LFT stands for liver function test, a routine blood panel that measures enzymes, proteins, and other substances to assess how well your liver is working. Despite the name, an LFT doesn’t just test liver “function.” It also detects liver damage, inflammation, and problems with bile flow. The panel typically includes between five and nine individual markers, each telling a different part of the story.
What an LFT Panel Includes
A standard liver function test measures several categories of blood markers. The most commonly ordered are:
- ALT (alanine transaminase) and AST (aspartate transaminase): Enzymes that help process amino acids. They exist inside liver cells, and when those cells are injured, the enzymes leak into the bloodstream.
- ALP (alkaline phosphatase): An enzyme found in the liver and bones. Elevated levels often point to blocked bile ducts or bone disease.
- GGT (gamma-glutamyl transferase): Another enzyme that rises when the liver or bile ducts are damaged. It’s often used alongside ALP to confirm whether an ALP elevation is liver-related or bone-related.
- Bilirubin: A yellow waste product created when old red blood cells break down. High levels cause jaundice, the yellowing of skin and eyes.
- Albumin and total protein: Proteins manufactured by the liver. Low levels suggest the liver isn’t producing proteins as it should.
- Prothrombin time (PT): A measure of how quickly your blood clots. The liver makes most of the proteins involved in clotting, so a slow clotting time can signal liver failure.
Some panels also include lactate dehydrogenase (LDH), an enzyme present in many tissues but found in high concentrations in the liver.
Damage Markers: ALT and AST
ALT and AST are the workhorses of liver damage detection. Both enzymes sit inside liver cells and participate in energy metabolism. When liver cells are injured, whether from infection, medication, alcohol, or reduced blood supply, these enzymes spill into the bloodstream. The higher the level, the more widespread the cell damage tends to be. Normal levels for both generally fall between 0 and 40 IU/L.
ALT is more specific to the liver than AST. AST also exists in heart muscle, skeletal muscle, and red blood cells, so an elevated AST doesn’t automatically mean a liver problem. A hard workout, a muscle injury, or even a heart attack can push AST levels up. When both ALT and AST are elevated together, the liver is the most likely source. When AST rises alone, your doctor will consider non-liver causes.
Bile Flow Markers: ALP and GGT
ALP and GGT rise for a different reason than ALT and AST. Rather than reflecting direct cell damage, elevated levels of these enzymes typically signal a problem with bile flow, a pattern called cholestasis. Bile is produced by the liver and stored in the gallbladder. When a bile duct is blocked, whether by a gallstone, a tumor, or inflammation, ALP and GGT levels climb.
ALP has a quirk: it’s also produced by bone. Growing children, people with fractures, and those with bone diseases like Paget’s disease can have elevated ALP without any liver involvement. GGT helps clarify this. If both ALP and GGT are high, the problem is almost certainly hepatic. If ALP is high but GGT is normal, the cause is more likely bone.
Synthetic Function: Albumin and Clotting Time
While enzymes reveal damage, albumin and prothrombin time reveal whether the liver can still do its job. The liver is a protein factory, producing albumin (the most abundant protein in blood) and the clotting factors that prevent uncontrolled bleeding.
Albumin has a long half-life of two to three weeks, meaning levels drop slowly. By the time albumin is measurably low, the liver has typically been struggling for a while. This makes albumin most useful for detecting chronic liver failure rather than an acute problem. Low albumin isn’t always the liver’s fault, though. Malnutrition, kidney disease, and chronic inflammatory conditions can also suppress it.
Prothrombin time works on a shorter timeline. Clotting factors have much shorter half-lives than albumin, so PT becomes prolonged earlier in the course of liver disease. This makes it valuable for detecting both acute and chronic liver failure. A key clinical detail: when a prolonged PT is caused by liver failure, giving vitamin K won’t fix it. If vitamin K does correct the clotting time, the problem is more likely malabsorption or dietary deficiency rather than liver damage.
Bilirubin: Direct vs. Indirect
Bilirubin is what gives bruises their yellowish tinge and jaundice its color. It forms naturally when old red blood cells are recycled. Normally, the liver processes bilirubin in two steps: it captures the raw, water-insoluble form (called indirect or unconjugated bilirubin), converts it into a water-soluble form (called direct or conjugated bilirubin), and then excretes it into bile, which eventually leaves the body through stool.
When the total bilirubin is elevated, knowing which type is high helps pinpoint where the problem lies. High indirect bilirubin suggests the liver isn’t capturing or processing bilirubin efficiently, or that too many red blood cells are being destroyed at once. High direct bilirubin points to a blockage downstream, preventing processed bilirubin from draining into the intestine. Normal total bilirubin ranges from roughly 5 to 21 micromol/L.
Why Your Doctor Orders an LFT
LFTs are among the most frequently ordered blood tests. Doctors request them for a wide range of reasons: unexplained fatigue, abdominal pain, nausea, dark urine, yellowing of the skin or eyes, or swelling in the abdomen. They’re also ordered routinely to monitor people taking medications known to stress the liver, including certain cholesterol drugs, pain relievers, and anti-seizure medications. People with a history of heavy alcohol use, hepatitis, or obesity-related fatty liver disease often have LFTs checked at regular intervals.
LFTs also show up in routine health screenings and pre-surgical evaluations, sometimes even when there’s no specific concern about the liver. An abnormal result on a routine panel is often the first clue that something is going on.
What to Expect During the Test
An LFT is a simple blood draw, usually from a vein in your arm. The process takes a few minutes, and results are typically available within a day or two. Fasting is not required. Unlike cholesterol or blood sugar tests, liver function markers are not meaningfully affected by recent meals.
A single abnormal result doesn’t necessarily mean liver disease. Strenuous exercise, certain supplements, and even a recent illness can temporarily shift values. If results come back abnormal, a repeat test or additional imaging is the usual next step to determine whether the elevation is persistent and clinically significant.