Liver function tests: ALT, AST, ALP, GGT and bilirubin

“Liver function tests” or a “liver panel” means a group of blood tests, not one measurement. It commonly includes ALT, AST, alkaline phosphatase (ALP), GGT, bilirubin and albumin. Prothrombin time or INR may be ordered separately. The exact panel depends on the laboratory, country and clinical question.

The name is slightly misleading. ALT, AST, ALP and GGT mainly signal cell injury or altered bile flow; they do not measure a percentage of liver function. Albumin and clotting factors provide more information about protein synthesis, but they also change for reasons unrelated to the liver. Clinicians therefore interpret the overall pattern alongside symptoms, medicines, alcohol exposure, previous results and other investigations.

What is included in a liver panel?

There is no single international panel. A typical profile may include:

ALT (alanine aminotransferase), the routine enzyme most closely associated with liver cells;
AST (aspartate aminotransferase), found in the liver but also in muscle and other tissues;
ALP (alkaline phosphatase), associated with bile ducts but also produced in bone;
GGT (gamma-glutamyl transferase), used to support a hepatobiliary source for raised ALP;
total and direct bilirubin, reflecting bilirubin production, processing and excretion;
albumin and total protein, providing protein and synthesis context;
prothrombin time/INR, a separate test that may reflect clotting-factor synthesis.

Several of these measurements are included in a US comprehensive metabolic panel, while UK services often order a dedicated liver profile. Check the itemised order: the same panel name does not guarantee the same analytes.

Injury markers are not the same as function markers

ALT and AST enter the bloodstream when cells are injured. Their activity does not quantify the liver’s functional reserve, and the height of an enzyme result does not by itself measure the severity of disease. Conversely, normal aminotransferases do not rule out every chronic liver condition.

Albumin is made in the liver but has a relatively long half-life. A low level can also reflect inflammation, kidney or gastrointestinal protein loss, poor nutrition or fluid overload. A prolonged prothrombin time or raised INR can occur with severely impaired clotting-factor synthesis, but vitamin K deficiency, anticoagulants and other conditions can produce the same finding.

In practice, the panel combines markers of cell injury, cholestasis, bilirubin handling and synthesis. No single number covers all four roles.

Reading the pattern

Pattern	Predominant results	What it suggests	Important checks
Hepatocellular	ALT is disproportionately elevated relative to ALP after each is divided by its own upper limit of normal (ULN); AST may rise alongside ALT but is not part of the formal ALT/ALP comparison	A predominant signal of liver-cell injury	Degree of elevation, symptoms, medicines and supplements, alcohol, infection and muscle exertion
Cholestatic	ALP with GGT; bilirubin may rise	A possible bile-formation or bile-flow pattern	Whether ALP is hepatic rather than bone-derived; whether imaging is needed
Mixed	Both aminotransferases and ALP are meaningfully raised	Features of both injury patterns	Full clinical assessment rather than a conclusion from one ratio
Isolated bilirubin	Bilirubin with relatively quiet enzymes	Total bilirubin should be split into direct and indirect fractions	Haemolysis, conjugation differences, liver or biliary causes
Synthesis context	Low albumin and/or altered PT/INR	May reflect reduced synthesis, but is not specific	Nutrition, inflammation, protein loss, vitamin K and anticoagulants

This table guides the next stage of assessment; it does not list diagnoses. Formal pattern classification may compare ALT and ALP as multiples of their respective upper reference limits. A clinician uses that calculation, the history and follow-up tests together.

ALT and AST: the hepatocellular pattern

ALT is more liver-specific, while AST is also present in skeletal muscle, the heart and other tissues. An isolated AST rise after strenuous exercise can therefore have a muscle component. Both results should be judged against the upper limit printed by the reporting laboratory rather than a generic online range.

The AST-to-ALT ratio occasionally adds context, but it cannot establish the cause by itself. Our guide to ALT and AST liver tests explains units, reference intervals and the ratio’s limitations in more detail.

ALP and GGT: the cholestatic pattern

When ALP rises disproportionately, clinicians consider a cholestatic pattern. ALP is not liver-specific: bone is another major source, and pregnancy can raise placental ALP. A raised GGT alongside ALP supports a hepatobiliary source. GGT on its own is nonspecific and is not recommended as a stand-alone screening test.

Once a cholestatic pattern is confirmed, the clinician reviews symptoms, medicines and history and decides whether ultrasound or additional blood tests are appropriate. ALP and GGT alone cannot show where a bile-flow problem is located.

