A blood test looks back weeks, a urine test days. But what if you need to be able to demonstrate months, for example that you drank little or nothing over a longer period? That is where the EtG hair test comes in, the only alcohol test that looks back that far. The only question is: how far exactly, and what does the result say and not say?
Hair grows and records traces along the way. That makes a hair analysis work like a timeline. Below you will read how hair length translates into a look-back period, which threshold values are common, and where the limits of the method lie.
What does an EtG hair test measure?
EtG stands for ethyl glucuronide, a direct breakdown product formed when the body processes alcohol. A small part of it deposits in the hair shaft as it grows. By analysing a strand of hair, a lab measures how much EtG has accumulated over that growth period. Unlike CDT, which reflects a pattern in blood, EtG in hair captures use over a much longer period.
Our position: the hair test is powerful for a long look back, but anyone hoping for a day-by-day reconstruction will be disappointed. It is an average, not a calendar.
How does hair length translate into a look-back period?
Scalp hair grows on average about one centimetre per month. As a result, the length of the analysed strand determines how far back the test looks. The table below makes that concrete.
| Hair length (from the scalp) | Indicative look-back period |
|---|---|
| 1 cm | ± the past month |
| 3 cm | ± the past 3 months |
| 6 cm | ± the past 6 months |
In practice, a segment of three centimetres close to the scalp is often analysed, good for roughly three months. Longer strands can look back further, but reliability decreases toward the end. Body hair grows differently from scalp hair and therefore gives a less precise timeline.
When is a hair test usually requested?
The hair test is not a test for the question "did I drink yesterday". It comes into play precisely when someone needs to be able to support a longer period. Common reasons are a process in which abstinence has to be made plausible, a personal wish for certainty over several months, or a situation in which a snapshot is not enough.
Our position: for demonstrating abstinence over months, no other alcohol test is a serious alternative. Blood and urine simply do not capture that window. At the same time: anyone who only wants to rule out recent use overshoots their goal with a hair test and is better served by EtG in urine.
Which threshold values are used?
Laboratories use internationally agreed thresholds to estimate the degree of use. These reference values, partly based on the guidelines of the Society of Hair Testing, are often interpreted as follows:
| EtG in hair (pg/mg) | Indicative interpretation |
|---|---|
| Below ± 5 pg/mg | Consistent with abstinence or very low use |
| ± 5 to 30 pg/mg | Consistent with moderate, repeated use |
| Above ± 30 pg/mg | Consistent with chronic heavy use |
Important: these are reference values for interpretation, not hard limits. The Trimbos Institute and the RIVM stress that individual factors, such as hair colour, hair care and cosmetic treatments, can influence the measured EtG. An elevated or indeed low value should therefore always be placed in context by a doctor.
Where are the limits of the hair test?
The hair test is strong at looking back, but has limitations:
- No day-level detail: the result is an average over the measured length, not a day or week.
- Cosmetic influence: intensive washing, bleaching or dyeing can lower EtG.
- Late starting point: very recent use (the last few days) is not yet in the measured segment; EtG in urine is more suitable for that.
In addition, EtG in hair measures the average over the analysed length, but not an exact amount of alcohol: the threshold values give a direction, not a number of drinks. A reliable interpretation therefore always needs the full picture, including the type of hair taken and the way the sample was collected.
For very recent use or a pattern of a few weeks, other markers make more sense. A CDT test for alcohol, for example, looks back two to three weeks. Which marker fits which question is fully set out in the pillar proving alcohol use: what CDT, EtG and a blood test show, and the complete time windows are in how long is alcohol detectable in blood, urine and hair.
Every blood test result at Zuivertest includes a professional assessment from a BIG-registered doctor. A test can provide clarity, but does not replace medical advice. For treatment decisions, discuss your results with your GP.
Sources
- Trimbos Institute, information on alcohol markers (trimbos.nl)
- RIVM, alcohol and health (rivm.nl)
- Society of Hair Testing, consensus on EtG in hair (soht.org)
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