Blood alcohol testing methods

Marie Law Alphabiolabs

By Marie Law, Head of Toxicology at AlphaBiolabs
Last reviewed: 20/09/2022

Legally-instructed alcohol testing is performed every day across the UK, and is especially useful for Family Court, where important decisions need to be made about children’s welfare.

Blood alcohol testing is one of the most reliable forms of alcohol testing for legal purposes, with PEth testing being the most conclusive blood alcohol test available.

However, there are several options when it comes to blood testing for chronic and excessive alcohol consumption. All of which can be used together to provide an in-depth insight into an individual’s alcohol use.

In this article, we look at the different types of blood alcohol test, what each test looks for, and how these tests can be used alongside other methods.

Table of contents
  • How do blood alcohol tests work?
  • What are the different types of blood alcohol test?
  • What is a Phosphatidylethanol (PEth) test?
  • What is Carbohydrate Deficient Transferrin (CDT) testing?
  • What is a Liver Function Test (LFT)?
  • What is a Mean Corpuscular Volume (MCV) test?
  • Which blood alcohol test is best?
  • Can blood alcohol testing be used alongside other alcohol tests?
  • Where can I order a blood alcohol test for court?

How do blood alcohol tests work?

Blood alcohol tests work in different ways, depending on the test type.

There are several ways in which such tests can be used, including:

  • To detect the presence of alcohol metabolites, above cut-off levels, indicating chronic and excessive use
  • To assess the functionality of the liver: with damage to this organ being a common indicator of alcohol misuse
  • To measure the percentage of transferrin – a protein that transports iron around the body – that is deficient of carbohydrate. Alcohol consumption can affect the binding of carbohydrate molecules to transferrin
  • To measure the size of red blood cells

We will explore the different types of blood alcohol tests and their uses in this article.

What are the different types of blood alcohol test?

There are four main types of blood alcohol test that can be called upon for evidence in court.

Each test provides a four-week historic overview of alcohol use, and all serve different purposes when attempting to gain an insight into an individual’s alcohol consumption.

  • Phosphatidylethanol (PEth) test – the gold standard of blood alcohol tests, used to detect Peth, which is a direct biomarker of alcohol
  • Carbohydrate Deficient Transferrin (CDT) test – CDT is an indirect marker of alcohol. A minimum of two weeks’ sustained alcohol use is needed to increase CDT above cut-off levels. However, it usually returns to normal levels within four weeks after drinking ends
  • Liver Function Test (LFT) – also known as a liver blood test, this test is used to assess the functionality of the liver. It’s performed by checking the levels of certain enzymes and proteins in the blood
  • Mean Corpuscular Volume (MCV) test – an MCV test is used to measure the average size of red blood cells. These cells carry oxygen around the body, supporting the function of tissues and organs

Although each test serves its own purpose, PEth testing is by far the most conclusive of all the blood alcohol tests.

Unlike CDT, LFT and MCV tests, the results of which can also indicate other underlying medical conditions, a PEth test looks for the presence of Phosphatidylethanol (PEth) in blood.

PEth is a direct biomarker of alcohol and can only be produced in the body when ethanol – the intoxicating agent in alcoholic drinks – is consumed.

This makes PEth testing an extremely reliable indicator of chronic and excessive alcohol consumption.

What is a Phosphatidylethanol (PEth) test?

A Phosphatidylethanol (PEth) test looks for the presence of PEth, a direct biomarker of alcohol, in a blood sample, and can provide an overview of historic alcohol consumption of up to four weeks.

An abnormal phospholipid, PEth requires ethanol (the intoxicating agent in alcoholic drinks) for its production. This means that it is only present in the body when alcohol has been consumed.

One of the biggest benefits of PEth testing versus other alcohol blood tests (CDT, LFT and MCV), is that it can be used to detect chronic and single-drinking episodes.

Daily alcohol consumption of more than 60g of ethanol can be clearly distinguished from lower alcohol consumption, with PEth being detectable in the blood after 1-2 hours and for up to 12 days after a single drinking episode.

PEth testing can also be used to monitor drinking behaviour/abstinence, to identify relapse, and to verify whether an individual has changed their patten of alcohol consumption.

The table below shows the range of PEth levels that can be measured using an AlphaBiolabs’ blood alcohol PEth test.

PEth level (ng/ml)
<20 Abstinence or low alcohol consumption in the past month prior to sample collection
20-210 Social/moderate alcohol consumption in the past month prior to sample collection
>210 Excessive alcohol consumption in the past month prior to sample collection

Learn about PEth blood alcohol tests vs EtG hair alcohol tests

What is Carbohydrate Deficient Transferrin (CDT) testing?

Second only to PEth testing for its accuracy and reliability, Carbohydrate Deficient Transferrin testing is used to establish the percentage of transferrin in the bloodstream that is carbohydrate deficient.

Transferrin is a protein largely made in the liver that regulates an individual’s iron absorption into the blood. It attaches iron molecules and transports them to the bone marrow, spleen, and liver.

Alcohol consumption can significantly impact the levels of carbohydrate deficient transferrin in the body, meaning laboratory analysis can be used to measure these levels and draw a conclusion about alcohol use over a defined period.

People who do not drink, or drink moderately, will have lower CDT levels in their blood.

However, people who consume 50-80 grams of alcohol at least five days a week for two weeks prior to a CDT test will have significantly greater levels.

As with all the blood alcohol tests, we would always recommend that CDT testing is used alongside other blood tests (including PEth) for the most accurate insight into an individual’s alcohol use.

This is because CDT is an indirect biomarker of alcohol, meaning that levels in the blood can be outside the normal range for many reasons.

Causes can include liver disease, previous alcohol use, certain medications, or other underlying health conditions.

What is a Liver Function Test (LFT)?

