Hair testing accuracy for drug and alcohol use and why it matters

In court, witness integrity can make or break a case. For trials involving substance use, the accuracy of a drug test for instance is paramount; any element of doubt means it can’t be depended on as evidence. In turn, cases like the one involving Plymouth mother Rose Jones – accused of poisoning her children with Tramadol – can go on indefinitely.

Rose Jones was only finally convicted after hair samples were taken from her young daughter, bringing her case to a close.

Hair strand testing for alcohol or drugs is among the most accurate tests for drug and alcohol use over long periods. But how accurate is accurate?

The answer is, it depends: on the laboratory conducting the test and their interpretation of the results, the type of hair test, and the circumstances of the sample collection.

How hair testing works, and what this means

Hair tests can be used to detect drug and alcohol use over months and years.

When drugs or alcohol are ingested, the body creates metabolite substances which pass into the blood. These metabolites are taken up by hairs; as hair grows above the scalp, traces of the substance used are exposed for testing.

Because hair strands on the scalp typically grow one centimetre each month, a one-centimetre section of hair can be used to detect drug or alcohol use over one month. A six-centimetre sample will indicate use over six months, and so on. As such, the window of detection is limited only by the length of the hair.

Performed professionally and precisely, a hair drug test can offer the most accurate means of detecting for drugs and alcohol use over months. However, accuracy and reliability of results can be affected by several factors. These include the type of hair test, the conditions for the test, and the interpretation of the laboratory performing the test.

Types of test

After the hair sample arrives at a laboratory and the sample is liquefied, it can then be subject to different rounds of testing. To an extent, the type of test determines how accurate the result will be.

Some laboratories test hairs using an immunoassay only; an immunochemical test which indicates the presence of a substance in a sample. The immunoassay method is useful for screening out negatives for drugs and alcohol – that is, for showing when a donor has not ingested a substance tested for. For this reason, it is most appropriate in circumstances like workplace testing, where the majority of donors will be not have ingested the target substances.

Other laboratories – including Cansford – use more sensitive and specific test methods, which are appropriate for circumstances where the donor is likely to have ingested a substance.

We test using LC-MS/MS; Liquid Chromatography with Mass Spectrometry. This two-part method separates the compounds in the hair extract, and then breaks the compounds into fragments – in turn identifying traces of cocaine, heroin, and other specified substances beyond any margin of doubt. Test results are then measured against calibrators and controls and reviewed by a second toxicologist to confirm the result.

The importance of interpretation

Review by a second toxicologist – a process assessed and accredited by UKAS according to ISO 17025 – is important given the role that interpretation plays in hair testing.

Toxicologists must judge whether any external factors have affected the amount of drug or alcohol metabolites in a hair sample, and whether this effect is enough to change the result of the test.

The process by which testing laboratories gather information about the hair sample, and the experience and expertise of the toxicologist, therefore affects the accuracy of hair tests. In turn, the issue could make or break a case.

Factors affecting test results

The more detail a donor can provide about their hair sample, the more reliable and accurate their test result will be. As such, a reliable laboratory will seek to gather as much information as possible about each sample – and the lawyer or the donor themselves will want to provide everything they know about the circumstances of collection. Factors which can affect a hair test result include:

• The window of detection. Hairs on the scalp only show after approximately seven days. Using hair tests to detect acute, or very recent, drug or alcohol use is therefore not appropriate.

• Use of medication. Some types of medication – including codeine – can give the appearance of drug use.

• Hair colour. Some substances bind with the melanin in dark hair more than in lighter hair, affecting trace levels.

• Bleaching. Hair samples can be damaged by colour treatments. As such, a donor may repeatedly bleach and wash their hair to attempt to remove traces of substances from their scalp.

• Ambient exposure. A hair strand drug test can detect exposure to drugs or alcohol which have not been ingested – for example, when a donor has been in the same room as someone smoking cannabis. In this way, testing for the presence of metabolites that only appear after a drug has been ingested can help a toxicologist’s interpretation, as can  analysing the solvent that was used to wash the sample before analysis.

Expert toxicologists can build this information into their interpretation of hair test results. For example, knowing when a hair donor used medication helps the toxicologist to look for patterns in a donor’s test results, against other substance use.

Not having access to this information can affect the accuracy of a hair test result. As such, laboratories should request information about the circumstances of a donor’s sample before delivering test results. Failure to do so can indicate a lack of reliability on the laboratory’s part – and the accuracy of their interpretation should be questioned.

In past cases, the results of a drug test with hair have been re-evaluated and changed when new evidence about the conditions for the sample collection was made available. For example, a female donor was found to have traces of cocaine in her hair. When the donor revealed that she historically had been a heavy user of the drug, our laboratory was able to confirm that the traces were from previous months and were not current. This caused the result of the test to be reviewed.

The final word

How accurate are hair tests for detecting alcohol and drug use?

Answer: Highly accurate compared to other testing methods, providing the substance use to be detected is not acute; the laboratory performs tests above and beyond immunoassay, and the conditions for the sample are known and acted upon by the laboratory performing the test.

To find out more about hair testing – including the process and costs – download our free 20-page ebook, ‘The complete guide to hair testing for drug & alcohol’.