- Liver function tests (LFTs) measure liver enzymes (AST, ALT, GGT, ALP), bile pigments (bilirubin), and synthetic proteins (albumin).
- AST and ALT are the primary markers of liver cell injury; ALT is more specific to the liver than AST.
- GGT reflects bile duct stress and is elevated by alcohol, fatty liver disease, and certain medications.
- Bilirubin elevated with normal transaminases can indicate biliary obstruction or haemolysis rather than hepatocyte injury.
- Albumin reflects the liver's synthetic capacity; low albumin in the presence of normal or mildly elevated enzymes suggests chronic liver dysfunction.
- Mild, transient elevation of liver enzymes after intense exercise is normal and reverses within 24-48 hours.
- Interpretation requires clinical context. A single elevated value rarely indicates disease; pattern and trend matter more.
What Are Liver Function Tests?
Liver function tests (LFTs) are a panel of blood markers that assess liver synthetic function, evidence of liver cell injury, and biliary flow. The main markers are:
- Enzymes: AST (aspartate aminotransferase), ALT (alanine aminotransferase), GGT (gamma-glutamyl transferase), ALP (alkaline phosphatase)
- Bile pigments: Bilirubin (total, direct, indirect)
- Synthetic proteins: Albumin
- Other markers: Sometimes prothrombin time (PT/INR) for synthetic function, and gamma-globulins for immune status
Together, these markers give a picture of liver health and can signal various patterns of liver dysfunction or biliary obstruction.[1]
Main Liver Function Test Markers
AST (Aspartate Aminotransferase)
AST is an enzyme found primarily in the liver but also in heart, skeletal muscle, and kidneys. When liver cells are damaged, AST leaks into the bloodstream. Typical reference range is 10 to 40 IU/L, though ranges vary by laboratory and sex (men tend to be slightly higher).
An elevated AST can indicate:
- Hepatocyte (liver cell) injury
- Alcohol-related liver disease
- Fatty liver disease
- Viral hepatitis
- Muscle damage (from trauma or strenuous exercise)
The challenge with AST is that it is not specific to the liver; muscle injury, heart injury, or kidney disease can also elevate it.
ALT (Alanine Aminotransferase)
ALT is more specific to the liver than AST, though it is also present in other tissues in smaller amounts. Typical reference range is 10 to 40 IU/L. Elevated ALT more reliably indicates liver-specific damage.[2]
An elevated ALT with normal or mildly elevated AST suggests hepatocyte injury. An elevated AST with normal ALT might indicate muscle damage rather than liver injury.
The AST-to-ALT ratio: An AST-to-ALT ratio above 2.0 often indicates alcoholic liver disease or cirrhosis. A ratio below 1.0 (where ALT is higher) is more consistent with non-alcoholic fatty liver disease or viral hepatitis.
GGT (Gamma-Glutamyl Transferase)
GGT is an enzyme involved in bile acid metabolism. It is elevated by:
- Alcohol consumption (even moderate regular intake)
- Fatty liver disease
- Certain medications (phenytoin, barbiturates)
- Biliary obstruction
GGT is less specific than ALT but can be a sensitive indicator of alcohol consumption or metabolic stress.[3] Elevated GGT with normal transaminases can indicate early-stage liver stress or medication effects rather than acute hepatocyte injury.
ALP (Alkaline Phosphatase)
ALP is produced primarily in the liver and bone. Elevated ALP can indicate:
- Cholestasis (blockage of bile flow)
- Bone disease
- Pregnancy
- Childhood (growing children have higher ALP)
ALP is less specific to liver disease and must be interpreted alongside other markers. If ALP is elevated with normal transaminases and bilirubin is normal, bone disease or other causes are likely.
Bilirubin
Bilirubin is a pigment produced when haemoglobin from old red blood cells is broken down. The liver conjugates (processes) bilirubin for excretion in bile. Elevated bilirubin can indicate:
Unconjugated (indirect) hyperbilirubinaemia: Suggests haemolysis (red blood cell breakdown) or impaired hepatic uptake of bilirubin.
Conjugated (direct) hyperbilirubinaemia: Suggests cholestasis (blockage of bile flow) or impaired hepatic excretion.
Typical total bilirubin range is 5 to 20 mcmol/L. Values above 20 warrant investigation, but context matters greatly.[4]
Albumin
Albumin is a protein synthesised by the liver. It reflects the liver's synthetic capacity (ability to manufacture proteins). Low albumin (under 35 g/L) in the presence of normal or mildly elevated transaminases often indicates chronic liver dysfunction rather than acute injury.[5]
Low albumin can also occur with malnutrition, kidney disease, or inflammatory states. It is interpreted alongside other markers, not in isolation.
Typical Liver Function Test Ranges
| Marker | Typical Range | Borderline | Elevated |
|---|---|---|---|
| AST | 10-40 IU/L | 40-80 IU/L | Above 80 IU/L |
| ALT | 10-40 IU/L | 40-80 IU/L | Above 80 IU/L |
| GGT | 9-48 IU/L (men); 9-32 IU/L (women) | 50-80 IU/L | Above 80 IU/L |
| ALP | 30-120 IU/L | 120-200 IU/L | Above 200 IU/L |
| Total bilirubin | 5-20 mcmol/L | 20-50 mcmol/L | Above 50 mcmol/L |
| Albumin | 35-50 g/L | 30-35 g/L | Below 30 g/L |
Reference range variation: Liver function test reference ranges vary between laboratories. Always compare your result against the specific range on your report, particularly for bilirubin and ALP which have wider variation.
