If you want to demonstrate alcohol use, for a CBR procedure, a new job, or your own peace of mind, a lab looks not at a faint line but at specific biomarkers. CDT in blood reflects a pattern of chronic use over roughly the past two to three weeks. EtG in hair can look back around three months. Which test suits you depends entirely on the question you want answered, and on the time window you need to cover.
Most people searching for this have a concrete reason: a CBR referral, a job where sobriety matters, or simply wanting to show themselves they are clean. Below you will read what each alcohol marker measures, how far back it looks, and what an abnormal value can and cannot prove. Our position up front: no single marker is the right one for every question, and anyone who ignores that almost always chooses the wrong test.
How do you prove alcohol use with a test?
Alcohol itself leaves the body quickly. The Trimbos Institute, the Dutch knowledge centre for substance use, describes an average breakdown of roughly one standard drink per hour. As a result, alcohol as a molecule is usually only measurable in blood for a few hours up to about a day. If you want to demonstrate a pattern over a longer period, a lab looks not at the alcohol itself but at the traces that prolonged use leaves behind.
That can be done in three ways, depending on the time window. CDT in blood points to sustained heavy use over the past two to three weeks. EtG in hair captures use over a longer period, roughly one month per centimetre of hair. PEth in blood sits between those windows. A home test strip only gives a yes/no signal and is unsuitable here: a lab quantifies an exact value per marker with a reference range, and in a borderline case that distinction can make the difference between doubt and a result you can support.
What is a CDT test and how far back does it look?
CDT stands for carbohydrate-deficient transferrin, a form of the iron-transport protein transferrin that rises with prolonged heavy alcohol use. The most common result is a percentage (%CDT), usually with the same reference value of below 2.8% for men and women. The half-life of CDT is around 14 to 17 days, so with full abstinence an elevated value usually falls back toward normal within two to three weeks.
CDT is therefore mainly a marker of a pattern, not of a single drink. If you want one marker measured specifically, a CDT test for alcohol can quantify that value for you. Why a value can be elevated without heavy drinking is covered in our article on a CDT value that is too high and its causes.
What is the difference between CDT, EtG, PEth and a blood-alcohol reading?
This is the core of the question, and it is often confused online. CDT is an indirect marker in blood, EtG is a direct breakdown product measured in urine or hair, PEth is a direct marker in blood, and a blood-alcohol reading (comparable to a breath test) shows only the most recent use. The table below sets out what each method measures, how far back it looks, and what an elevated value can and cannot prove.
| Method | Matrix | Look-back window | What it can show | What it does not prove |
|---|---|---|---|---|
| Blood alcohol (BAC) | Blood or breath | A few hours up to ± 1 day | Very recent use at that moment | Anything about the weeks before |
| EtG (urine) | Urine | ± 1 to 3 days | Recent use, even after the alcohol itself has cleared | A pattern over weeks |
| CDT (%CDT) | Blood | ± 2 to 3 weeks | Pattern of chronic heavy use | A single drink or one-off evening |
| PEth | Blood | ± 2 to 4 weeks | More recent pattern, sensitive to moderate use | The exact number of drinks |
| EtG (hair) | Hair | ± 1 month per cm, up to ± 3 months | Use or abstinence over a long period | When within those months drinking occurred |
What follows from this: a breath test or blood-alcohol reading says nothing about the past month, while a hair test does not tell you when within those months drinking occurred. For a long look back, hair analysis may fit better; for a pattern over recent weeks, CDT is often used; and for very recent use, EtG in urine is more suitable. How long each form stays detectable is detailed in how long is alcohol detectable in blood, urine and hair.
Which blood values does the CBR use for alcohol?
In an investigation into driving fitness, the CBR does not look at a single number but at a combination of blood values. A frequently used set is gamma-GT, CDT and MCV. If all three are elevated, that points to a pattern of chronic excessive use; one abnormal value alone says less.
- Gamma-GT (GGT): a liver enzyme, reference roughly 10 to 40 in men and 6 to 25 in women. Can also rise from liver or gallbladder conditions and some medications.
- CDT (%CDT): reference below 2.8%. The most specific of the three for alcohol, with a look-back window of two to three weeks.
- ASAT/ALAT: liver enzymes that can indicate strain; markedly elevated values point to greater pressure on the liver.
- MCV: the average size of red blood cells, which can be elevated after prolonged heavy use.
The CBR weighs these values together with an interview and the reason for the referral. The result of a single test therefore does not decide the outcome of a procedure on its own. What a separate test does do is give you insight beforehand into where you stand. How this assessment works is covered in CDT value and your driving licence: what the CBR assessment involves.
How far back does an EtG hair test look?
