Liver Function Tests (LFTs), also known as a liver panel, are a group of blood tests that measure different enzymes, proteins, and other substances made by the liver. These tests are crucial tools used in diagnosing, monitoring, and evaluating the health and function of the liver. While the name suggests they measure function, many LFTs primarily indicate liver inflammation or damage rather than the liver’s overall capacity to perform its many tasks.
The liver is a vital organ with numerous critical roles, including:
- Producing bile, essential for digestion.
- Synthesizing proteins, such as albumin and clotting factors.
- Metabolizing fats, carbohydrates, and proteins.
- Detoxifying waste products, drugs, and toxins.
- Storing glycogen, vitamins, and minerals.
Because the liver performs such diverse functions, no single test can capture its complete status. Therefore, LFTs are typically ordered as a panel, with the collective results providing a more comprehensive picture.
Introduction to the Core Categories of LFTs
LFTs can broadly be categorized based on what they primarily assess:
- Tests Indicating Hepatocellular Injury (Liver Cell Damage): These tests measure enzymes that are normally found within liver cells and are released into the bloodstream when the cells are damaged or inflamed.
- Tests Indicating Cholestasis (Impaired Bile Flow): These tests measure substances that are either produced in response to blocked bile ducts or are components of bile that back up into the bloodstream.
- Tests Evaluating the Liver’s Synthetic Function: These tests measure substances that the liver produces; levels can be altered when the liver’s ability to synthesize is compromised, often indicating more chronic or severe disease.
Understanding these categories helps in interpreting patterns seen in LFT results.
Key Tests for Detecting Liver Injury – Aminotransferases (ALT and AST)
The aminotransferases, Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST), are the most sensitive indicators of liver cell injury.
- Test Name: Alanine Aminotransferase (ALT), formerly known as SGPT (Serum Glutamic-Pyruvic Transaminase).
- What it Measures: ALT is an enzyme found predominantly in the liver. Smaller amounts are found in the kidneys, heart, and muscles.
- Clinical Significance: Elevated levels of ALT in the bloodstream indicate hepatocellular injury. When liver cells are damaged (due to inflammation, toxins, lack of blood flow, etc.), ALT leaks out into the circulation. It is often considered more specific to the liver than AST. Markedly elevated ALT levels (often hundreds or thousands of units/L) typically suggest acute liver damage, such as viral hepatitis, drug-induced liver injury, or ischemic liver injury. Moderately elevated levels can be seen in various chronic liver diseases like fatty liver disease, chronic hepatitis B or C, or alcoholic liver disease.
- Test Name: Aspartate Aminotransferase (AST), formerly known as SGOT (Serum Glutamic-Oxaloacetic Transaminase).
- What it Measures: AST is an enzyme found in high concentrations in the liver, but also in the heart, muscles, kidneys, brain, and red blood cells.
- Clinical Significance: Like ALT, elevated AST levels suggest cellular injury. However, because AST is also found in other organs, elevated levels are not always specific to liver damage alone. They can be high following a heart attack, muscle injury, kidney damage, or even strenuous exercise. When both AST and ALT are elevated, it strongly suggests liver involvement. In alcoholic liver disease, AST levels are often significantly higher than ALT levels (AST:ALT ratio ≥ 2:1), which is a helpful clue in diagnosis. In non-alcoholic fatty liver disease, ALT is often higher than AST.
Key Tests for Assessing Bile Duct Function – ALP, GGT, and Bilirubin
These tests are crucial for evaluating the liver’s ability to excrete bile and identifying potential blockages in the bile ducts (cholestasis).
- Test Name: Alkaline Phosphatase (ALP).
- What it Measures: ALP is an enzyme found in the liver, bile ducts, bone, kidneys, and placenta.
- Clinical Significance: Elevated ALP levels can suggest several conditions. The most common cause related to LFTs is cholestasis (obstruction of the bile ducts), which causes ALP to back up into the bloodstream. Conditions like gallstones in the bile duct, tumors pressing on bile ducts, or certain liver diseases affecting the small bile ducts within the liver (e.g., primary biliary cholangitis) can cause high ALP. However, because ALP is also found in bone, elevated levels can result from bone growth (in children and adolescents), bone healing after a fracture, vitamin D deficiency, or bone diseases like Paget’s disease. Pregnancy also causes elevated ALP. Therefore, ALP elevation needs to be interpreted in conjunction with other tests, particularly GGT.
- Test Name: Gamma-Glutamyl Transpeptidase (GGT).
- What it Measures: GGT is an enzyme found in the liver, bile ducts, kidneys, pancreas, and other organs.
- Clinical Significance: Elevated GGT levels are often associated with liver and bile duct conditions. When ALP is elevated, measuring GGT helps differentiate between liver/bile duct causes and bone causes. If both ALP and GGT are elevated, it strongly suggests a hepatobiliary (liver or bile duct) origin for the ALP elevation, usually indicating cholestasis. GGT is also a sensitive indicator of alcohol intake, though its elevated levels are not specific to alcohol-induced liver injury; they can be raised by many drugs and liver conditions.
