Hair test results: the levels chart, and what pg/mg means

• Context — self-reports, medical records, prescriptions.
• Individual factors — metabolism, hair pigmentation, and cosmetic treatment such as dyeing, bleaching or straightening.
• Confirmatory testing where warranted — segmental hair analysis, or a different sample type.

Accurate interpretation needs a qualified forensic toxicologist weighing these against the specific case. Dr Michael Robertson provides written interpretation reports.

A cut-off is the level at or above which the laboratory reports a substance as detected. Most drugs begin at 20 pg/mg, with these exceptions:

• 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid — 1 pg/mg
• Psychoactives — 2 pg/mg
• Alfentanil, Norfentanyl, Fentanyl — 2 pg/mg
• Steroids — 1 – 10 pg/mg, depending on the compound
• Alcohol (EtG) — 5 pg/mg

The Psychoactives group is the NBOMe compounds, LSD and psilocin — the full list is on our drugs tested page.

A cut-off decides whether something appears on the report at all. The tables below are a separate matter — they describe how a concentration is read once it has been reported, and for most substances they begin well above these figures. A result can sit above the cut-off and still fall inside the band that means infrequent or incidental.

Hair takes time to grow through the scalp before it can be collected, so the most recently grown hair may not yet be available for testing. The testing period shown on your report does not necessarily extend right up to the date your sample was taken.

If a court or agency has named a period, tell us the dates when you book and we will work out the sample length that covers them.

The laboratory includes a statement about bleaching as a precautionary measure when chemical treatment of the hair has been disclosed. It records the treatment so that it can be considered when the results are interpreted.

It does not mean your results are unreliable. The statement itself is not a finding that bleaching affected anything. Chemical treatment such as bleaching can affect hair and is therefore taken into account during interpretation — but whether it affected a particular result is a question only a qualified forensic toxicologist can answer, on the specifics of that case.

On what the research shows about treated hair generally, see does dyeing or bleaching your hair change a drug test result?

No. Drug testing and EtG alcohol testing use different substances, different testing methods and different interpretation criteria, and your drug results are reported separately on the laboratory report. The tables below give the levels for each.

We can explain the testing process and the recognised reference information on this page, and reception will happily walk you through either. A formal interpretation of an individual result is a different thing: it is a forensic opinion, and it comes from a qualified forensic toxicologist.

Dr Michael Robertson does exactly that work — verbal opinions and written reports on toxicology results from any source, hair, blood, urine, saliva or breath, including results this clinic did not collect. See the expert medico-legal service.

• Hair growth rates differ between people. Faster growth dilutes concentration; slower growth concentrates it over the same period.
• Hair type and colour matter. Darker, more porous hair incorporates more drug, which can skew a result upward.

• Passive exposure — second-hand smoke, handling a substance — can produce detectable levels without ingestion.
• Drugs can adhere to the outside of the hair, particularly in environments where they are present.

• Lipophilic drugs such as THC accumulate more readily than hydrophilic ones, so the same usage produces different concentrations across substances.
• Individual metabolic rates change how much of a substance reaches the hair at all.

• Where a laboratory sets its cut-off determines whether infrequent use registers at all.
• Distinguishing 'low' from 'medium' with precision is difficult for sporadic use.

• Without segmenting the sample, a single result averages months and hides periods of heavy or light use.
• A chronic user who recently stopped may still be classified as 'high'.

• One substance may sit at 'high' while another sits at 'low', which makes a single overall classification meaningless.

• Hair analysis cannot distinguish intentional use from a prescription or accidental ingestion.
• It measures deposition, not how frequently or in what quantity a substance was consumed.

• Simplistic categories invite unfair assumptions in legal and employment contexts.
• Courts may question a categorisation based on hair testing alone, given the variability above.