EtG (ethyl glucuronide) is a direct breakdown product of alcohol that deposits in the hair shaft. Because scalp hair grows on average about one centimetre per month, a hair analysis can build a timeline: a three-centimetre strand covers roughly the past three months. That makes the hair test the marker with the longest window, and the only one that can make months of abstinence plausible.
The downside is that EtG in hair does not tell you on which day or in which week within that period drinking occurred. It gives an average over the whole length. Bear in mind too that intensive washing, bleaching or dyeing can lower the measured EtG, and that hair colour and hair care influence the result. What the common threshold values are and how hair length translates into a look-back period is detailed in the EtG hair test: how far back does hair analysis look.
Can a value be elevated without heavy drinking?
Yes, it can. An elevated %CDT usually relates to sustained alcohol use, but not always. Rare inherited disorders in the sugar chains of proteins (CDG variants), severe liver conditions and a recent blood transfusion can influence the value without heavy drinking. That is why a reliable lab never looks at CDT alone, but always at the whole picture.
A quantified lab value with a reference range offers more grip here than a home strip, because the result can be placed in context. What an abnormal value means for you personally is best discussed with a doctor. The causes of an abnormal CDT are fully set out in CDT too high: causes of an abnormal value.
Why a lab is more reliable than a home strip
Home alcohol test strips are sold online, and the appeal is understandable: fast, cheap, no appointment. The problem is what such a strip measures. A test strip gives a yes/no signal for alcohol in saliva or urine at that moment, comparable to a breath test. That says something about the past few hours, but nothing about a pattern over weeks or months, and the latter is usually the question.
A lab, by contrast, quantifies a marker such as CDT or EtG down to a number, with a reference range and a professional interpretation. The difference is not only in accuracy, but in evidential value: a quantified value within the reference range supports a pattern, while a line on a strip does not. Our position: for any question that goes beyond "do I have alcohol in my system now", a laboratory measurement is the only serious option.
In a borderline case, that distinction counts double. A lab can place a repeated measurement after a few weeks alongside the first and so show a line over time. A home strip only delivers separate snapshots with no comparability between them.
How does an alcohol test at Zuivertest work?
The process is built around privacy and control. You order online the marker or panel that fits your question. We arrange the lab referral, after which you have blood drawn at an affiliated lab nearby. Nothing is shipped to you and no one comes to your home: it is a service, not a package. The result appears in your own secured account, with an assessment by a BIG-registered doctor.
Who sees the result is up to you. A test ordered through Zuivertest is not automatically open to an employer or the CBR; only you decide whether to share the result. That makes the test usable as preparation for a procedure, or purely for your own certainty, without the outcome ending up outside your view.
Which test fits your question?
The choice depends on the period you want to cover, not on which test sounds most familiar:
- Very recent use (hours to days): a blood-alcohol reading or EtG in urine.
- A pattern over recent weeks: CDT in blood, optionally supplemented with gamma-GT and MCV.
- A long look back or demonstrating abstinence (months): an EtG hair test.
If you are unsure between two options, start from the longest window you definitely want to cover. For a CBR process, for example, a pattern over weeks is often relevant, while someone wanting to support months of abstinence is better off looking at hair. A combination is also possible: a CDT measurement now and a repeat after a few weeks shows a line instead of one isolated number.
At Zuivertest you choose which marker to have measured, only you see the result, and every result comes with an assessment by a BIG-registered doctor. You can have the CDT marker measured on its own or as part of a broader panel. The WHO stresses that alcohol use is an important health factor at population level; an individual test result, however, remains a snapshot that is best interpreted together with a doctor.
Frequently asked questions
How long does alcohol stay detectable in blood?
Alcohol itself is usually measurable in blood for a few hours up to about a day. Markers of a use pattern, such as CDT, look back further: roughly two to three weeks.
Does a CDT test show whether I drank yesterday?
No. CDT responds to sustained heavy use over weeks, not to a single evening. For very recent use, other markers, such as EtG in urine, are more suitable.
Do I need a referral for an alcohol test?
Not at Zuivertest. You order online, we arrange the lab referral, and you have blood drawn at a lab near you.
Can my employer or the CBR see my result?
No. A test ordered through Zuivertest sits behind your own secured account. Only you decide whether to share the result.
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.
Further reading
- How reliable a breathalyser really is
- The limits the RIVM applies
- Why one glass makes you flush
- What alcohol does to your recovery and blood values
Sources
- Trimbos Institute, National Drug Monitor, alcohol and substance use in the Netherlands (trimbos.nl)
- RIVM, information on alcohol and health (rivm.nl)
- World Health Organization (WHO), alcohol and health (who.int)
- CBR, assessment of driving fitness (cbr.nl)
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