A Liver Function Test is used to determine the health of the liver, by measuring certain enzymes and proteins created by the liver and found in the blood.

Alcohol can be toxic to the liver, meaning that a person who consumes excessive amounts of alcohol will damage this vital organ and may experience decreased liver function.

An AlphaBiolabs LFT is used to assess liver damage that may have been caused by alcohol consumption, and by measuring three enzymes in the blood: aspartate transaminase (AST), alanine aminotransferase (ALT) and gamma glutamyl transferase (GGT).

While the LFT is used as a form of blood alcohol testing at AlphaBiolabs, it serves many other purposes for the medical profession.

It is also used to check for damage caused by infections or diseases, to diagnose symptoms of a liver disorder, and to monitor the side effects of medications.

Because other factors such as certain medications and pre-existing medical conditions can affect the results of an LFT, the test should not be used in isolation to assess alcohol use.

We would always recommend that LFTs be used alongside other blood alcohol tests including PEth, CDT and MCV for a more complete picture of alcohol a person’s historic alcohol consumption.

What is a Mean Corpuscular Volume (MCV) test?

A Mean Corpuscular Volume (MCV) test is used to measure the average size of red blood cells in a blood sample.

Red blood cells play a vital role in the function of organs and tissues, carrying oxygen in the blood to all parts of the body. If red blood cells are too big or too small, this can be an indicator of underlying health problems.

If the MCV marker in the blood is higher than normal, this can indicate enlarged red blood cells and, in turn, chronic and excessive alcohol consumption.

It takes several weeks of heavy alcohol consumption for MCV to become elevated, and it may also take several months before MCV returns to a normal level after abstinence.

However, there are many reasons why an MCV test might show abnormalities in the size of red blood cells, including the use of certain medications, vitamin deficiencies and other medical conditions.

This means that, as a standalone indicator of alcohol misuse, MCV has somewhat low sensitivity.

However, when combined with other blood tests, it can support a diagnosis of excessive drinking.

Which blood alcohol test is best?

Which blood alcohol test you choose, and which test is best for your client, can vary, depending on the circumstances of the case and which tests have been ordered by the court.

However, out of the four blood alcohol tests available from AlphaBiolabs, we would always recommend that Phosphatidylethanol (PEth) testing be the number one choice when deciding which blood alcohol tests should be used.

This is because PEth is a direct biomarker of alcohol, which means that it can only be detected when alcohol has been consumed.

Its high specificity (48-89%) and sensitivity of 88-100% is because it is directly related to alcohol consumption.

However, we would also recommend that PEth be used in conjunction with other blood alcohol tests including CDT, LFT and MCV, to provide the fullest picture of alcohol use from which to draw conclusions.

Can blood alcohol testing be used alongside other alcohol tests?

When considering which alcohol tests would best suit your client, it’s important to understand which tests provide the most conclusive insight.

One important thing to note, is that PEth blood alcohol testing is second only to the detection of ethyl glucuronide (EtG) and ethyl palmitate (EtPa) – a fatty acid ethyl ester (FAEE) – in the hair.

Both EtG and EtPa are direct biomarkers of alcohol and give us a highly accurate insight into patterns of drinking, with a three- or six-month overview of usage.

Each is absorbed into the hair via different routes, and their levels can assist in assessing excessive alcohol consumption using hair alcohol testing.

EtG is water soluble, produced in the liver and can be impacted by various hair treatments including excessive washing.

EtPa is formed from ethanol diffusing from the blood circulation and then deposited into hair primarily from sebum (from the oil glands on the scalp). EtPa (FAEEs) are lipophilic so while not water soluble, its presence can be affected using alcohol-containing hair products such as sprays, gels, and wax.

Because of their respective strengths and weaknesses, both EtG and EtPA tests should be performed, and their findings used to support each other, as shown in LB Richmond v B & W & B & CB [2010] EWHC (2903) Fam, where Mr Justice Moylan gave guidance on the evidential worth of hair strand testing. Within his guidance, it was also stressed that hair strand tests should not be used in isolation to reach evidential conclusions.

By performing these tests in conjunction with PEth and other blood tests, a more holistic conclusion can be drawn as to a person’s alcohol use.

A good option for customers looking for an in-depth insight into an individual’s historic alcohol consumption is our Comprehensive Alcohol Analysis package.

This package combines head hair alcohol testing with blood alcohol tests, and includes sample collection, along with a comprehensive Statement of Witness report.

Where can I order a blood alcohol test for court?

Using highly accurate scientific analysis in the form of PEth blood testing, along with CDT, LFT and MCV blood alcohol tests, a case can be made to protect those in need – and reunite families where abstinence has been achieved.

AlphaBiolabs has extensive experience working with family law professionals, social workers, local authorities, and Family Court, providing a range of legal testing services including court-approved blood alcohol testing.

It’s easy to request a quote for legally instructed alcohol testing online now.

Alternatively, call our Legal sales team on 0333 600 1300 or email testing@alphabiolabs.com and a member of the team will be in touch to discuss your case.

Legal alcohol testing

Learn more about alcohol testing for legal matters, direct from our award-winning laboratory.

Marie Law

Marie Law

Head of Toxicology at AlphaBiolabs

A highly-skilled and respected scientist with over 13 years’ experience in the field of forensics, Marie joined AlphaBiolabs in 2022 and oversees the company’s growing toxicology team.

As Head of Toxicology, Marie’s day-to-day responsibilities include maintaining the highest quality testing standards for toxicology and further enhancing AlphaBiolabs’ drug and alcohol testing services for members of the public, the legal sector, and the workplace sector.

Prior to joining AlphaBiolabs, Marie held roles with LGC Forensics, Cheshire Constabulary and Eurofins Forensic Services.

Connect with Marie on LinkedIn

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