Liver Function Tests Explained: Understanding ALT, AST, ALP, GGT, Bilirubin and Albumin

This article provides general health education only. It does not diagnose liver disease, explain the cause of an individual's abnormal liver blood tests, or recommend personalised treatment, alcohol limits, medication changes or supplementation.

Liver blood tests must be interpreted using the reference intervals supplied by the laboratory that performed the analysis, together with symptoms, medical history, alcohol exposure, medications and supplements, metabolic risk factors, previous results, imaging and other investigations.

Seek prompt medical assessment if abnormal liver tests occur with symptoms such as new jaundice, confusion, significant drowsiness, vomiting blood, black stools, rapidly increasing abdominal swelling, severe abdominal pain or significant deterioration in general health. These can indicate complications requiring urgent clinical evaluation.

The term liver function tests suggests that every marker directly measures how well the liver is functioning.

That is not quite true.

ALT and AST primarily indicate cellular injury.

ALP and GGT frequently indicate cholestasis or biliary involvement.

Albumin and clotting measures provide information closer to actual synthetic function.

Bilirubin reflects the liver's ability to process and excrete a breakdown product of haemoglobin, but bilirubin can also rise because red blood cells are being broken down excessively or because bile flow is obstructed.

This distinction matters because someone may have markedly elevated ALT yet still have preserved liver synthetic function, particularly during an acute episode.

Conversely, a person with advanced chronic liver disease may have ALT within the reference interval even though substantial fibrosis or cirrhosis is present.

BSG guidance makes this point explicitly: the extent of liver blood-test abnormality is not necessarily a measure of how clinically important the underlying disease is.

This is one of the most important concepts to understand before interpreting an LFT panel.

The liver performs hundreds of biochemical functions essential to normal physiology.

Among its major roles are:

processing nutrients absorbed from the gastrointestinal tract, regulating glucose and lipid metabolism, synthesising proteins including albumin and many clotting factors, metabolising drugs and toxins, producing bile, processing bilirubin, storing nutrients, and participating in immune and metabolic regulation.

Because these processes are diverse, no single blood test can provide a complete assessment of liver health.

That is why clinicians use combinations of liver blood tests, clinical history, physical examination, imaging and—when necessary—fibrosis assessment or other specialist investigations. BSG guidance similarly emphasises context and recommends a structured evaluation rather than relying on any one analyte.

Alanine aminotransferase (ALT) is an enzyme found in high concentrations inside liver cells, or hepatocytes.

When hepatocytes are injured, ALT can leak into the bloodstream.

For this reason, ALT is commonly used as a marker of hepatocellular injury. NHS guidance lists several recognised causes of ALT elevation, including viral hepatitis, metabolic fatty liver disease, alcohol-related liver disease, autoimmune hepatitis and drug-induced liver injury.

ALT is relatively liver-focused compared with AST, although it is not completely liver-specific.

A raised ALT tells you:

There may be hepatocyte injury.

It does not tell you:

Exactly what caused it.

And it does not by itself tell you:

How much fibrosis or permanent liver damage is present.

Reference ranges differ between laboratories and assays.

NHS Specialist Pharmacy Service gives a broad illustrative adult interval of approximately:

0–40 IU/L

for ALT and AST, but explicitly advises clinicians to use local laboratory reference ranges.

This is important because laboratories may use sex-specific thresholds, different upper limits or different analytical methods.

A result of 42 IU/L, for example, may be slightly above one laboratory's range and within another's.

Do not replace the laboratory's own upper limit of normal with a generic internet figure.

Common possibilities include:

Metabolic dysfunction-associated steatotic liver disease

MASLD is associated with metabolic risk factors such as excess body weight, insulin resistance and type 2 diabetes. ALT may be elevated, but NICE warns that normal liver enzymes do not exclude the condition.

Alcohol-related liver disease

Alcohol can cause a spectrum ranging from hepatic steatosis through alcoholic hepatitis to fibrosis and cirrhosis. Blood-test patterns vary, and enzyme concentrations alone do not determine disease severity.

Viral hepatitis

Acute or chronic hepatitis B, C and other viral infections can raise aminotransferases. Importantly, chronic viral hepatitis may sometimes exist with relatively modest or intermittently normal enzymes, which is why appropriate risk-based testing remains important.

Medicines and supplements

Prescription medicines, over-the-counter products and herbal or nutritional supplements can cause liver injury.

