⚕️ Educational content only. This article explains what liver function tests measure and what results generally mean. It is not medical advice. If you have received a liver function test result, speak with your doctor or healthcare provider who can interpret it in the context of your full clinical picture.
A liver function test (LFT) — also called a liver panel or hepatic function panel — is one of the most commonly ordered blood tests in clinical practice. It measures a range of proteins, enzymes, and substances produced by or processed through the liver. The results help clinicians detect liver damage, monitor ongoing liver conditions, and assess how well the liver is working.
But what exactly does each marker measure? And what does it mean if a result is high or low? This article explains the key components of a liver function test in plain language.
Key Takeaways
- LFTs measure enzymes, proteins, and bilirubin to assess liver health and function.
- ALT and AST are the main markers of liver cell damage; elevated levels suggest hepatocyte injury.
- ALP and GGT are associated with bile duct function and can indicate cholestatic disease.
- Albumin and total protein reflect the liver’s synthetic capacity — how well it makes proteins.
- Bilirubin is the breakdown product of haemoglobin; elevated levels can cause jaundice.
- A single abnormal result does not always mean liver disease — context and pattern matter.
What Is a Liver Function Test?
Despite its name, an LFT does not directly measure liver function in a single, comprehensive way. Rather, it is a panel of individual tests that, taken together, provide a clinical picture of liver health. The typical components vary slightly between laboratories and countries, but most panels include alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), albumin, total protein, and bilirubin.
Some laboratories also include prothrombin time (PT) or international normalised ratio (INR) as part of an extended liver screen, as the liver produces most clotting factors.
ALT (Alanine Aminotransferase)
ALT is an enzyme found predominantly in liver cells (hepatocytes). When hepatocytes are damaged or destroyed, ALT leaks into the bloodstream, causing a rise in measured levels. Because ALT is relatively liver-specific, it is considered the most sensitive marker of hepatocyte injury.
Elevated ALT is associated with conditions such as viral hepatitis, non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease, autoimmune hepatitis, and drug-induced liver injury (DILI). Mild, transient elevations can also occur with strenuous exercise or after taking certain medications. Typical reference range: 7–56 U/L, though this varies by laboratory and sex.
AST (Aspartate Aminotransferase)
AST is another enzyme released during cell damage, but it is less liver-specific than ALT — it is also found in cardiac muscle, skeletal muscle, kidneys, and red blood cells. This means an elevated AST alone could reflect liver injury, muscle damage (e.g., after intense exercise or a heart attack), or haemolysis.
The AST:ALT ratio can be diagnostically useful. A ratio greater than 2:1 is often associated with alcoholic liver disease, whereas viral hepatitis tends to produce a higher ALT than AST. Typical reference range: 10–40 U/L.
ALP (Alkaline Phosphatase)
ALP is an enzyme present in many tissues including the liver, bone, intestine, and placenta. In the liver, it is particularly concentrated in the cells lining the bile ducts. Elevated ALP often indicates a problem with bile flow (cholestasis), rather than hepatocyte destruction.
Causes of raised ALP include bile duct obstruction (e.g., gallstones, strictures), primary biliary cholangitis, and drug-induced cholestasis. However, ALP can also be elevated in bone disorders such as Paget’s disease, and is physiologically raised in children and adolescents (due to bone growth) and during pregnancy. If ALP is raised in isolation, GGT is often checked to determine whether the elevation is of liver or bone origin. Typical reference range: 44–147 U/L (adults).
GGT (Gamma-Glutamyl Transferase)
GGT is particularly sensitive to liver pathology related to alcohol and bile ducts. It is often elevated in people who drink alcohol heavily and is used as an indirect marker of alcohol consumption, though it is not specific to alcohol alone — medications, obesity, and non-alcoholic fatty liver disease can also raise GGT.
A raised GGT alongside a raised ALP strongly suggests a hepatobiliary (liver/bile duct) cause rather than a bone cause. Typical reference range: 9–48 U/L for adult men; 7–25 U/L for adult women (values vary widely by laboratory).
Albumin
Albumin is the most abundant protein in the blood and is synthesised exclusively by the liver. Unlike enzymes (which rise when liver cells are damaged), albumin reflects the liver’s synthetic function — its ability to produce proteins. A low albumin suggests that the liver is not making enough protein, which is a sign of significant, often chronic, liver impairment.
However, albumin is also affected by malnutrition, kidney disease (nephrotic syndrome), and acute illness. It is not a specific marker of liver disease alone. Typical reference range: 35–50 g/L.
Bilirubin (Total and Conjugated)
Bilirubin is a yellow pigment produced when red blood cells break down (haemoglobin is metabolised to bilirubin). The liver processes bilirubin into a water-soluble form (conjugated or direct bilirubin) and excretes it into bile. If this process is disrupted, bilirubin accumulates in the blood and can cause jaundice — the yellow tinge to skin and eyes most people recognise.
High bilirubin can arise from three broad categories: pre-hepatic (e.g., haemolytic anaemia — too much bilirubin being produced), hepatic (e.g., hepatitis or cirrhosis — the liver cannot process it), or post-hepatic (e.g., bile duct obstruction — bilirubin cannot be excreted). The ratio of conjugated to unconjugated bilirubin helps distinguish between these. Typical reference range: total bilirubin 3–17 µmol/L (0.2–1.0 mg/dL).
Total Protein
Total protein measures all proteins in the blood, including albumin and globulins (immune proteins). Together with albumin, it gives an indication of the liver’s protein-manufacturing capacity. The albumin-to-globulin (A:G) ratio can help identify conditions such as cirrhosis, multiple myeloma, or autoimmune disease, where globulins may be disproportionately elevated. Typical reference range: 60–80 g/L.
