A practical guide to selecting an appropriate hair testing period and understanding what a hair test can — and cannot — tell you.
When arranging a hair drug or alcohol test, one of the most important questions is:
What period of time do you need the test to cover?
Hair testing is different from many other forms of drug testing because it can provide information about drug or alcohol exposure over a period of months rather than focusing only on recent exposure.
However, simply requesting a “hair drug test” is not always enough. The length of hair analysed, whether the hair is tested as one section or divided into smaller segments, and the purpose of the test can all influence the information provided by the final laboratory report.
At Toxicology Solutions, we help clients understand the testing period and collection options so that the test selected is appropriate for the question being asked.
How far back can a hair drug test detect drug use?
For the purposes of interpreting scalp-hair testing, an average growth rate of approximately 1 centimetre per month is commonly used as a practical guide in forensic toxicology. Scientific research has reported an average head-hair growth rate of approximately 1.06 cm per month, while also demonstrating considerable variation between individuals.
The Society of Hair Testing (SoHT) recommends that hair growth rates and growth cycles be considered when establishing the timeframe represented by analysed scalp or body hair. This is important because a hair segment should not be interpreted as representing an exact calendar period.
As a general guide:
- 1 cm of scalp hair represents approximately 1 month
- 3 cm represents approximately 3 months
- 6 cm represents approximately 6 months
- 9 cm represents approximately 9 months
- 12 cm represents approximately 12 months
These time periods are approximate rather than exact. Individual hair growth rates and growth cycles vary, and the collection point and other factors can influence the timeframe represented by the analysed hair.
This becomes particularly important when interpreting segmented hair analysis or attempting to relate a result to a particular period.
Different tests provide different detection windows
Hair Testing Provides a Longer Detection Window
The specimen selected should therefore reflect the question the testing is intended to answer. Oral fluid and urine testing generally provide information about comparatively recent exposure, whereas hair testing can provide a much longer retrospective window.
Hair testing does not, however, establish impairment at a particular point in time.
Overview testing versus segmented hair testing
Overview hair analysis
An overview analysis examines a longer section of hair as a single sample.
For example, analysing the proximal 3 cm of scalp hair provides one result representing approximately the previous three months.
This can be useful where the question is whether there is evidence of drug exposure during the overall period being examined.
However, an overview result does not establish when within that approximate three-month period the exposure occurred.
Segmented hair analysis
Scalp hair can also be divided into smaller sections and analysed separately.
For example, a 6 cm sample may potentially be divided into six 1 cm sections, with each section representing an approximate one-month period.
This can provide considerably more information about changes over time.
Segmented testing may be particularly useful where there is a need to investigate:
- whether drug exposure appears to have changed over time
- periods of reported abstinence or cessation
- changes in the concentration of a detected substance
- consistency between a person’s reported history and the laboratory findings
This type of analysis can be particularly valuable in family law and other legal matters where the pattern over time may be as important as whether a substance was detected.
It is important, however, not to assign exact dates to individual hair segments solely on the basis of the 1 cm/month approximation. The SoHT recommends that growth rates, growth cycles and all relevant circumstances of the case be considered when interpreting hair-analysis results.
Why choosing the correct testing period matters
Consider a person who reports that they stopped using a particular drug approximately two months ago.
If a 3 cm section of hair is analysed as one overview sample, that section may contain hair representing both the period when the person reports using the drug and the period after they report stopping.
The laboratory therefore produces one combined result for that approximate three-month period.
If the same length of hair is divided into smaller segments and analysed separately, the results may provide considerably more information about how drug concentrations vary across the period.
This is why the purpose of the test should ideally be established before the laboratory analysis is selected.
Can hair testing identify a single occasion of drug use?
Hair testing is primarily used to investigate historical drug exposure and patterns over time.
In some circumstances, specialised sectional analysis using smaller hair segments may assist in investigating intermittent or potentially isolated exposure.
However, hair testing should not automatically be interpreted as proving the exact date, dose or frequency of drug consumption.
Factors including hair growth, collection technique, drug incorporation, cosmetic treatment, individual physiology, external contamination and the analytical characteristics of the substance being tested can affect interpretation. The SoHT similarly recommends that hair results be interpreted in the context of all relevant information surrounding the case.
