The liver rarely complains. It filters the blood, helps with digestion, stores energy and makes proteins, usually without giving any sign that it is under strain. That is one reason liver function tests are among the most frequently requested blood tests in both GP surgeries and hospitals. This guide explains what each marker on a standard panel measures, what can shift the numbers and how results are usually read.
What liver function tests are
"Liver function tests" is a slightly misleading name. Most of the markers do not measure how well the liver works; they measure enzymes and substances that rise or fall when liver cells or bile ducts are under stress. LML's description puts it simply: the panel measures enzymes, proteins and other substances whose levels can reflect how well the liver is working, and abnormal results can point to damage, inflammation or other problems.
British Society of Gastroenterology guidance on abnormal liver blood tests sets out what a standard panel should contain and how results should be followed up in primary care (Newsome et al., 2018).
The markers, one by one
ALT (alanine transaminase)
ALT is an enzyme found mainly inside liver cells. When those cells are irritated or damaged, ALT leaks into the blood. It is one of the most useful markers of liver cell stress.
AST (aspartate aminotransferase)
AST is also found in liver cells but is present in muscle, heart and other tissues too. That makes it less specific: a hard workout or a muscle injury can raise AST without any liver issue. It is most informative when read alongside ALT.
GGT (gamma glutamyl transferase)
GGT is very sensitive to liver injury but has poor specificity for any particular cause, according to a review in Liver International (Brennan et al., 2022). The same review notes its long history as a marker of regular alcohol intake, and its links with metabolic and cardiovascular risk.
ALP (alkaline phosphatase)
ALP comes from the bile ducts and from bone. A raised ALP with a raised GGT suggests the bile ducts; a raised ALP with a normal GGT can simply reflect bone activity, which is common in teenagers and after fractures.
Bilirubin, albumin and total protein
Bilirubin is a yellow pigment from the normal breakdown of red blood cells, which the liver processes and excretes. Albumin and the other proteins are made by the liver, so they say something about its making capacity over time. The LML panel also reports globulin.
How the numbers are read
A review for clinicians in the World Journal of Hepatology describes two broad patterns (Kalas et al., 2021):
- Hepatocellular pattern: ALT and AST rise more than ALP. Common causes include fatty liver linked to metabolic health, alcohol, viral hepatitis and some medicines.
- Cholestatic pattern: ALP and GGT rise more, pointing towards the bile ducts and bile flow.
The same review groups ALT rises by size, from mild to severe, measured against the upper limit of the reference range. Mild changes are common and often settle; the pattern, the size of the rise and whether it persists matter more than one value.
Several everyday things can move the numbers:
- Alcohol in the days before the test, especially for GGT.
- Intense exercise, for AST and sometimes ALT.
- Medicines and some supplements. LML notes that liver tests are often used to watch for side effects of certain medications.
- Weight and metabolic health, linked to fatty liver.
What liver function tests do not show well
Normal liver blood tests do not always mean a healthy liver, and abnormal ones do not tell the stage of any liver condition. NICE guidance on metabolic dysfunction-associated steatotic liver disease (MASLD) advises against using routine liver blood tests to assess advanced liver fibrosis, and recommends a specific test such as the Enhanced Liver Fibrosis (ELF) test for that question (NICE, 2016, updated). Liver blood tests are a broad first look; scarring needs its own assessment.
When a liver check usually makes sense
People often discuss liver tests with a professional when they:
- Drink alcohol regularly, or want a baseline before cutting down.
- Carry extra weight around the waist or live with type 2 diabetes, both linked to fatty liver.
- Take long-term medicines that can affect the liver.
- Have a family history of liver conditions, or have been told of a past abnormal result.
The NHS lists common types of liver disease, including alcohol-related liver disease, fatty liver disease, hepatitis and haemochromatosis, and notes that many of them cause no symptoms in the early stages.
How often? As a general guide, adults keeping an eye on their health often check liver function once a year, and more often if a professional is following an abnormal result or a medicine. Talk to a professional about the interval that suits you.