Drug-induced liver injury can range from mild enzyme elevations to severe acute hepatitis. EASL guidance classifies biochemical liver injury partly using the relationship between ALT and ALP relative to each test's upper limit of normal.

Autoimmune hepatitis

Autoimmune hepatitis can produce substantially raised aminotransferases and requires specialist diagnosis using biochemical, immunological and sometimes histological information.

Ischaemic injury

A major reduction in liver blood supply, often in the setting of severe circulatory illness, can cause very large rises in AST and ALT.

The degree of elevation can help narrow the possibilities even though it does not directly measure disease severity.

NHS guidance states that ALT or AST greater than approximately 1,000 IU/L suggests a relatively limited group of severe acute processes, including:

drug-induced liver injury such as paracetamol toxicity, acute viral hepatitis, ischaemic hepatitis, and autoimmune hepatitis.

Such results need a different level of attention from a mildly elevated ALT of 50 or 60 IU/L.

However, the reverse conclusion is dangerous:

A mildly raised ALT does not necessarily mean mild disease.

BSG guidance describes chronic hepatitis C, fatty liver disease and alcohol-related liver disease as conditions that can produce only mild or moderate abnormalities despite clinically important chronic disease.

Healio360 Insight Enzyme height is not the same as liver damage.

Consider two hypothetical people:

Person A: ALT 1,200 IU/L during acute viral hepatitis, with preserved albumin and clotting.

Person B: ALT 32 IU/L with advanced chronic cirrhosis and portal hypertension.

Person A has the much higher ALT.

Person B may have the much more advanced long-term liver disease.

This is why clinical interpretation must separate current cell injury from underlying liver reserve and fibrosis.

Aspartate aminotransferase (AST) is another enzyme released during cellular injury.

Unlike ALT, AST is widely present outside the liver, including in skeletal muscle and cardiac tissue.

A raised AST therefore does not automatically mean the liver is the source.

NHS guidance notes that AST and ALT can rise in liver disorders but also in other conditions, including heart-related disease.

Clinical context becomes particularly important when AST is elevated disproportionately or when there has been:

strenuous exercise, muscle injury, muscle disease, or another reason for extra-hepatic cellular injury.

ALT tends to be more liver-specific.

AST can originate from liver and non-liver tissues.

Clinicians sometimes consider the AST-to-ALT relationship when interpreting patterns.

For example, an AST/ALT ratio greater than one can occur in advanced fibrosis, and a relatively high AST compared with ALT is often seen in alcohol-related liver disease. However, the ratio is not sufficiently specific to diagnose alcohol-related disease by itself. EASL guidance notes that a high AST/ALT ratio can reflect advanced alcohol-related liver disease rather than simply proving heavy alcohol consumption.

No responsible interpretation should conclude "alcohol-related liver disease" from the ratio alone.

Alkaline phosphatase (ALP) is an enzyme found in several tissues.

Within the liver, it is particularly associated with cells lining the biliary system.

But ALP is also produced in bone, making isolated ALP elevation more complicated than ALT elevation.

NHS guidance states that high ALP can occur with:

cholestasis, biliary obstruction, primary biliary cholangitis and other hepatobiliary disease, but also bone disorders and vitamin D-related bone disease.

This is where GGT becomes useful.

Suppose ALP is high.

The question is:

Is the source liver/bile ducts or bone?

When ALP and GGT are both elevated, a hepatobiliary source becomes more likely.

When ALP is high but GGT is normal, clinicians may consider non-hepatic sources such as bone, although interpretation still depends on the clinical setting.

This is one of the reasons liver blood tests are interpreted as a panel.

Cholestasis means impaired bile formation or bile flow.

The classic biochemical pattern is:

ALP ↑ + GGT ↑

with ALT and AST often elevated to a lesser extent.

BSG guidance lists causes such as:

primary biliary cholangitis, primary sclerosing cholangitis, bile-duct obstruction from stones or strictures, malignancy affecting the biliary system, hepatic congestion, and drug-induced liver injury.

Imaging—often beginning with ultrasound—can become important when obstruction or structural biliary disease is suspected.

Gamma-glutamyl transferase (GGT) is an enzyme concentrated particularly within the hepatobiliary system.

GGT is sensitive but not very specific.

It may increase with:

cholestasis, alcohol exposure, MASLD, chronic hepatitis, and some medications.

NHS guidance notes that GGT can be elevated by alcohol and that raised GGT accompanying raised ALP supports a hepatobiliary source.

This is why an isolated GGT elevation should not automatically be labelled "alcohol-related liver disease."

No.