Total and direct bilirubin

Bilirubin is produced when red blood cells are broken down. It initially circulates as indirect, or unconjugated, bilirubin. The liver conjugates it into the direct form so it can be excreted in bile.

If total bilirubin is raised in isolation, dividing it into direct and indirect fractions helps narrow the possibilities. Predominantly indirect bilirubin can reflect increased red-cell breakdown or reduced conjugation; direct bilirubin can rise when excretion is impaired or liver cells are injured. Neither fraction identifies one cause without the rest of the clinical picture.

US reports commonly express bilirubin in mg/dL, while UK and many other laboratories use µmol/L. Use the units and reference interval on your own report; do not compare the number directly with a range in a different unit.

Albumin and clotting context

Albumin changes relatively slowly and should not be read as an immediate response to a single day’s events. An isolated low albumin with otherwise unremarkable liver tests often has a non-hepatic explanation. Clinicians review it alongside total protein, kidney findings, inflammation, nutritional status and fluid balance.

PT/INR can change more quickly, but interpretation requires knowing whether the person takes warfarin or another anticoagulant and whether vitamin K deficiency is possible. A rising INR in someone not taking anticoagulants, together with acute liver injury, needs prompt professional assessment.

Alcohol, medicines, supplements and exercise

Before interpreting a panel, note factors that may have altered it:

recent and usual alcohol intake;
newly started prescription or over-the-counter medicines and dose changes;
paracetamol/acetaminophen contained in more than one combination product;
herbal, bodybuilding or “detox” supplements;
unusually strenuous resistance or endurance exercise;
acute infection, vomiting or dehydration.

Do not stop prescribed treatment on your own. A clinician can compare the timing of an exposure with the laboratory trend and decide whether treatment should change, another test is needed or the panel should be repeated.

When is a repeat test useful?

A clinician may repeat the panel to confirm an unexpected result, see its direction or check whether a temporary influence has resolved. The interval depends on the degree of abnormality, symptoms and clinical risk; there is no single safe schedule for everyone.

For a cleaner comparison, use the same laboratory where practical, verify the units and collect samples under similar conditions. Report recent exercise, alcohol, illness and every medicine or supplement. Repeating a significantly abnormal result without investigating its possible cause is not a substitute for assessment.

Metabolic health is often reviewed at the same time through LDL and HDL cholesterol, blood glucose and HbA1c. That wider pattern may guide a clinician’s assessment, but it does not diagnose fatty liver disease on its own.

When to seek urgent help

New jaundice—yellowing of the whites of the eyes or skin—needs prompt medical assessment, especially with dark urine, pale stools, fever, severe abdominal pain or repeated vomiting.

Seek emergency help for new confusion, marked drowsiness, disorientation, fainting or bleeding. Acute liver injury accompanied by impaired clotting and altered mental status can indicate acute liver failure and should not wait for a routine repeat test.

Tracking liver tests with HealthLab

HealthLab keeps laboratory PDF reports together, extracts biomarkers and displays their trends over time. This can make it easier to see whether ALT, AST, GGT or bilirubin changed once or continued in the same direction across several panels.

For valid comparisons, check units, reference intervals, laboratory and date. HealthLab organises results and helps prepare a clearer history for a clinical appointment; it does not diagnose a liver condition or determine how urgently it should be treated.

Frequently asked questions

Do I need to fast for liver function tests?

Preparation varies by laboratory, the specific order and any tests collected alongside the panel. Follow the instructions on the order or from the laboratory; if they are unclear, ask about food, drinks and medicines before the blood draw.

Does a high GGT result prove alcohol use?

No. Alcohol can raise GGT, but the marker is nonspecific and can also change with biliary disease, medicines and other conditions. It is interpreted with ALP, ALT, AST, bilirubin and the history, not as a stand-alone alcohol test.

Can liver disease be present with normal ALT and AST?

Yes. Normal aminotransferases do not exclude every chronic process. If symptoms, risk factors or other abnormalities are present, a clinician may order more blood tests or imaging regardless of ALT and AST.

Can a liver panel diagnose fatty liver disease?

No. Abnormal enzymes alongside glucose and lipid changes may prompt a metabolic risk assessment, but they do not confirm liver fat or inflammation. Diagnosis requires clinical evaluation and, when indicated, imaging or other tests.