How hair testing works

How drugs get into hair

Drugs and their metabolites reach hair by one route or a combination of four.

Blood. Blood vessels feed the growing hair at the root. Drug and metabolites carried in the blood are deposited into the hair as it forms and are trapped in the shaft as it grows. This is the main route.

Sebum. The sebaceous glands in the follicle coat the growing hair with an oily substance. Drug and metabolites in sebum are deposited on the shaft and absorbed by it.

Sweat. Sweat glands in the scalp saturate the hair. Metabolites carried in sweat bind to the hair shaft.

Environment. Where a drug is smoked, handled, manufactured, or distributed, residue in that environment can be deposited on the hair and incorporated into it.

The laboratory reports what it measured in the sample. It does not report which route put it there, and neither does TrueTest.

The sample

Hair is always collected in person by a TrueTest collector, in view — never mailed in, never self-collected. The collector cuts a lock close to the scalp and keeps the root end marked, because the laboratory tests the 1.5 inches closest to the root. An optimal sample is about 200 strands, roughly the width of a #2 pencil. Larger panels need more hair, and the collector takes more than the minimum rather than risk a sample the laboratory cannot test.

What the sample covers

On average, hair grows about half an inch every thirty days. The laboratory tests only the inch and a half closest to the head, so head hair covers approximately ninety days. It takes ten to fourteen days for drug use to show up in a hair sample, so use in the days just before collection is not yet in the hair.

Body hair grows to a set length, stops, and sheds on a longer cycle. A body hair sample can reflect up to about a year, with a less precise timeline. Standard practice is head hair; body hair is collected when head hair is not available.

Reading the report

A hair report lists every category on the panel. Each category row is a screening test with its own screening cutoff. When a category screens positive, the report breaks it out and lists each compound in that category, confirmed individually by mass spectrometry, with its own quantitative value and confirmation cutoff. A category that screens negative shows no compound rows.

The two cutoffs describe different things. The screening cutoff belongs to the category; the confirmation cutoff belongs to one named compound, and a quantitative value is always read against the confirmation cutoff on its own row.

A TrueTest results notice names a positive by the category as the laboratory prints it and the compound confirmed — for example, Cannabinoids (Carboxy-Delta-9-THC).

What a positive establishes

A confirmed positive establishes that the drug or its metabolite was present in the hair at or above the confirmation cutoff, over the period the sample covers.

Drug testing cannot determine time, dose, or frequency. A hair result does not establish how the drug reached the hair, and it does not establish impairment at any point in time.

A negative hair result is not proof that no use occurred. Hair testing is built to show a pattern over months; a single or infrequent use may fall below what the test detects.

EtG in hair

Ethyl glucuronide (EtG) is a direct alcohol metabolite that can be tested in hair as an add-on to a drug panel or on its own, at a cutoff of 20 pg/mg. It gives a long look-back for drinking, but it is poorly suited to recent or infrequent drinking, and bleaching, coloring, or perming can reduce the EtG measurable in hair. Where the question is drinking in recent weeks, a PEth blood test may be a better-suited test.

What the order has to say

The order should name the panel; “hair test” alone leaves the choice to whoever is collected. It should specify head or body hair — if not specified, the collector will attempt head hair first and move to body hair if head hair is not available. Nail testing can be a good alternative if neither head nor body hair is available. The order should require the donor not to cut, dye, or chemically treat their hair for four weeks before collection when possible.

The model order carries this language.