Understanding Liver Function Tests: A Free Health Guide

What Liver Function Tests Are and Why They Matter

Liver function tests (LFTs) are blood tests that measure how well your liver is working. Your liver is one of the largest organs in your body, weighing about three pounds in adults. It performs hundreds of different jobs every single day, from filtering waste out of your blood to making proteins that help your blood clot. When your doctor suspects something might be wrong with your liver, or when you have a condition that could affect liver health, these tests provide valuable information.

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The liver sits just below your right rib cage and processes nearly everything you eat, drink, and take as medicine. It breaks down nutrients, stores energy, produces bile to help digest fats, and removes toxins from your bloodstream. Because the liver does so much work, damage to it can affect many different body systems. That's why doctors order liver function tests in various situations—when patients have symptoms like jaundice (yellowing of skin and eyes), unusual fatigue, abdominal swelling, or dark urine. These tests may also be ordered before starting certain medications, to monitor chronic conditions like hepatitis or cirrhosis, or as part of routine health screenings.

Liver function tests don't diagnose a specific disease on their own. Instead, they reveal whether liver cells are being damaged, whether the liver can produce proteins normally, and whether bile is flowing properly. The results give doctors a snapshot of liver health at one moment in time. Multiple tests performed over weeks or months can show trends and help track whether a condition is getting better or worse. Understanding what these tests measure helps you have better conversations with your healthcare provider about your health.

Practical takeaway: Liver function tests are routine blood work that show how well your liver is filtering waste, making proteins, and processing other substances. They're not diagnostic tests but rather screening and monitoring tools. If your doctor has ordered these tests, ask which specific tests are included and why they're being done.

The Main Liver Enzymes and What They Indicate

Several enzymes are commonly measured in liver function tests, and each one tells a different story about liver health. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are the two most important enzymes. These are proteins inside liver cells that help the liver process nutrients. When liver cells are damaged or dying, they release these enzymes into the bloodstream, where they can be measured. Think of it like this: if bricks are falling out of a wall, you know the wall has damage. Similarly, high levels of these enzymes in blood indicate liver cell damage.

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ALT is found mostly in the liver, while AST is found in several organs including the heart and muscles. Because ALT is more specific to the liver, doctors often focus more on ALT levels when assessing liver health. Normal ALT levels range from about 7 to 56 units per liter (U/L), though this can vary slightly between laboratories. When someone has hepatitis, a viral infection of the liver, ALT levels can spike to several hundred or even thousands of units per liter. People with fatty liver disease typically show mild to moderate elevation in both ALT and AST. Heavy alcohol use can also raise these enzyme levels significantly.

Another important enzyme is alkaline phosphatase (ALP). This enzyme is involved in breaking down proteins and is found in bone, liver, and bile ducts. When bile ducts become blocked or damaged, ALP levels rise. Gamma-glutamyl transferase (GGT) is another enzyme that becomes elevated when there's bile duct damage or alcohol-related liver disease. Normal GGT levels range from 0 to 65 U/L, but these ranges vary by lab. Bilirubin, though technically not an enzyme, is also measured in liver function tests. It's a yellow pigment created when old red blood cells are broken down. A healthy liver processes bilirubin and sends it into bile. When the liver isn't working properly, or when bile ducts are blocked, bilirubin builds up in the blood and causes yellowing of skin and eyes.

Practical takeaway: High ALT and AST suggest liver cell damage, elevated ALP and GGT indicate possible bile duct problems, and elevated bilirubin means bilirubin isn't being processed normally. Your doctor will look at the pattern of which enzymes are high and by how much to narrow down what might be happening. Keep a copy of your results and note the normal ranges used by your lab, as these can vary.

Understanding Albumin and Protein Measurements

While enzyme tests show whether liver cells are being damaged, protein measurements show whether your liver can still do its job of making proteins. Albumin is the main protein made by your liver, making up more than half of all blood proteins. It carries hormones, vitamins, and medications through your bloodstream and helps keep fluid in your blood vessels instead of letting it leak into body tissues. When your liver doesn't work properly for weeks or months, albumin production drops. Low albumin levels can cause swelling in the legs, abdomen, and other areas because fluid leaks out of blood vessels.

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Normal albumin levels range from 3.4 to 5.4 grams per deciliter (g/dL), though labs may have slightly different ranges. Albumin levels don't change quickly—they drop slowly over time as liver disease progresses. This makes albumin a useful marker for chronic liver disease but not helpful for detecting acute (sudden) liver problems. Someone with newly injured liver cells might have normal albumin levels even though liver function tests show significant damage. Conversely, people with advanced liver cirrhosis almost always have low albumin because their livers have been damaged for a long time and can't make enough of this critical protein.

Doctors also measure total protein, which includes albumin plus other blood proteins like globulins. These globulins include antibodies that fight infection and other proteins that carry substances through the blood. The ratio of albumin to globulins (called the A/G ratio) provides additional information. In liver disease, globulin levels often rise while albumin falls, which changes this ratio. Another related test measures prothrombin time (PT or INR), which shows how long it takes for blood to clot. Since the liver makes clotting factors, people with liver disease may have prolonged clotting times and bleed more easily. This is why it's dangerous to have liver failure—your blood won't clot properly, and even small cuts or injuries can cause serious bleeding.

Practical takeaway: Protein measurements show your liver's ability to manufacture essential substances. Low albumin develops slowly with chronic liver disease and can cause fluid buildup in legs and abdomen. If you have low albumin levels, your doctor will want to monitor how well your liver is functioning over time and watch for complications like easy bruising or bleeding.

Interpreting Your Results and What Different Patterns Mean

Liver function test results form patterns that help doctors understand what's happening. One common pattern is when ALT and AST are very high (sometimes 1,000 to 5,000 U/L or higher) while ALP and bilirubin are normal or only slightly elevated. This pattern typically indicates acute (sudden) liver injury, such as from viral hepatitis, medication toxicity, or autoimmune hepatitis. When acute liver injury occurs, liver cells die quickly and release large amounts of enzymes. The good news is that if the cause is treated or removed, the liver often heals itself because liver cells can regenerate.

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A different pattern appears with cholestasis, which means bile isn't flowing normally. In this pattern, ALP and bilirubin are significantly elevated while ALT and AST are normal or only mildly elevated. This suggests the problem is in the bile ducts rather than the liver cells themselves. Bile duct blockage from gallstones, tumors, or scarring causes this pattern. Pregnancy-related cholestasis and certain medications can also cause this pattern. The bilirubin buildup is what causes the yellowing of skin and eyes, while the ALP elevation indicates bile duct irritation.

Fatty liver disease shows yet another pattern: mild elevation of ALT and AST (usually less than 200 U/L), with ALT typically higher than AST. This differs from alcohol-related liver disease, where AST is usually higher than ALT. About 25-30% of adults in the United States have fatty liver disease, and most don't even know it. The condition often has no symptoms and only shows up on blood tests or ultrasound. When liver function tests are normal but fatty