How Does Hair Drug Testing Work?

Hair drug testing examines a sample of hair in a laboratory for evidence of exposure to specified drugs. It can provide information about a longer period than a typical urine test, but the period covered and the meaning of a result depend on the sample, the substances tested and the laboratory’s methods.

Contents

• What Is Hair Drug Testing?
• What Time Period Does Hair Testing Cover?
• Does Body Hair Give Similar Results?
• How Soon After Use Can a Drug Be Detected?
• Can Environmental Exposure Affect Results?
• Can Hair Treatments Affect a Test?
• Does Hair Colour Affect Results?
• Can Hair From a Brush Be Used?
• Hair or Urine Drug Testing?

What Is Hair Drug Testing?

A small sample of hair is usually cut close to the scalp and sent to a laboratory. The laboratory analyses it for the particular drugs or drug metabolites included in the requested test. A hair test is used to investigate past exposure; it cannot establish whether someone is currently impaired.

Different laboratories offer different testing panels and procedures. If the result could have serious consequences, ask which substances were tested, what confirmation method was used and how the laboratory interprets its findings.

What Time Period Does Hair Testing Cover?

A sample taken from the scalp can provide information about a period of months. As a rough guide, laboratories often use the hair closest to the scalp to assess approximately the previous three months. The precise period is an estimate because hair growth varies between people and the sample may include hair at different stages of growth.

A positive result does not identify an exact date of use, and a negative result does not prove that no drug was used during the period. The substance, amount and pattern of use, test sensitivity and condition of the hair can all affect what is detected.

Does Body Hair Give Similar Results to Head Hair?

If there is insufficient scalp hair, a laboratory may be able to analyse body hair. Body hair grows and rests differently from scalp hair, so it is harder to assign its result to a particular period. Tell the laboratory where the sample was collected and follow its instructions on the amount required.

How Soon After Use Can a Drug Be Detected in Hair?

Hair testing is generally unsuitable for detecting very recent use. It takes time for the part of the hair containing a drug or metabolite to emerge above the scalp and become available for collection. If the question concerns recent exposure, a different specimen, such as urine or oral fluid, may be more appropriate.

Can Environmental Exposure to Drugs Affect Hair Test Results?

It can. Drugs may reach hair through the environment as well as through the body. Laboratories use procedures such as washing the sample and analysing relevant metabolites to help assess possible contamination, but washing alone does not make contamination irrelevant in every case.

For example, interpretation of a cannabis test depends on the substances and metabolites the laboratory measures. If environmental exposure is a concern, give the laboratory the relevant circumstances so they can be considered alongside the analytical result.

Can Hair Treatments Affect a Hair Drug Test?

Bleaching, dyeing, perming and other chemical or physical treatments can change the concentrations of substances measured in hair. Ordinary washing is different from these treatments, but no shampoo or treatment can be relied upon to produce a particular test result. The collector should record relevant treatments so the laboratory can consider them when interpreting the sample.

Does Hair Colour Affect Hair Drug Test Results?

Natural hair characteristics, including melanin content, can influence how some substances enter and remain in hair. Hair colour alone cannot determine what a test result means. It is one of several factors a laboratory may need to consider, alongside the substance tested, hair treatments and the circumstances of the case.

Can Hair Collected From a Brush Be Used?

A laboratory may be able to analyse hair taken from a brush, but it generally cannot verify who supplied each strand or when it was shed. A brush sample is therefore unsuitable when the identity of the donor and a documented collection process matter. Ask the laboratory about its sample requirements before arranging a test.

Hair or Urine Drug Testing?

The choice depends on the question being asked. Hair is generally used to investigate a longer history of exposure. Urine is generally better suited to more recent use. Neither test, by itself, establishes impairment at the time the sample was collected.

Detection Window

Urine detection times vary considerably by drug, pattern of use, test cut-off and individual circumstances. Some substances may be detectable for only a few days, while cannabis metabolites can remain detectable for longer after frequent use. Hair testing can cover months when enough suitable hair is available, but it usually misses the most recent period before collection.

Sample Collection

Hair collection involves cutting a small sample and documenting its source and orientation. Urine collection uses a different process, with measures to check sample identity and validity where required. Neither collection method should be described as automatically tamper-proof: the procedures used matter.

In a UK workplace, drug testing should be carried out with the employee’s consent and within an appropriate policy. Testing and the handling of results should be proportionate to the role and the reason for testing.

Cost and Drugs Detected

Hair testing usually requires specialist laboratory analysis and may cost more than a rapid urine screen. The two methods may also use different testing panels. Check the exact substances included before ordering a test, particularly if a prescribed medicine or a specific drug is relevant.

Interpreting Results

A result needs to be interpreted in context. A hair concentration does not reliably show an exact dose, a precise date of use or current impairment. An initial screening result with serious medical, legal or employment implications should be appropriately confirmed and reviewed by a qualified professional rather than treated as a decision on its own.

For workplace testing, employers should also consider consent, their testing policy, reliable collection and analysis, and a fair process for discussing the result with the employee. Neither a hair nor a urine test automatically carries greater legal weight in every situation.

Summary

Hair and urine tests answer different questions. Hair can provide information about exposure over a longer period, while urine is more useful for investigating recent use. The value of either result depends on the chosen test, proper collection, laboratory analysis and careful interpretation. If the outcome could affect someone’s job or legal position, seek appropriate confirmation and professional advice.

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