their effectiveness as standards for real hair samples. Procedures have been developed to remove the majority of external contamination and criteria have been developed to distinguish external from internal contamination.

An attempt to determine cut-off values for positive and negative results, has been conducted.[13] A cut-off value is the value which is the highest concentration of cocaine in a sample allowed without a positive result of drug use. Reference hair, which was contaminated in the laboratory (via soaking, vide supra), was used in this study. Using hair soaked in cocaine solutions allows internal calibration and optimization of the method.[14]

Cocaine is found at concentrations in the range of nanogram of cocaine per milligram of hair. In cases of cocaine abuse, more cocaine than any of the cocaine metabolites is usually found in the hair. This is due to the quick incorporation time and not to the half lives of the substances themselves as the half life of cocaine is far shorter than the half life of any of its metabolites.[15]

Because of the ambiguity of actual drug ingestion and the possibility of external contamination, hair testing can come back with one of three results. These results are positive, negative, or contamination but no use. The contamination but no use category is split into two more categories: trivial contamination and extensive contamination. These results are returned when the drug level is near the cut-off value set by the testers, but no metabolites can be identified. Trivial contamination exceeds the cut-off value by a small measure. These results are usually found for subjects who spend a lot of time in the presence of drug users. Extensive contamination is found in cases of drug dealers and others who are heavily involved in the drug scene, but who are not active users. These values exceed the cut-off value significantly.[13]

The major metabolites of interest formed from cocaine and found in hair are benzoylecgonine and cocaethylene. Benzoylecgonine is the metabolite which is most useful in hair analysis because it is always formed by the body in the presence of cocaine. In order for cocaine to be metabolized into cocaethylene, the drug user must also drink alcohol while cocaine is present in their system. Because this is not the universal case, cocaethylene levels cannot be used as an absolute marker of cocaine. Metabolites are helpful in determining if high levels of cocaine in the sample are due to external contamination. If the cocaine detected is due to drug use, and the hair is therefore internally contaminated, then metabolites will be present, albeit, often in small amounts. Externally contaminated hair, will contain no metabolites since the drug never entered the subject's system where it could be metabolized. Metabolites (e.g. benzoylecgonine, cocaethylene) are typically found in hair at 10-35 % of the concentration than cocaine is.[13] One concern in using metabolites to avoid false positives with respect to illicit drug use is that some of the cocaine in a hair sample may be converted to benzoylecgonine during the extraction phase (vide infra). It has been found that 5% of the cocaine present may be converted in the extraction phase if acid or base treatment is used, while no change takes place if enzymatic treatment is used.[16] A problem with using benzoylecgonine in a routine analysis along with cocaine detection is that benzoylecgonine, the most prevalent metabolite, is more polar than cocaine and therefore is much less soluble in many common extractors.[12] Other metabolites of cocaine include norcocaine and ecgonine methyl ester which are the results of oxidation of cocaine by P450(CYPs) by serum cholinesterases. Norcocaine can be further oxidized to N-hydroxy-norcocaine. Norcocaine is the only pharmacologically active metabolite of cocaine.[17]

The method of incorporation of drugs into hair has also been one of much debate and is still uncertain. The simplest model proposes that passive diffusion of the drugs from the blood stream into the growing cells of the hair occurs. The drug is then tightly bound into the matrix of the hair shaft through keratogenesis, or binding to the keratin fibers of the hair. It is also believed that considerations must be given to incorporation through sweat and external adhesion.[9] Other constituents of hair to which cocaine may bind which must be considered are melanin, cheratin, and cell membrane complexes.[5] The complicated matrix of the hair not only causes difficulties in determining the binding of the drug, but also in determining how to get the drug out of the hair. Experiments done on teflon wool indicate that supercritical fluid extraction with pure CO2 is sufficient to remove all cocaine from the sample, but when using real hair a modifier must be added to the CO2 in order to release the cocaine trapped in the matrix. A modifier slightly alters the properties of polarity and solvent abilities of an extracting fluid. Modifier tuning has been used to target specific binding sites and may help determine where the drug is actually bound to the hair.[12]

Although there is a great deal of controversy surrounding the hair analysis procedure, great strides have been made in recent years to eliminate many of the problems with the procedure. Although the method of incorporation of drugs into hair and the direct relationship between cocaine and its metabolites has not been determined, research is being conducted in these fields to help eliminate these problems. In addition to the ongoing metabolite and incorporation research, washing procedure research has been fine tuned and improved to give a fairly reliable system of washing to ensure that most external contamination is removed. Hair testing has been recognized and regulated by the Society of Forensic Toxicologists and accepted by courts of law in some types of cases. It is also important to determine whether the positive result stems from the person taking the drug or whether an innocent person was exposed to the smoke of the drug which yielded a positive result through external entry. Although it is not routinely accepted in a court of law, hair analysis for drugs of abuse may soon be permitted in court regularly since techniques are becoming universal and standards for preparation and detection for the drugs of abuse are being developed.