Alcohol can increase GGT, but GGT is not sufficiently specific to diagnose harmful alcohol use.

It may also rise in metabolic fatty liver disease, cholestatic disorders, chronic hepatitis and medication-related enzyme induction.

A high GGT can be a useful clue when considered alongside:

alcohol history, ALT, AST, ALP, metabolic risk, medications, and imaging.

But it is not a biochemical lie detector for alcohol consumption.

Bilirubin is a yellow pigment generated during normal breakdown of haemoglobin from ageing red blood cells.

It is transported to the liver, processed and excreted into bile.

NHS guidance gives a broad illustrative total bilirubin interval of approximately:

5–21 µmol/L

although laboratory ranges vary.

Elevated bilirubin can occur because of:

increased red-cell breakdown, impaired hepatic processing, hepatocellular disease, impaired bile excretion, or bile-duct obstruction.

The clinical meaning depends partly on whether conjugated/direct or unconjugated/indirect bilirubin predominates.

One of the commonest explanations for isolated raised bilirubin is Gilbert syndrome.

BSG guidance notes that Gilbert syndrome affects approximately 5–8% of the population and typically produces isolated unconjugated hyperbilirubinaemia without evidence of liver disease.

Bilirubin may fluctuate and can rise during fasting, illness or other physiological stress.

However, an isolated raised bilirubin should not automatically be assumed to be Gilbert syndrome.

Haemolysis and other causes may need exclusion depending on the clinical picture. BSG recommends considering full blood count and appropriate haemolysis tests when anaemia or other features make increased red-cell destruction possible.

Albumin is a major circulating protein synthesised by the liver.

Because the liver makes albumin, low albumin can occur with chronic liver dysfunction.

However, albumin is not liver-specific.

NHS guidance gives a broad adult reference interval of approximately:

35–50 g/L

and notes that albumin can also fall with:

sepsis, systemic inflammation, nephrotic syndrome, malabsorption, gastrointestinal protein loss, and other conditions.

Albumin also has a relatively long half-life, roughly 20 days, meaning it changes more slowly than liver enzymes during an acute event.

This misconception is common.

A low albumin result may reflect liver disease, but it may also result from:

inflammation, kidney disease causing urinary protein loss, gastrointestinal loss, malabsorption, severe systemic illness, or other nutritional and metabolic problems.

NHS guidance specifically notes that isolated low albumin without other abnormal liver blood tests is unlikely to be hepatic in origin.

The combination matters.

Low albumin plus other evidence of chronic liver disease is considerably more concerning than a slightly low isolated albumin during an acute inflammatory illness.

The liver produces many proteins involved in blood clotting.

Consequently, prothrombin time (PT) and its standardised expression, the INR, can provide important information about liver synthetic function.

A prolonged INR can occur when the liver is unable to synthesise clotting factors adequately.

But INR can also be affected by:

anticoagulant medication such as warfarin, vitamin K deficiency, severe nutritional problems, and other coagulation disorders.

INR therefore requires clinical interpretation.

In suspected acute liver failure, worsening coagulation combined with other clinical abnormalities is particularly important because it may indicate impaired liver synthetic capacity rather than merely hepatocyte enzyme leakage. EASL acute liver-failure guidance incorporates coagulation status into assessment of severe acute hepatic dysfunction.

Platelet count is not technically a liver function test.

But it is extremely useful in chronic liver assessment.

NHS guidance notes that thrombocytopenia is a common blood abnormality in chronic liver disease, although low platelets have many other possible causes.

In advanced chronic liver disease, platelets may decline partly because portal hypertension enlarges the spleen and alters platelet sequestration, while other mechanisms may also contribute.

This is one reason platelet count is incorporated into non-invasive fibrosis tools such as FIB-4.

EASL guidance recommends using AST, ALT and platelet count in non-invasive evaluation strategies for liver fibrosis.

A simplified pattern-based approach is useful.

1. Hepatocellular pattern

Typical pattern:

ALT ↑↑ AST ↑↑ ALP normal or less elevated.

Possible causes include:

viral hepatitis, MASLD, alcohol-related liver disease, autoimmune hepatitis, drug-induced liver injury, ischaemic injury.

2. Cholestatic pattern

Typical pattern:

ALP ↑↑ GGT ↑↑ ALT/AST less elevated.

Possible causes include:

bile-duct stones, biliary obstruction, primary biliary cholangitis, primary sclerosing cholangitis, malignancy affecting bile drainage, drug-induced cholestasis.

