Liver Function Tests Explained: ALT, AST, Bilirubin and What Elevated Levels Mean

Your liver is the body's largest internal organ and its primary chemical processing plant. It performs over 500 vital functions including filtering toxins, metabolizing medications, producing bile for digestion, storing nutrients, and manufacturing essential proteins. Liver function tests (LFTs) are blood tests that evaluate how well your liver is working and whether it's damaged or inflamed. ## Core Liver Function Tests ### ALT (Alanine Aminotransferase) ALT is the most liver-specific enzyme. It's found primarily in liver cells and is released into the bloodstream when liver cells are damaged. Normal range: 7–56 U/L (though many labs consider above 35 U/L for men and 25 U/L for women as elevated based on updated guidelines). ALT is particularly sensitive to liver cell injury from conditions like hepatitis, fatty liver disease, and drug toxicity. It's often the first enzyme to rise when the liver is under stress. ### AST (Aspartate Aminotransferase) AST is found in the liver, heart, muscles, kidneys, and brain. Normal range: 10–40 U/L. Because AST exists in multiple tissues, elevated AST alone is less specific to the liver than ALT. However, the AST/ALT ratio provides valuable diagnostic clues: - **AST/ALT ratio < 1**: Typical of non-alcoholic fatty liver disease (NAFLD) and viral hepatitis - **AST/ALT ratio > 2**: Strongly suggests alcoholic liver disease - **AST/ALT ratio > 1 with high values**: May indicate cirrhosis or advanced liver disease ### ALP (Alkaline Phosphatase) ALP is found in the liver, bile ducts, and bones. Normal range: 44–147 U/L. Elevated ALP, especially with elevated GGT, suggests bile duct obstruction or cholestatic liver disease. Isolated ALP elevation without GGT elevation often indicates bone-related conditions. ### GGT (Gamma-Glutamyl Transferase) GGT is found primarily in the liver and bile ducts. Normal range: 9–48 U/L. It's highly sensitive to alcohol use and bile duct issues. GGT helps distinguish whether elevated ALP is from liver or bone origin. ### Bilirubin Bilirubin is a yellow pigment produced when red blood cells break down. The liver processes bilirubin for excretion in bile. Normal total bilirubin: 0.1–1.2 mg/dL. - **Unconjugated (indirect) bilirubin**: Elevated in conditions with excessive red blood cell breakdown (hemolysis) or liver uptake deficiency (Gilbert's syndrome) - **Conjugated (direct) bilirubin**: Elevated in bile duct obstruction, hepatitis, and liver disease ### Albumin Albumin is the most abundant protein in blood, produced exclusively by the liver. Normal range: 3.5–5.0 g/dL. Low albumin indicates chronic liver disease or severe malnutrition, as the liver can no longer produce adequate amounts. ## Common Causes of Elevated Liver Enzymes ### Non-Alcoholic Fatty Liver Disease (NAFLD/MASLD) The most common cause of chronic ALT elevation worldwide. Associated with obesity, insulin resistance, and metabolic syndrome. Often produces mild ALT elevation (40–100 U/L) with AST/ALT ratio below 1. ### Alcohol Use Regular or heavy alcohol consumption is a leading cause of liver damage. GGT is the most sensitive marker. The classic pattern is AST/ALT ratio above 2 with elevated GGT. ### Medications Many common medications can cause liver enzyme elevation: acetaminophen (paracetamol), statins, NSAIDs, antibiotics, and antifungals. Always review medication lists when interpreting liver tests. ### Viral Hepatitis Hepatitis B and C can cause both acute and chronic liver inflammation. ALT is often significantly elevated (5–50x normal) in acute hepatitis. Chronic hepatitis may show persistent mild elevation. ### Autoimmune Hepatitis The immune system attacks liver cells. Characterized by very high ALT and AST, elevated immunoglobulins, and positive autoantibodies. ### Strenuous Exercise Intense exercise, especially endurance events, can temporarily raise AST (and to a lesser extent ALT) due to muscle breakdown. This typically normalizes within 48–72 hours. ## Protecting Your Liver - Limit alcohol consumption (or avoid entirely if enzymes are elevated) - Maintain a healthy weight and exercise regularly - Avoid unnecessary medications and supplements - Get vaccinated against Hepatitis A and B - Manage metabolic risk factors (blood sugar, cholesterol, blood pressure) - Stay hydrated and eat a balanced, plant-rich diet - Avoid exposure to industrial chemicals and toxins ## Conclusion Liver function tests are a window into one of your most essential organs. Even mild, persistent elevations deserve investigation. Early detection of liver conditions—particularly fatty liver disease—allows intervention before irreversible damage occurs. The liver has remarkable regenerative capacity, but only if the cause of damage is identified and addressed in time.