Liver Function Tests (LFT): Understanding ALT, AST, ALP, and Bilirubin

Introduction: Your Liver's Silent Service

Your liver is your body's unsung hero, quietly performing over 500 vital functions every day. It filters toxins, produces proteins, stores energy, regulates blood clotting, aids digestion, and much more. Yet liver disease is remarkably common in Australia – chronic liver disease affects approximately 1 in 10 Australians, and it's the 13th leading cause of death.

The concerning reality is that liver disease often has no symptoms until it's quite advanced. Your liver can lose up to 75% of its function before you feel unwell. This is why liver function tests (LFTs) are so important – they're your early warning system, catching problems before irreversible damage occurs.

This comprehensive guide explains liver function tests – ALT, AST, ALP, GGT, bilirubin, albumin, and total protein – in plain English. You'll learn what these tests measure, how to interpret your results, what causes abnormal liver enzymes, and what you can do to protect your liver health.

Your Liver: The Body's Processing Plant

Located in your upper right abdomen beneath your ribs, your liver weighs about 1.5kg and is your largest internal organ. Its remarkable functions include:

• Detoxification: Filters blood to remove toxins, drugs, alcohol, and metabolic waste
• Protein production: Makes albumin (regulates blood volume), clotting factors (stops bleeding), and immune proteins
• Bile production: Essential for digesting fats and absorbing fat-soluble vitamins (A, D, E, K)
• Glucose regulation: Stores glucose as glycogen and releases it when blood sugar drops
• Fat metabolism: Produces cholesterol and processes dietary fats
• Vitamin and mineral storage: Stores vitamins A, D, E, K, B12, iron, and copper
• Old red blood cell breakdown: Processes hemoglobin into bilirubin
• Hormone regulation: Breaks down excess hormones

Your liver has an amazing ability to regenerate – it can regrow from just 25% of its original size. However, repeated or chronic damage can lead to scarring (fibrosis) and eventually cirrhosis, where damage becomes irreversible.

Understanding Liver Function Tests: The Key Enzymes

1. ALT (Alanine Aminotransferase)

ALT is an enzyme found primarily in liver cells. When liver cells are damaged or inflamed, ALT leaks into the bloodstream, making it one of the most specific indicators of liver injury.

Normal range: 10-40 U/L (units per litre) for men, 7-35 U/L for women

High ALT indicates:

• Viral hepatitis: Hepatitis A, B, C, or E (can cause very high ALT, >1000 U/L)
• Non-alcoholic fatty liver disease (NAFLD): Australia's most common liver condition, affecting 25-30% of adults
• Alcoholic liver disease: From excessive alcohol consumption
• Medications: Paracetamol overdose, statins, antibiotics, anti-epileptics, and many others
• Autoimmune hepatitis: Immune system attacks liver
• Hemochromatosis: Iron overload damaging liver
• Wilson disease: Copper accumulation (rare)
• Recent intense exercise: Can temporarily elevate ALT

How high is high?

• Mild elevation (1-2x normal): Common with fatty liver, mild hepatitis, medications
• Moderate elevation (2-5x normal): More significant liver inflammation or injury
• Severe elevation (>10x normal): Acute liver injury – viral hepatitis, drug toxicity, autoimmune hepatitis
• Very severe (>1000 U/L): Acute hepatitis, paracetamol poisoning, shock liver – requires immediate medical attention

Important note: ALT can remain elevated even after the cause is removed. For example, after stopping excessive drinking, ALT may take weeks to months to normalize.

2. AST (Aspartate Aminotransferase)

AST is found in liver cells, but also in heart, muscle, kidneys, and brain. This makes it less specific for liver disease than ALT.

Normal range: 10-40 U/L

High AST can indicate:

• Liver disease (same causes as ALT)
• Heart attack (myocardial infarction)
• Muscle injury or disease
• Hemolysis (red blood cell breakdown)

The AST/ALT Ratio: A Diagnostic Clue

The ratio of AST to ALT helps identify specific liver conditions:

• AST/ALT ratio <1: Typical of acute viral hepatitis, NAFLD
• AST/ALT ratio >2: Suggests alcoholic liver disease
• AST much higher than ALT: May indicate cirrhosis, muscle damage, or hemolysis

In alcoholic liver disease, AST is often 2-3 times higher than ALT because alcohol damages mitochondria where AST is located, and alcohol also depletes vitamin B6 needed for ALT production.