Interpreting Patterns in LFT Results
No single LFT marker should be interpreted in isolation. Clinicians look at the overall pattern of results to distinguish between different types of liver pathology:
| Pattern | Markers most elevated | Possible causes |
|---|---|---|
| Hepatocellular damage | ALT, AST (high) | Viral hepatitis, NAFLD, DILI, alcohol |
| Cholestatic | ALP, GGT (high); bilirubin raised | Bile duct obstruction, PBC, PSC |
| Synthetic failure | Low albumin, raised bilirubin, raised PT | Cirrhosis, acute liver failure |
| Isolated raised bilirubin | Bilirubin only | Gilbert’s syndrome, haemolysis |
Gilbert’s syndrome is a common, benign inherited condition where unconjugated bilirubin is mildly elevated — often noticed incidentally on a blood test. It is not harmful and requires no treatment.
What Causes Abnormal Liver Function Tests?
Abnormal LFT results have many causes, ranging from benign to serious. Common causes include non-alcoholic fatty liver disease (increasingly prevalent due to obesity and metabolic syndrome), alcohol-related liver disease, viral hepatitis (A, B, C, D, E), autoimmune hepatitis, primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC), drug-induced liver injury (including common medications such as paracetamol/acetaminophen, statins, and antibiotics), gallstones and biliary obstruction, and hereditary conditions such as haemochromatosis or Wilson’s disease.
It is worth noting that mildly elevated LFTs are common and do not always indicate serious disease. They often require repeat testing and clinical correlation before a cause is established.
Why Might Liver Function Tests Be Ordered?
LFTs are ordered for a wide range of clinical reasons, including investigation of symptoms (jaundice, abdominal pain, fatigue, nausea), monitoring of known liver disease (e.g., chronic hepatitis, cirrhosis, NAFLD), monitoring of medications that can affect the liver (e.g., statins, methotrexate, antiepileptics), screening in the context of other conditions (e.g., thyroid disease, coeliac disease), pre-operative assessment, and as part of a general health screen.
References
- National Library of Medicine. Liver Function Tests. MedlinePlus. https://medlineplus.gov/lab-tests/liver-function-tests/
- NHS. Liver Function Tests. https://www.nhs.uk/conditions/liver-disease/
- Pratt DS, Kaplan MM. Evaluation of abnormal liver-enzyme results in asymptomatic patients. New England Journal of Medicine. 2000;342(17):1266–1271.
- Giannini EG, Testa R, Savarino V. Liver enzyme alteration: a guide for clinicians. CMAJ. 2005;172(3):367–379.
- European Association for the Study of the Liver (EASL). Clinical practice guidelines on the management of abnormal liver blood tests. Journal of Hepatology. 2023;79(2):457–512.
- American College of Gastroenterology (ACG). ACG clinical guideline: the diagnosis and management of idiosyncratic drug-induced liver injury. American Journal of Gastroenterology. 2014;109(7):950–966.
More questions answered
What type of sample is used for the liver function test panel?
The liver function test panel is commonly performed using serum or plasma, although laboratories may specify a particular tube, container or processing method. Correct collection matters because the analytical method is validated for defined specimen conditions. A wrong or poorly handled specimen can produce unreliable results or be rejected. The collection instructions from the testing laboratory should therefore be followed.
What happens to the sample before the liver function test panel is analysed?
Before analysis, the laboratory confirms patient and specimen identification and checks whether the sample meets acceptance criteria. The specimen may then be centrifuged, separated, mixed or otherwise prepared according to the test method. These pre-analytical steps are controlled because they can affect the final result before the analyser even begins measuring. Unsuitable samples are managed according to laboratory procedure.
What is the laboratory actually measuring in the liver function test panel?
The liver function test panel is designed to determine a group of enzymes, proteins and pigments associated with liver and biliary function. Laboratories use a clinical chemistry analyser so that the analytical signal or observation can be converted into a reportable result. The exact chemistry, optical signal, timing step or detection system varies by test and manufacturer. Students should separate the biological or analytical target from the technology used to measure it.
How does quality control help make the liver function test panel reliable?
Laboratories run quality-control material to check whether the analytical system is performing within accepted limits. Results are assessed using defined rules before patient testing is trusted, and problems can trigger investigation or recalibration. Laboratories may also participate in external quality assessment to compare performance with other services. These systems reduce avoidable analytical error but do not remove normal biological variation.
Why can the liver function test panel use different methods or units between laboratories?
Different laboratories may use different analytical platforms, reagents or calibration systems for the liver function test panel. Some tests can therefore have method-specific units, cut-offs or reference intervals even when they assess the same biological target. Direct comparison is safest when the method and units are compatible. The range and units printed on the reporting laboratory’s result should be used.
Why are previous liver function test panel results useful?
Previous liver function test panel results provide a personal trend that can show whether the value is stable, gradually changing or different from the person’s earlier pattern. A trend can sometimes be more informative than comparing one result only with a population reference interval. Changes still need to be considered alongside method changes, timing and clinical context. Where possible, comparable methods make longitudinal interpretation easier.
What should students remember when interpreting the liver function test panel?
Students should first identify what the test measures, then check the specimen, method, units and reference information. Next, consider medicines, alcohol, muscle or bone sources, haemolysis and many liver or biliary conditions and how ALT, ALP, bilirubin, albumin and other locally reported markers might change the interpretation. A laboratory value should not be converted directly into a diagnosis without context. This stepwise approach connects analytical science with safe result interpretation.