Understanding Hair Growth Rate: How It Affects Drug Detection Accuracy

Understanding Hair Growth Rate: How It Affects Drug Detection Accuracy

When it comes to drug testing methods, hair strand testing (HST) offers a unique advantage in detecting drug use over an extended period. However, it comes with limitations, particularly due to variations in hair growth rates. In this blog, we’ll explore how individual differences in hair growth impact the accuracy of hair drug testing, with insights from our recent webinar.

How Drugs are Detected in Hair

When drugs are ingested, they circulate in the bloodstream, eventually reaching the scalp. Drug metabolites—the byproducts created as drugs are broken down in the body—get embedded in the hair shaft as hair grows. This means that a section of hair represents a timeline of drug exposure.

According to the Society of Hair Testing (SoHT) and the European Workplace Drug Testing Society (EWDTS)hair grows at an approximate rate of 1 cm per month. As such, a 3 cm strand taken from the scalp can provide a three-month window of drug use. Longer hair can provide an even longer timeline to 6 months, and some cases 12 months.

The Importance of Hair Growth Rate in Drug Detection Accuracy

Hair drug testing generally assumes a standard growth rate, but individual differences can create variation in the timeline that a test represents. Hair goes through growth cycles, and these phases (anagen, catagen, and telogen) can vary in length among individuals. Hair growth rates and patterns vary among individuals. Factors like genetics, race, age, health, diet, and hormonal levels all play a role in how quickly hair grows and its overall health. For example, Afro-Caribbean hair growth is known to be slower when compared to cocaine hair growth. Certain areas of the scalp and body also experience different rates of growth, thickness, and density. 

Here’s how:

1. Faster Growth Rates: In people whose hair grows faster than average, such as those with Asian hair,  the same length of hair may cover a shorter time frame. For example, if someone’s hair grows at 1.5 cm per month instead of 1 cm, a 3 cm strand might reflect only two months of drug use instead of three.

2. Slower Growth Rates: For people with slower hair growth, such as those with African hair,  the same length of hair may represent a longer period. For instance, if hair grows at 0.8 cm per month, a 3 cm sample may reflect nearly four months of history instead of three.

These variations in hair strand testing can lead to discrepancies in test results interpretations, especially if the test is intended to represent a specific timeframe.

Real-Life Scenarios: How Hair Growth Rate Affects Drug Detection

To illustrate how hair growth rates influence drug test results, consider this scenario with Tandi and Sue, both of whom are newly pregnant and have a history of drug taking.

Tandi, of African descent, and Sue, who is Caucasian, both stopped using drugs at the beginning of their pregnancies. Three months later, a hair drug test was conducted using a 3 cm sample from each.

Sue’s hair growth rate aligns with the average of 1 cm per month, so her 3 cm sample reflects approximately three months of drug history. Since Sue had stopped using drugs over three months prior, her test returned a negative result.

Tandi, however, has a slower hair growth rate—only 0.0.8 cm per month. For her, 3 cm of hair captures roughly four months of drug history. Although Tandi also stopped using drugs at the start of her pregnancy, her slower growth rate meant her hair still showed drug presence from more than three months ago, leading to a false positive hair result, despite her cessation during pregnancy.

This example highlights a critical consideration in hair follicle drug testing: growth rate differences can extend or shorten the detection window. For professionals in social services, child care, and family law proceedings, understanding such nuances is essential for the fair interpretation of hair drug test results.

What Hair Growth Rate Means in Court: Key Takeaways for Fair Interpretation

In a courtroom, to avoid false positive and false negative hair drug test results, understanding the impact of growth rates on these hair drug test results is essential for ensuring just outcomes. Since slower hair growth can extend the detection window and lead to false positive results for drug use that occurred far in the past, courts should interpret hair follicle test results with caution.

Ultimately, the influence of hair growth rate on drug detection emphasises the need for a nuanced approach in court, where science and individual biological differences are factored into legal decisions.

If you missed our webinar on hair testing limitations, you can rewatch it here: Watch the Webinar. Contact us to learn how hair follicle and nail drug testing can impact your case, or reach out to our experts for guidance.

 

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