3. ALP (Alkaline Phosphatase)

ALP is found in liver bile ducts, but also in bones, intestines, and placenta (during pregnancy). Elevated ALP suggests problems with bile flow (cholestasis) or bone disease.

Normal range: 30-120 U/L (varies with age and sex; higher in children and adolescents due to bone growth)

High ALP indicates:

• Bile duct obstruction: Gallstones, pancreatic cancer, bile duct cancer
• Primary biliary cholangitis (PBC): Autoimmune destruction of bile ducts
• Primary sclerosing cholangitis (PSC): Inflammation and scarring of bile ducts
• Infiltrative liver diseases: Sarcoidosis, tuberculosis, lymphoma
• Bone disorders: Paget's disease, osteomalacia, bone fractures, bone metastases
• Medications: Some antibiotics, anti-epileptics
• Pregnancy: Third trimester (normal physiological rise)

If ALP is elevated but other liver enzymes are normal, your doctor may order bone-specific ALP or GGT to determine if the elevation comes from liver or bone.

4. GGT (Gamma-Glutamyl Transferase)

GGT is found primarily in liver and bile ducts. It's particularly sensitive to bile duct problems and alcohol consumption.

Normal range: 10-50 U/L for men, 5-30 U/L for women

High GGT indicates:

• Alcohol consumption: Even moderate amounts raise GGT; it's one of the most sensitive markers
• Non-alcoholic fatty liver disease
• Bile duct obstruction or disease
• Medications: Phenytoin, barbiturates, some antibiotics
• Pancreatitis
• Diabetes and metabolic syndrome

GGT is particularly useful for:

• Confirming that elevated ALP comes from liver rather than bone
• Assessing alcohol-related liver disease (though it can't distinguish current drinking from past heavy use)
• As a general marker of liver disease severity

5. Bilirubin

Bilirubin is a yellow pigment produced when red blood cells break down. Your liver processes bilirubin and excretes it in bile. When bilirubin builds up, you develop jaundice (yellowing of skin and eyes).

Total bilirubin normal range: 3-20 μmol/L

Direct (conjugated) bilirubin: <5 μmol/L

Indirect (unconjugated) bilirubin: 3-15 μmol/L

High Unconjugated (Indirect) Bilirubin

Suggests problems before the liver:

• Hemolysis: Rapid red blood cell breakdown (hemolytic anemia)
• Gilbert's syndrome: Benign genetic condition affecting 3-10% of population; bilirubin rises with fasting or illness but is harmless
• Crigler-Najjar syndrome: Rare genetic disorder

High Conjugated (Direct) Bilirubin

Suggests problems with bile flow:

• Bile duct obstruction: Gallstones, cancer, strictures
• Hepatitis: Viral or autoimmune
• Cirrhosis: Advanced liver scarring
• Primary biliary cholangitis or primary sclerosing cholangitis
• Medications: Many drugs can cause cholestasis

Jaundice appears when bilirubin exceeds about 50 μmol/L. If you notice yellowing of eyes or skin, seek medical attention promptly.

6. Albumin

Albumin is the most abundant protein in blood, made exclusively by the liver. It maintains blood volume, transports hormones and drugs, and provides nutrition to tissues.

Normal range: 35-50 g/L

Low albumin indicates:

• Chronic liver disease: Cirrhosis, chronic hepatitis (liver can't produce enough)
• Malnutrition: Insufficient protein intake
• Kidney disease: Nephrotic syndrome (losing protein in urine)
• Inflammation: Severe infection, inflammatory bowel disease
• Protein-losing conditions: Burns, extensive wounds

Low albumin is a serious finding suggesting significant liver dysfunction. It can cause:

• Edema (swelling in legs and abdomen)
• Ascites (fluid accumulation in abdomen)
• Increased risk of infections

7. Total Protein

Measures all proteins in blood – albumin plus globulins (including antibodies).

Normal range: 60-80 g/L

Low total protein: Usually due to low albumin (liver disease, malnutrition, kidney disease)

High total protein: Usually due to high globulins (chronic infection, autoimmune disease, multiple myeloma)

Common Liver Conditions in Australia

Non-Alcoholic Fatty Liver Disease (NAFLD)

NAFLD affects 25-30% of Australian adults, making it our most common liver condition. Fat accumulates in liver cells, typically in people who drink little or no alcohol.