How to Interpret Your Results
A single elevated liver enzyme is rarely diagnostic of disease. Interpretation depends on the pattern and clinical context.
Normal transaminases with elevated GGT: May indicate alcohol consumption, fatty liver, or medication effects. Not typically a sign of acute liver disease.
Elevated transaminases (AST and ALT) with normal ALP and bilirubin: Suggests hepatocyte injury without cholestasis. Could reflect acute viral hepatitis, non-alcoholic fatty liver disease, or medication effects.
Elevated ALP with normal transaminases: Suggests cholestasis or bone disease. Further investigation may include imaging (ultrasound) to look for bile duct obstruction.
Elevated bilirubin with normal transaminases: May indicate haemolysis, biliary obstruction, or Gilbert syndrome (a benign genetic variant). The pattern of direct vs indirect bilirubin helps distinguish.
Elevated transaminases with low albumin: Suggests chronic liver dysfunction rather than acute injury. More concerning pattern that warrants specialist assessment.
What Affects Liver Function Tests
Alcohol consumption: Even moderate regular intake elevates GGT. Heavy consumption elevates all liver enzymes, with AST-to-ALT ratio above 2.0 being characteristic of alcoholic liver disease.
Fatty liver disease: Non-alcoholic fatty liver disease (NAFLD) affects over 25% of Australian adults and elevates transaminases, particularly ALT, and GGT.[6] It is associated with obesity, type 2 diabetes, and metabolic syndrome.
Intense exercise: Strenuous exercise, particularly eccentric training and long-duration endurance activities, can transiently elevate AST, ALT, and GGT for 24-48 hours post-exercise.[7] The elevation is typically modest and reverses quickly. Athletes should have this context documented when testing.
Medications: Numerous medications elevate liver enzymes. Paracetamol (acetaminophen), statins, antibiotics, and many others can cause mild transaminase elevation.
Viral and bacterial infections: Acute hepatitis from viruses elevates transaminases substantially. Bacterial infections can elevate several markers.
Body weight: Excess adipose tissue is strongly associated with elevated transaminases, particularly with fatty liver disease.
Sleep: Poor sleep is associated with elevated transaminases and fatty liver risk.
Who Should Consider Liver Function Testing
Request liver function tests if you:
- Drink alcohol regularly (a baseline is reasonable)
- Are overweight or have been told you have fatty liver
- Have abdominal pain, jaundice, dark urine, or pale stools
- Are about to start a medication known to affect liver function
- Have a family history of liver disease
- Want a baseline assessment as part of a comprehensive health check
Routine testing is reasonable for anyone seeking a health baseline. If results are abnormal, follow-up with your GP is important.
When to See a GP
Discuss your results with a GP if:
- Transaminases are significantly elevated (above 3x the upper limit of normal)
- Bilirubin is elevated above 50 mcmol/L
- Albumin is low (under 35 g/L)
- You have symptoms including jaundice, right upper quadrant pain, or unexplained fatigue alongside abnormal results
- You are contemplating significant lifestyle changes (weight loss, dietary changes) and want clinical guidance on the best approach
Liver Function Testing and Longitudinal Health Monitoring
For athletes or highly active people, baseline liver function testing is reasonable before starting a new training phase or supplement regimen. Retesting 6-12 weeks later provides reassurance that training or supplements are not causing hepatic stress.
For anyone with risk factors for fatty liver (obesity, metabolic syndrome, type 2 diabetes), annual or biennial liver function testing helps track liver health over time and motivates improvement in metabolic markers and weight.
FAQ
What do liver function tests measure?
Liver function tests measure enzymes and proteins made by the liver, plus bile pigments it processes. The main markers are AST, ALT, GGT, ALP, bilirubin, and albumin. Together they assess liver synthetic function, injury, and biliary flow.
What is the difference between AST and ALT?
Both are enzymes released when liver cells are damaged. AST is found in the liver but also in heart, muscle, and kidney. ALT is more specific to the liver. An elevated ALT-to-AST ratio typically indicates liver-specific damage rather than muscle or other tissue damage.
Why is GGT elevated and what does it mean?
GGT is elevated by alcohol consumption, fatty liver disease, and certain medications. An elevated GGT with normal AST and ALT can indicate less acute liver stress but is worth investigating with your GP.
Can intense exercise temporarily elevate liver enzymes?
Yes. Strenuous exercise, particularly eccentric exercise and long-duration endurance training, can temporarily elevate AST, ALT, and GGT for 24-48 hours post-exercise. The elevation is typically modest and reverses quickly.
Do I need to fast before liver function tests?
Fasting is not required for most liver markers. However, if LFTs are ordered alongside a lipid panel or glucose, fasting may be required for those components. Check your request form.