3. Isolated bilirubin elevation

Typical pattern:

bilirubin ↑ other liver enzymes largely normal.

Possible causes include:

Gilbert syndrome, haemolysis, other disorders of bilirubin metabolism.

Real patients do not always fit neatly into one category.

A mixed pattern occurs when both aminotransferases and ALP are substantially elevated.

Drug-induced liver injury is one important situation in which clinicians formally compare ALT and ALP relative to their upper limits of normal to classify injury as hepatocellular, cholestatic or mixed. EASL describes this approach using the biochemical R ratio.

This is specialist-level interpretation and should not be used by patients to self-diagnose medication toxicity.

Metabolic dysfunction-associated steatotic liver disease—MASLD—is the contemporary term increasingly used for the condition historically called NAFLD.

It is strongly associated with metabolic risk, including obesity, insulin resistance and type 2 diabetes.

One of the most dangerous misconceptions is:

"My ALT is normal, therefore I don't have fatty liver disease."

NICE specifically warns against this conclusion.

Routine liver blood tests may be normal in people with fatty liver disease, and NICE states that they should not be used to rule out the condition.

The clinical question often needs to shift from:

"Are my enzymes abnormal?"

to:

"Is there evidence of liver fibrosis?"

Fibrosis is the accumulation of scar tissue in the liver following persistent or repeated injury.

Advanced fibrosis can progress to cirrhosis.

Routine LFTs are poor tools for accurately staging fibrosis.

For people with established MASLD, guidelines therefore recommend specific fibrosis-risk assessment rather than relying on ALT alone.

NICE recommends assessment for advanced fibrosis in people with fatty liver disease, while EASL supports non-invasive fibrosis strategies incorporating tools such as FIB-4 and liver stiffness measurement in appropriate settings.

FIB-4 is a non-invasive fibrosis-risk score based on:

age, AST, ALT, and platelet count.

It is not a liver enzyme.

It is a mathematical risk tool intended to help identify people who may require further fibrosis assessment.

BSG guidance recommends fibrosis risk stratification in people with fatty liver disease, and EASL supports FIB-4 as a first-line non-invasive test in several risk pathways.

A FIB-4 result should be interpreted according to the population, age and pathway in which it is being used rather than as a standalone diagnosis of cirrhosis.

FibroScan is a device using vibration-controlled transient elastography to estimate liver stiffness.

Liver stiffness correlates with fibrosis, although inflammation and other factors can sometimes affect measurements.

NICE describes FibroScan as a non-invasive method capable of distinguishing minimal fibrosis from more advanced liver scarring and cirrhosis in appropriate clinical settings.

FibroScan answers a different question from ALT.

ALT asks:

Is there biochemical evidence of hepatocyte injury?

FibroScan asks:

How stiff—and therefore potentially scarred—is the liver?

Healio360 Insight Normal liver enzymes do not equal zero liver risk.

A person can have:

ALT: normal AST: normal advanced fibrosis: present

especially in chronic metabolic or other liver disease.

This is why fibrosis assessment has become increasingly important in modern hepatology.

Alcohol can influence liver enzymes, particularly GGT and AST.

But blood tests cannot determine exactly how much alcohol someone drinks, and no single enzyme can diagnose alcohol-related liver disease.

BSG guidance emphasises assessing the entire clinical context rather than interpreting an abnormal analyte alone.

A person with alcohol-related cirrhosis may have relatively modest enzymes, while another person may have much higher enzymes from a reversible acute insult.

Again:

enzyme height ≠ fibrosis severity.

Medication history is essential when interpreting LFTs.

Potentially relevant substances include:

prescription medicines, over-the-counter medicines, herbal products, bodybuilding supplements, weight-loss supplements, and other nutraceuticals.

Drug-induced liver injury can mimic many other liver conditions and may present with hepatocellular, cholestatic or mixed biochemical patterns.

Do not stop prescribed medication solely because a liver enzyme is mildly elevated unless advised by the prescribing clinician.

The correct response depends on:

the medication, magnitude of elevation, symptoms, bilirubin, previous results, competing causes, and the clinical reason the medicine is being used.

Statins are a frequent source of unnecessary concern.

A mildly elevated ALT in someone taking a statin does not automatically mean serious liver injury or that the medication must be stopped.

BSG guidance even uses the example of a well patient taking a statin with ALT around 80 IU/L to illustrate why biochemical numbers must be interpreted in context rather than automatically equated with severe disease.

Medication decisions should be based on established monitoring guidance and individual clinical assessment.

