How does external contamination affect the results of hair drug tests?

For drug users, the prospect of an upcoming hair test can lead to some questionable reactions. While some will quietly – but reluctantly – accept their fate, others believe that they can circumvent the system by bleaching their hair, wearing a wig or shaving their head.

These tactics don’t work.

For others, claims of external contamination are the chosen approach. “I never touched it, I just went to a party where others were using!” is a common refrain.

External factors can lead to hair contamination, it’s true. However, it’s rare that such a situation could sway the results of a court case. Here’s why.

What is external contamination?

While a drug test with hair aims to establish patterns of both longer and shorter-term use, it is possible for samples to be externally contaminated by a subject’s environment. Drugs can enter the hair in three different ways: via the blood, via sweat or sebum, and as a result of external exposure.

Such exposure can lead to the hair being contaminated with small traces of the drug, resulting in a false conclusion of drug use or false positive if hair testing procedures are not accurately followed.

How can external contamination happen?

The most common cause of external contamination is through the subject coming into contact with cocaine traces or cannabis smoke as a result of close contact with drug users. It may be that the subject has spent time at a party where friends have been smoking marijuana in their vicinity – or that their hands have come into contact with cocaine, either intentionally or accidentally, before being rubbed through their hair.

Contamination can also be caused via sweat or sebum, making it difficult to differentiate from single or multiple use in some specific situations.

What contamination means for hair testing results

It is likely that external contamination will have very little impact on hair testing results – providing procedures are followed properly.

Hair testing laboratories must wash all hair samples before testing begins, to minimise the effects of external contamination. While this wash is efficient, it is not possible to be sure that all has been removed, whilst excessive washing will start the extraction process if ingested drugs are present, affecting results.

However, by analysing the wash residue and comparing it with the levels detected in hair, we can determine conclusive results. If the levels in the wash residue are higher than those in the hair, the conclusion is that most of the drug is from external contamination. If the drugs are detected in the hair, however, and not the wash residue, this indicates drug use.

There are numerous scientific studies of wash protocols that show their efficiency in hair drug testing – but their validity can be argued due to the use of artificially contaminated samples. Our own assessments using real hair samples, however, show how rarely external contamination can have a significant impact on the interpretation of results.

What to look out for

There may be instances where misdiagnosis is possible in a drug test with hair – but steps can be taken to avoid this.

It may be, for example, that analysing wash residues shows detectable drug levels, but that these are below the stated analytical cut-off levels and therefore unquantifiable. In these instances, it could be that a small dose of drugs was ingested – or that an environmental contamination may not have been completely removed by the washing process.

To avoid such confusion, best laboratory practice should be combined with diagnostic tools to give as accurate a picture of the situation as possible, before external contamination is even considered. When drugs are detected in a hair sample, it is clear that the subject must have come into contact with drugs within the timeframe being tested: the challenge is establishing the root cause of the positive result.

By employing rigorous pre-testing and testing methods, it is possible to establish whether a positive result comes from ingestion or from external contamination in the majority of cases. However, in those very few cases where the results are unclear, they must be interpreted alongside corroborating evidence from social context, clinical data or both to ensure a degree of accuracy that will hold up in court.