The importance of expert interpretation in hair testing

Hair testing is an established tool in family proceedings, providing a retrospective view of drug and alcohol exposure over several months and is particularly useful when combined with tests for recent use, using samples such as blood, oral fluid, or urine.

However, laboratory analysis is only part of the process.

Here, we look at why expert interpretation matters, and how drawing on the broader circumstances of the case ensures hair test results are interpreted fairly and accurately for family law cases.

A positive result alone doesn’t tell the whole story

A common misconception is that a positive hair drug test result automatically indicates illicit drug use.

However, correct interpretation depends on several factors, including:

• Substance used and concentration
• Pattern across hair segments
• Relevant background information

For example, codeine metabolises in the body to morphine. Consequently, the detection of morphine in hair does not necessarily indicate direct morphine use and may simply reflect the use of a codeine-containing medication.

Similarly, ADHD medications such as dexamphetamine can produce findings that overlap with illicit amphetamine use.

Incorrect conclusions could therefore be drawn if medical and prescription histories were not considered during interpretation.

A negative result doesn’t always prove no substances were used

Hair testing is effective at identifying repeated or ongoing use, but a single, low-dose exposure – such as one-off cannabis or cocaine use – can be harder to detect.

Similarly, hair alcohol testing for the biomarkers Ethyl Glucuronide (EtG) and Ethyl Palmitate (EtPa) is designed to identify chronic, excessive consumption, so a single drinking episode is unlikely to be detected by hair testing alone.

Combining hair and blood alcohol testing gives a more complete picture. PEth blood testing, for example, can detect a single drinking episode for up to 12 days after consumption.

Learn more: Can hair testing be used to tell the difference between one-time and regular drug use for court?

Other factors that shape interpretation

Cosmetic treatments

Hair treatments such as excessive bleaching, dyeing and chemical straightening can alter the concentration of certain substances in hair.

For a hair alcohol test, EtG concentrations can be decreased by excessive bleaching or dyeing, while EtPa levels can be elevated by alcohol-based styling products such as sprays, gels or wax.

This does not mean these factors eliminate evidence of misuse or produce a false positive result. However, they must be considered by the Reporting Scientist during interpretation, and the sample collector will therefore always ask about and log any use of hair treatments and styling products when collecting the sample.

Environmental exposure

A child living in a household where drugs are regularly smoked or handled may show low-level findings that do not necessarily indicate active consumption. Modern laboratory procedures, including washing protocols and metabolite analysis, help distinguish environmental exposure from ingestion.

Segmented analysis

Head hair can be segmented into an approximate timeline of use, based on an average growth rate of 1cm per month.

However, individual hair growth rates can vary with ethnicity, diet, health and other factors, which should all be considered when interpreting results.

Learn more: What affects a hair drug test result?

Context is essential

Perhaps the most important principle in hair drug and alcohol testing is that results should never be interpreted in isolation.

The laboratory result is just one piece of a wider evidential picture, with clinical history, prescriptions, disclosures and witness statements all contributing to a balanced assessment.

Similar analytical results may be obtained for two different people, yet the interpretation could be very different depending on the wider circumstances of each case.

Learn more: Hair drug testing vs Hair alcohol testing

Accredited hair testing you can trust

At AlphaBiolabs, we offer Ireland’s most comprehensive portfolio of legally-instructed drug and alcohol testing services, backed by over 20 years’ experience supporting family law professionals and social workers.

We are ISO 17025-accredited, employ in-house toxicologists and Reporting Scientists with decades of collective experience, and offer FREE sample collection for legally-instructed tests at our walk-in centres.

To request a quote for court-approved drug and alcohol testing, contact our New Enquiry team on 01 402 9466, email testing@alphabiolabs.com, or complete our online quote form.