Risk factors:

• Obesity, especially abdominal obesity
• Type 2 diabetes or prediabetes
• High cholesterol or triglycerides
• Metabolic syndrome
• Rapid weight loss
• Certain medications

Progression:

1. Simple fatty liver (steatosis): Fat accumulation but no inflammation – usually benign
2. NASH (non-alcoholic steatohepatitis): Fat plus inflammation – 20-30% of NAFLD patients develop NASH
3. Fibrosis: Scarring from chronic inflammation
4. Cirrhosis: Advanced scarring – occurs in 10-20% of NASH patients
5. Liver cancer (hepatocellular carcinoma): Increased risk with cirrhosis

Diagnosis:

• Elevated ALT (often mildly, 1-2x normal)
• Ultrasound showing fatty liver
• Liver biopsy (gold standard but invasive – reserved for uncertain cases)
• FibroScan (elastography) – measures liver stiffness to assess fibrosis

Treatment:

• Weight loss: 7-10% weight loss can reverse fatty liver and improve NASH
• Exercise: 150-300 minutes weekly, even without weight loss, improves liver fat
• Mediterranean diet: Reduce refined carbs, increase healthy fats, vegetables, fish
• Manage diabetes, cholesterol, blood pressure
• Avoid alcohol
• Medications: No approved medications yet, but several in development; some doctors use diabetes drugs like pioglitazone or GLP-1 agonists

Alcoholic Liver Disease

Alcohol is directly toxic to liver cells. Chronic excessive drinking causes progressive liver damage:

1. Alcoholic fatty liver: Develops within weeks of heavy drinking; reversible with abstinence
2. Alcoholic hepatitis: Inflammation and liver cell death; can be life-threatening
3. Alcoholic cirrhosis: Irreversible scarring; occurs in 10-20% of heavy drinkers

Lab findings:

• AST/ALT ratio >2 (AST typically 2-3 times higher than ALT)
• Elevated GGT (very sensitive to alcohol)
• High MCV (mean corpuscular volume – red blood cells become larger)
• Low platelets (cirrhosis)
• High bilirubin (advanced disease)
• Low albumin (cirrhosis)

Australian safe drinking guidelines:

• Healthy adults: No more than 10 standard drinks per week, and no more than 4 on any day
• For liver health: Many hepatologists recommend no more than 2 standard drinks daily for men, 1 for women
• With existing liver disease: Complete abstinence

Treatment:

• Stop drinking: Essential; even advanced disease can improve with abstinence
• Nutritional support: B vitamins, especially thiamine
• Medications for severe alcoholic hepatitis: Corticosteroids, pentoxifylline
• Liver transplant: For end-stage disease with demonstrated abstinence

Viral Hepatitis

Hepatitis A

Transmitted through contaminated food/water. Causes acute hepatitis but doesn't become chronic. Vaccine available and recommended for travelers.

Hepatitis B

Transmitted through blood, sexual contact, or mother-to-baby. Can cause chronic infection leading to cirrhosis and liver cancer. Approximately 232,000 Australians live with chronic hepatitis B.

Testing: HBsAg (surface antigen) indicates current infection; Anti-HBs (antibody) indicates immunity

Treatment: Antiviral medications (tenofovir, entecavir) suppress virus

Prevention: Vaccination (included in childhood immunization schedule since 2000)

Hepatitis C

Transmitted through blood contact. About 80% of infections become chronic, causing cirrhosis in 15-30% over 20-30 years. Approximately 143,000 Australians have chronic hepatitis C.

Good news: New direct-acting antivirals (DAAs) cure >95% of cases with 8-12 weeks of treatment. Since these became PBS-funded in 2016, hepatitis C is increasingly being eliminated.

Testing: Anti-HCV antibody (indicates exposure); HCV RNA (indicates active infection)

Treatment: DAAs like sofosbuvir, ledipasvir, glecaprevir

Autoimmune Liver Diseases

Autoimmune Hepatitis

Immune system attacks liver cells. More common in women. Causes chronic hepatitis that can progress to cirrhosis without treatment.

Lab findings: High ALT/AST, positive autoantibodies (ANA, SMA, anti-LKM), high immunoglobulins

Treatment: Immunosuppressants (prednisone, azathioprine)

Primary Biliary Cholangitis (PBC)

Immune system destroys small bile ducts in liver. Mainly affects middle-aged women.

