Liver function tests measure several enzymes and proteins that reflect liver health, injury, and synthetic function. The standard panel typically includes ALT, AST, alkaline phosphatase, total bilirubin, albumin, and total protein. Liver function tests are part of the comprehensive metabolic panel and are commonly ordered for routine screening, evaluation of unexplained symptoms, monitoring chronic liver disease, and assessing medication effects on the liver. This guide covers what each component measures, normal ranges, and what abnormal patterns suggest.
What liver function tests actually measure
The standard liver panel includes:
- ALT (alanine aminotransferase): Liver-specific enzyme released with hepatocyte injury
- AST (aspartate aminotransferase): Enzyme found in liver, heart, and muscle
- Alkaline phosphatase (ALP): Enzyme from liver bile ducts and bone
- GGT (gamma-glutamyl transferase): Sometimes added; liver and bile duct enzyme
- Total bilirubin: Bile pigment, marker of bile flow and red cell turnover
- Direct (conjugated) bilirubin: Bilirubin processed by the liver
- Albumin: Major liver protein
- Total protein: Albumin plus globulins
Reference ranges
Typical adult reference ranges:
- ALT: 7-56 U/L
- AST: 10-40 U/L
- ALP: 44-147 IU/L
- GGT: 9-48 U/L
- Total bilirubin: 0.1-1.2 mg/dL
- Direct bilirubin: 0.0-0.3 mg/dL
- Albumin: 3.5-5.0 g/dL
- Total protein: 6.0-8.3 g/dL
Patterns of abnormality matter more than individual values
Liver function tests are most useful when interpreted as patterns:
Hepatocellular pattern: ALT and AST predominantly elevated, with relatively mild ALP elevation. Suggests liver cell injury — viral hepatitis, fatty liver, alcohol-related liver injury, drug-induced liver injury.
Cholestatic pattern: ALP and bilirubin predominantly elevated, with relatively mild ALT/AST elevation. Suggests bile duct obstruction or cholestatic liver diseases — gallstones, primary biliary cholangitis, primary sclerosing cholangitis, drug-induced cholestasis.
Mixed pattern: Both transaminases and ALP elevated. May suggest various conditions including infiltrative liver diseases, certain drug reactions.
ALT and AST elevations
The most common cause of mildly elevated ALT and AST in the US is metabolic dysfunction-associated steatotic liver disease (MASLD/MAFLD, formerly NAFLD), which affects about 25-30% of US adults. Alcohol-related liver injury, viral hepatitis (B, C), medication effects, and autoimmune hepatitis are other common causes.
The AST:ALT ratio provides clues. ALT>AST suggests viral hepatitis or fatty liver. AST>ALT (often ratio 2:1 or higher) suggests alcohol-related liver disease or advanced fibrosis. Very high transaminases (1,000+ U/L) suggest acute viral hepatitis, drug-induced injury (acetaminophen toxicity), or ischemic injury. The American Association for the Study of Liver Diseases publishes detailed clinical guidance.
Alkaline phosphatase elevations
ALP is found in liver, bile ducts, bone, intestine, and placenta. Isolated ALP elevation requires distinguishing liver from bone source — GGT elevation alongside ALP suggests liver origin; isolated ALP without GGT elevation suggests bone source. Common causes include bile duct obstruction, primary biliary cholangitis, infiltrative liver disease, bone disorders, and pregnancy.
Bilirubin elevations
Total bilirubin elevations have several patterns:
Indirect (unconjugated) hyperbilirubinemia: Direct bilirubin proportionally low. Suggests increased red cell turnover (hemolysis), Gilbert syndrome (a benign genetic variant affecting 5-10% of the population), or impaired conjugation.
Direct (conjugated) hyperbilirubinemia: Direct bilirubin proportionally elevated. Suggests bile duct obstruction, hepatocellular dysfunction, or specific genetic syndromes.
Jaundice typically becomes visible at total bilirubin above 2.5-3.0 mg/dL.
Albumin and synthetic function
Low albumin may indicate chronic liver disease (impaired synthesis), kidney disease (protein loss in urine), malnutrition, or chronic inflammation. Albumin reflects long-term liver function rather than acute injury — albumin can be normal in acute hepatitis but low in cirrhosis.
The combination of low albumin, prolonged PT/INR (synthetic function), and elevated bilirubin suggests significant hepatic dysfunction. This pattern features in the Child-Pugh score for chronic liver disease.
Workup of mildly elevated liver enzymes
Many patients have mildly elevated ALT or AST on routine screening without symptoms. Common workup includes:
- Repeat testing to confirm
- Hepatitis B and C screening
- Iron studies for hereditary hemochromatosis
- Liver ultrasound for fatty liver and structural assessment
- Review of medications and alcohol intake
- Autoimmune liver disease screening if indicated
- Specialist referral if elevation persists or is significant
Frequently Asked Questions
Do I need to fast for liver function tests?
Not strictly required, though fasting may produce slightly more reliable results. Many providers schedule LFTs alongside other fasting tests.
What does mildly elevated ALT mean?
Most commonly fatty liver disease. Other causes include viral hepatitis, medications, alcohol, and various less common conditions. Repeat testing and additional workup are often appropriate.
Can medications cause abnormal liver tests?
Yes. Many medications (statins, certain antibiotics, NSAIDs, supplements) can cause liver enzyme elevations. Acetaminophen overdose can cause severe hepatic injury.
Is fatty liver serious?
Most cases are mild and don’t progress significantly. About 20% of patients with non-alcoholic fatty liver progress to NASH (now MASH), which can lead to fibrosis, cirrhosis, and liver cancer.
How often should I get liver function tests?
Annual screening is common. More frequent testing applies during medication monitoring, with chronic liver disease, or when investigating symptoms.
The bottom line on liver function tests
Liver function tests provide a snapshot of liver health, injury, and synthetic function. Patterns of abnormality matter more than individual values — hepatocellular vs cholestatic patterns suggest different diagnoses. Mild elevations are common and most often reflect fatty liver disease. Significant abnormalities warrant investigation. Discuss results with your healthcare provider for proper interpretation in clinical context.