Where a specific event or suspected isolated exposure is being investigated, the circumstances should be discussed before the collection and analytical strategy is determined.
What about body hair?
Where sufficient scalp hair is unavailable, body hair may sometimes be suitable for testing.
However, body hair has different growth characteristics from scalp hair. The SoHT specifically notes that when body hair is analysed, the greater proportion of dormant or telogen hair must be considered when interpreting the result.
For this reason, body hair generally cannot provide the same chronological information as segmented scalp hair.
Where identifying changes over time is important, scalp hair is generally preferred when a suitable sample is available.
Hair testing for pre-employment and workplace screening
The longer retrospective window offered by hair testing can also be valuable for employers.
Urine and oral-fluid testing generally focus on comparatively recent drug exposure. Hair testing can provide information relating to a substantially longer historical period.
For employers recruiting into safety-sensitive or high-risk positions, hair testing can therefore provide an additional tool within an appropriately designed pre-employment drug and alcohol testing program.
A three-month hair analysis, for example, can provide information relating to an approximate three-month period before collection rather than focusing only on very recent exposure.
Hair testing does not determine whether a person was impaired at work or impaired at a particular point in time. Its value lies instead in providing a longer-term view of drug exposure.
This distinction is important when deciding which form of testing is appropriate for a workplace program.
Hair testing in family law and legal matters
The testing period can be particularly important in family law, parenting, child protection and other legal matters.
Questions may include:
Has there been evidence of drug exposure during the previous six months?
Are the results consistent with a reported period of abstinence?
Have drug concentrations changed across the period examined?
Is there evidence relevant to a particular period within the proceedings?
These questions may require different testing approaches.
For example, a six-month overview analysis and six individual monthly segments examine approximately the same overall length of hair but provide very different levels of chronological information.
For this reason, lawyers, case workers and individuals arranging testing should consider not only which substances need to be tested, but also:
What question is the hair test intended to answer?
Choosing the right hair test
1. What period needs to be examined?
Is the relevant period approximately three months, six months, nine months or longer?
2. Is an overall result sufficient?
If the purpose is simply to investigate whether there has been exposure during an overall period, an overview analysis may be appropriate.
3. Do you need to understand changes over time?
Where cessation, reported abstinence or changing patterns are relevant, segmented analysis may provide more useful information.
4. Are particular drugs or medications relevant?
Declared prescription medications and the substances being investigated should be considered when selecting an appropriate laboratory testing profile.
5. What question are you trying to answer?
This is often the most important consideration. The appropriate test depends on whether you are investigating historical drug exposure, changes over time, reported abstinence, workplace risk or another specific issue.
Getting the testing strategy right
Hair testing can provide valuable information about historical drug and alcohol exposure, but the usefulness of the result depends on selecting an appropriate testing strategy.
The longest testing period or largest testing panel is not necessarily the best option.
The objective should be to select the testing period, segmentation and analytical profile that best address the question being asked.
Toxicology Solutions provides professional hair sample collection and chain-of-custody services for legal, workplace and private testing throughout Australia. Samples are submitted to specialist laboratories for analysis using validated analytical methods.
If you are unsure which testing period or hair analysis is appropriate, contact Toxicology Solutions before arranging your appointment. We can help you understand the available collection and testing options for your circumstances.
References
Society of Hair Testing (SoHT). Consensus on Hair Testing – Revision 2022. The consensus addresses collection, segmentation, interpretation and the importance of considering hair growth rates and growth cycles when establishing the timeframe represented by analysed hair.
Favretto D, Cooper G, Andraus M, et al. The Society of Hair Testing consensus on general recommendations for hair testing and drugs of abuse testing in hair. Drug Testing and Analysis. 2023;15(9):1042–1046. doi:10.1002/dta.3526.
LeBeau MA, Montgomery MA, Brewer JD. The role of variations in growth rate and sample collection on interpreting results of segmental analyses of hair. Forensic Science International. 2011;210(1–3):110–116. This study reported an average head-hair growth rate of approximately 1.06 cm/month and highlighted the variability that needs to be considered when assigning time periods to hair segments.