Lab findings: Elevated ALP and GGT, positive anti-mitochondrial antibody (AMA)

Treatment: Ursodeoxycholic acid (UDCA) slows progression

Primary Sclerosing Cholangitis (PSC)

Progressive inflammation and scarring of bile ducts. Often associated with inflammatory bowel disease (especially ulcerative colitis).

Lab findings: Elevated ALP, GGT, bilirubin

Diagnosis: MRCP (MRI of bile ducts) shows characteristic beading

Treatment: No effective medical therapy; liver transplant for end-stage disease

Hemochromatosis

Genetic condition causing iron accumulation in liver (and other organs), leading to cirrhosis and liver cancer. See Iron Studies article for details.

Cirrhosis

Cirrhosis is end-stage liver scarring from any chronic liver disease. The liver becomes nodular and hard, losing its ability to function.

Causes in Australia:

1. Alcohol (40-50%)
2. NASH (20-30%)
3. Hepatitis C (15-20%)
4. Hepatitis B (10-15%)
5. Other (autoimmune, hemochromatosis, etc.)

Complications:

• Portal hypertension: High pressure in portal vein causes enlarged spleen, variceal bleeding
• Ascites: Fluid accumulation in abdomen
• Hepatic encephalopathy: Confusion from toxin buildup
• Liver cancer: 1-5% annual risk with cirrhosis
• Liver failure: May require transplant

Lab findings:

• May have normal or elevated liver enzymes (depends on activity of underlying disease)
• Low albumin
• High bilirubin
• Prolonged INR (clotting time – liver makes clotting factors)
• Low platelets

Medications and Your Liver

Many medications can cause liver enzyme elevation or liver damage:

Common culprits:

• Paracetamol (acetaminophen): Safe at recommended doses (4g/day max for adults); overdose causes severe liver damage
• NSAIDs: Ibuprofen, diclofenac – usually mild elevation
• Statins: Mild ALT elevation in 1-3%; usually benign and doesn't require stopping
• Antibiotics: Amoxicillin-clavulanate, nitrofurantoin, isoniazid
• Anti-epileptics: Phenytoin, valproate
• Herbal supplements: Green tea extract, kava, many others (often unregulated and potentially hepatotoxic)

Always tell your doctor about ALL medications, supplements, and herbal remedies you're taking if you have abnormal liver tests.

Protecting Your Liver Health

Lifestyle Strategies

• Maintain healthy weight: BMI 18.5-24.9; lose weight gradually if overweight
• Exercise regularly: 150-300 minutes weekly of moderate activity
• Eat a balanced diet: Mediterranean-style diet; limit added sugars and refined carbs
• Limit alcohol: Follow safe drinking guidelines or avoid entirely
• Avoid unnecessary medications and supplements: Especially unregulated herbal products
• Practice safe sex and drug use: Prevent hepatitis B and C transmission
• Get vaccinated: Hepatitis A and B vaccines
• Manage chronic conditions: Diabetes, high cholesterol, high blood pressure

When to Test Liver Function

You should request liver function tests if you experience:

• Unexplained fatigue or weakness
• Jaundice (yellowing of skin or eyes)
• Dark urine or pale stools
• Abdominal pain, especially upper right quadrant
• Nausea, vomiting, loss of appetite
• Easy bruising or bleeding
• Itchy skin
• Swollen legs or abdomen

Regular screening recommended for:

• People with risk factors for liver disease (obesity, diabetes, heavy alcohol use)
• People taking potentially hepatotoxic medications
• Known hepatitis B or C carriers
• Family history of liver disease
• General health screening as part of annual check-up

Resources for Further Reading

Conclusion

Your liver works silently and tirelessly, and it's remarkably resilient – capable of regenerating and recovering from significant damage if caught early. But once cirrhosis develops, damage becomes permanent.

This is why understanding your liver function tests is so important. Simple blood tests can detect liver problems years before symptoms appear, when lifestyle changes or medical interventions can still make a difference.

Whether you're monitoring fatty liver disease, recovering from hepatitis, taking potentially hepatotoxic medications, or simply doing a health check, tracking your liver enzymes over time provides valuable insights. Are your lifestyle changes working? Is that new medication affecting your liver? Are trends moving in the right direction?

BloodResults allows you to store all your liver function tests securely, visualize trends over months and years, and share results easily with your healthcare team. Your liver health journey is a long-term commitment – having all your data in one place helps you stay on track.

Take care of your liver, and it will take care of you for a lifetime.