- Test Name: Bilirubin (Total and Direct/Conjugated).
- What it Measures: Bilirubin is a yellow pigment produced from the breakdown of red blood cells. The liver processes bilirubin, making it soluble in water (conjugated or direct bilirubin) so it can be excreted in bile. Total Bilirubin measures both unconjugated (indirect) and conjugated (direct) bilirubin.
- Clinical Significance: Elevated bilirubin levels cause jaundice (yellowing of the skin and eyes). High bilirubin can indicate:
- Pre-hepatic causes: Increased production of bilirubin (e.g., severe hemolysis or breakdown of red blood cells) where the liver cannot process it fast enough. This primarily elevates unconjugated (indirect) bilirubin.
- Hepato-cellular causes: Damage to liver cells impairs their ability to conjugate bilirubin. This can elevate both conjugated and unconjugated bilirubin, though conjugated often predominates.
- Post-hepatic causes: Obstruction of the bile ducts (cholestasis) prevents conjugated bilirubin from being excreted, causing it to back up into the bloodstream. This primarily elevates conjugated (direct) bilirubin. Measuring both total and direct bilirubin helps determine the likely cause of jaundice. Elevated levels of direct bilirubin are always clinically significant, indicating either impaired processing within the liver or obstruction of bile excretion. Elevated indirect bilirubin can be related to liver issues or conditions outside the liver like hemolysis.
Key Tests for Evaluating the Liver’s Synthetic Function – Albumin and Prothrombin Time (PT/INR)
These tests assess the liver’s ability to synthesize essential proteins. Changes in these levels often reflect the severity and chronicity of liver disease rather than acute injury.
- Test Name: Albumin.
- What it Measures: Albumin is the main protein produced by the liver and is responsible for maintaining osmotic pressure in the blood, among other functions.
- Clinical Significance: Low albumin levels (hypoalbuminemia) can indicate the liver’s impaired ability to synthesize protein. Because albumin has a relatively long half-life (about 20 days), levels typically remain normal in acute liver injury. Low albumin is more indicative of chronic liver disease, malnutrition, inflammation (which increases capillary permeability letting albumin leak out), kidney disease (protein loss in urine), or malabsorption issues. High albumin levels are uncommon and usually due to dehydration. In the context of LFTs, assessing albumin helps gauge the liver’s long-term synthetic health.
- Test Name: Prothrombin Time (PT) and International Normalized Ratio (INR).
- What it Measures: The prothrombin time measures how quickly blood clots. The liver synthesizes many of the clotting factors (Factors I, II, V, VII, IX, X, protein C, and protein S) required for blood coagulation. The INR is a standardized calculation derived from the PT, used to monitor patients on anticoagulant therapy (like warfarin) and to assess liver function.
- Clinical Significance: A prolonged PT or an elevated INR indicates that the blood is taking longer than normal to clot. This can be due to a deficiency in clotting factors. Since the liver produces most clotting factors, a prolonged PT/elevated INR in a patient with liver disease suggests impaired liver synthetic function. Unlike albumin, clotting factors have shorter half-lives (hours to days), so PT/INR can change relatively quickly and are better indicators of both acute severe liver injury (like acute liver failure) and chronic liver disease severity. Vitamin K deficiency and certain medications (like warfarin) can also prolong PT/INR, so these must be considered during interpretation.
Interpreting the Results: Putting It All Together
Interpreting LFTs is not simply looking at individual results in isolation. The pattern of abnormalities across the entire panel, combined with the patient’s clinical history, physical examination findings, symptoms, and results of other diagnostic tests (like imaging studies), is essential for accurate diagnosis and management.
For example:
- Markedly elevated ALT and AST with normal ALP and bilirubin might suggest acute viral hepatitis.
- Moderately elevated ALP and GGT with elevated conjugated bilirubin (indicating jaundice), but only mildly elevated ALT/AST, might point towards a bile duct obstruction.
- Mildly elevated AST and ALT (AST > ALT), with normal or slightly elevated ALP, low albumin, and a prolonged PT/INR, might suggest advanced alcoholic cirrhosis.
Many factors can influence LFT results, including medications (prescription, over-the-counter, and herbal supplements), alcohol consumption, recent viral infections, underlying medical conditions unrelated to the liver, and even strenuous exercise. Reference ranges for results can vary slightly between laboratories.
Seeking Professional Guidance
This guide provides a general overview of common LFTs. It is crucial to understand that interpreting LFT results requires medical expertise. Abnormal results do not automatically mean a diagnosis of liver disease; conversely, some liver conditions may present with normal LFTs. Patients should always discuss their LFT results with a qualified healthcare professional who can interpret them in the context of their individual health status and symptoms.
Conclusion
Liver function tests are invaluable diagnostic tools in clinical medicine. By measuring levels of specific enzymes, proteins, and other substances in the blood, they provide vital clues about the presence, type, and severity of liver injury, impaired bile flow, or compromised synthetic function. Understanding what each test measures and its clinical significance is a fundamental step in comprehending liver health, but the true power of LFTs lies in their collective interpretation by experienced clinicians.