AST—and to some extent ALT—can increase following significant skeletal muscle injury or very strenuous exercise because aminotransferases are not exclusively hepatic.

This can complicate interpretation, particularly when creatine kinase is also elevated.

The possibility of a muscle rather than liver source becomes more relevant when AST is disproportionately raised and other hepatobiliary markers remain normal.

This is another example of why LFT interpretation begins with history rather than with automated diagnosis.

A common approach to mildly abnormal liver tests is:

"We'll repeat them in a month and see what happens."

Sometimes that is reasonable.

But it should not substitute for appropriate investigation.

BSG guidance found that in the BALLETS study, 84% of adults with abnormal liver blood tests still had abnormal tests one month later. It therefore argues that abnormal results should often prompt assessment of the likely cause rather than repeated testing without a diagnostic plan.

Even normalisation does not necessarily prove that chronic liver disease has disappeared.

Hepatitis C and fatty liver disease, for example, can exist despite normal enzymes.

The investigation depends on the pattern and clinical context.

The BSG adult liver aetiology screen includes consideration of:

abdominal ultrasound, hepatitis B testing, hepatitis C testing, autoimmune markers, immunoglobulins, ferritin, and transferrin saturation.

Additional investigations may be needed depending on:

age, symptoms, family history, medication exposure, alcohol use, metabolic risk, travel, infection risk, and specific biochemical abnormalities.

This does not mean everyone with ALT 45 needs every liver investigation available.

It means unexplained abnormalities should be approached systematically.

Most mild abnormalities found incidentally are not medical emergencies.

However, some patterns require much faster investigation.

Examples include:

very large ALT or AST elevations, new jaundice, worsening bilirubin with other abnormal LFTs, impaired clotting, confusion or altered consciousness, significant abdominal swelling, vomiting blood, evidence of gastrointestinal bleeding, suspected paracetamol overdose, severe illness with possible acute liver injury, suspected bile-duct obstruction accompanied by fever or severe pain.

Marked ALT/AST elevations above approximately 1,000 IU/L are specifically recognised by NHS guidance as potentially reflecting severe acute liver injury requiring secondary-care assessment.

Blood Tests & Biomarkers HbA1c Explained: Understanding Your Blood Sugar Over Time Ferritin Explained: Understanding Your Iron Stores Vitamin B12 Explained Vitamin D Explained Thyroid Function Tests Explained High Creatinine: What an Elevated Creatinine Blood Test Means Complete Blood Count Explained Bilirubin Explained ALT Explained AST Explained ALP Explained: Understanding Alkaline Phosphatase GGT Explained Albumin Blood Test Explained Platelets Explained FIB-4 Liver Fibrosis Score Explained Liver & Metabolic Health Fatty Liver Disease Explained What Does "Hepatic Steatosis" Mean on an Ultrasound? Liver Fibrosis Explained FibroScan Explained Metabolic Syndrome Explained High Triglycerides Explained Understanding Medical Reports What Does "Unremarkable" Mean on a Scan? What Does "Mild Degenerative Changes" Mean? What Does "Clinical Correlation Recommended" Mean?

One abnormal ALT is a snapshot. Your liver health is a timeline.

Liver blood tests become substantially more useful when they are viewed alongside previous results.

An isolated report might say:

ALT: 68 IU/L — High

But a longitudinal record might reveal:

ALT 32 → 39 → 48 → 68

AST 27 → 31 → 37 → 45

GGT 36 → 51 → 71 → 89

Platelets 248 → 232 → 216 → 194

HbA1c 39 → 42 → 45 → 47 mmol/mol

Triglycerides 1.3 → 1.7 → 2.1 → 2.5 mmol/L

That connected view creates a much more meaningful discussion about metabolic health and potential liver risk than reading ALT in isolation.

Healio360 helps bring those pieces together.

Upload and organise health reports, understand medical terminology in plain language and follow biomarkers over time so that disconnected results become a longitudinal health story.

Instead of asking only:

"Why is my ALT high?"

you can begin asking:

"When did it start rising?"

"What happened to my other liver markers?"

"Did my metabolic markers change at the same time?"

"Were platelets stable?"

"Was fibrosis ever assessed?"

"Could medication or alcohol exposure explain the timing?"

"Is this a persistent trend or a temporary change?"

Turn fragmented medical information into a clearer, connected health story.

Healio360 provides educational health information and tools to organise and understand health records. It does not diagnose liver disease, replace professional medical assessment or provide personalised treatment recommendations.