An LFT, or liver function test, is a group of blood tests that measure enzymes, proteins, and substances produced or processed by your liver. A single blood draw gives your doctor a snapshot of how well your liver is working, whether liver cells are being damaged, and whether bile is flowing normally. The panel typically includes five to eight individual markers, each revealing something different about liver health.
What the Panel Measures
A standard LFT panel includes several markers that fall into three categories: enzymes that leak out when liver cells are injured, substances the liver processes, and proteins the liver manufactures. Here’s what each one tells you.
ALT (alanine aminotransferase) is found almost exclusively in liver cells. When those cells are damaged, ALT spills into the bloodstream. It’s considered the most liver-specific enzyme on the panel, so a high ALT level is a strong signal that something is affecting the liver directly.
AST (aspartate aminotransferase) is similar to ALT but less specific. AST lives in liver cells, but also in heart muscle, skeletal muscle, and red blood cells. That means an elevated AST can come from sources outside the liver, including intense exercise, a heart attack, or even the breakdown of red blood cells. When both ALT and AST are elevated together, the liver is the most likely source. When AST rises alone, other causes need to be considered.
ALP (alkaline phosphatase) sits on the inner lining of bile ducts within the liver. It rises when bile flow is blocked or slowed, a condition called cholestasis. ALP is also found in bone, so elevated levels sometimes reflect bone disease rather than a liver problem. A second marker called GGT (gamma-glutamyl transferase) can help sort this out: if both ALP and GGT are high, the liver is almost certainly the source. If ALP is high but GGT is normal, the cause is more likely bone-related.
Bilirubin is the yellow pigment created when old red blood cells break down. The liver processes bilirubin and sends it into bile for elimination. High bilirubin levels cause jaundice, the yellowing of skin and eyes. Labs often report total bilirubin, and sometimes split it into direct (processed by the liver) and indirect (not yet processed). Dark urine suggests the direct form is elevated, pointing toward a liver or bile duct problem. When only the indirect form is high, the issue may be excessive red blood cell breakdown rather than liver disease.
Albumin is the most abundant protein the liver produces. It keeps fluid balanced in your bloodstream and carries hormones and medications through the body. Low albumin signals that the liver isn’t manufacturing proteins effectively, which happens in chronic liver disease or liver failure. Normal albumin is roughly 3.5 to 5.0 grams per deciliter.
PT/INR (prothrombin time / international normalized ratio) measures how quickly your blood clots. The liver produces most of the proteins involved in clotting. When severe liver injury reduces production of these clotting factors, your blood takes longer to clot, and the PT or INR rises. This marker reflects serious liver dysfunction and is used to gauge the severity of liver disease.
Normal Reference Ranges
Results can vary slightly between laboratories, but the typical adult ranges are:
- ALT: 7 to 55 U/L (men tend toward the higher end; women’s upper limit is closer to 35)
- AST: 8 to 48 U/L (women’s upper limit is closer to 33)
- ALP: 40 to 129 U/L
- Bilirubin: 0.1 to 1.2 mg/dL
- Albumin: 3.5 to 5.0 g/dL
Ranges for children, teenagers, and pregnant women differ from these adult values. Your lab report will show its own reference range next to each result, and that’s the range to compare against.
What Abnormal Results Can Mean
The pattern of which markers are elevated matters more than any single number. Doctors look at the overall picture rather than reacting to one value in isolation.
When ALT and AST are the dominant elevations, the pattern points toward direct liver cell injury. Common causes include viral hepatitis, fatty liver disease, alcohol-related liver damage, and medication side effects. The ratio between ALT and AST also carries information. A higher ALT relative to AST is associated with fatty liver disease (NAFLD), and research shows this ratio correlates with the amount of fat in the liver and the degree of liver scarring. In alcohol-related liver disease, the pattern often flips, with AST rising higher than ALT.
When ALP and bilirubin lead the way, the pattern suggests a problem with bile flow rather than liver cell damage. This could mean a gallstone blocking a bile duct, a medication causing cholestasis, or less commonly, a tumor affecting the bile ducts or pancreas.
Low albumin and a prolonged clotting time together signal that the liver is struggling to do its manufacturing job. These markers tend to stay normal in early or mild liver disease and become abnormal as damage progresses, making them important indicators of how advanced a liver condition has become.
Non-Liver Causes of Abnormal Results
Not every abnormal LFT result means your liver is in trouble. AST is one of the most commonly misattributed markers. Skeletal muscle damage, whether from intense exercise, a fall, or a condition called rhabdomyolysis, is one of the most frequent non-liver causes of elevated AST. Heart muscle damage from a heart attack or inflammation of the heart can also push AST levels up significantly. Even the destruction of red blood cells (hemolysis) releases enough AST to skew results.
ALP can rise from bone growth or bone disease, which is why elevated ALP in a teenager may simply reflect normal development. Pregnancy raises ALP levels too, since the placenta produces its own version of the enzyme. GGT testing helps clarify whether an isolated ALP elevation is coming from the liver or from somewhere else.
Certain medications and supplements are well-known triggers for elevated liver enzymes without necessarily meaning serious liver damage. Over-the-counter pain relievers, cholesterol-lowering drugs, antibiotics, and herbal supplements can all temporarily raise ALT or AST. If you’re taking any of these, your doctor may recheck your levels after adjusting or pausing the medication.
Why Your Doctor Ordered the Test
LFTs are ordered in a wide range of situations. You might get one as part of a routine checkup, especially if you have risk factors like obesity, diabetes, or regular alcohol use. Doctors also order LFTs to monitor the side effects of medications known to stress the liver, to investigate symptoms like unexplained fatigue, nausea, abdominal pain, dark urine, or yellowing of the skin, and to track the progression of a known liver condition over time.
If you’ve already been diagnosed with a liver condition, repeated LFTs help your doctor see whether things are improving, stable, or worsening. In hepatitis treatment, for example, falling ALT levels signal that the infection is coming under control.
What to Expect From the Test
An LFT is a simple blood draw, typically from a vein in your arm. The process takes a few minutes, and results are usually available within one to two days, sometimes sooner if the lab is in-house. Some doctors ask you to fast for 8 to 12 hours beforehand, though fasting isn’t always required for every marker on the panel. Follow whatever instructions your doctor’s office gives you.
If your results come back abnormal, it doesn’t automatically mean you have liver disease. A single set of mildly elevated enzymes is often rechecked in a few weeks to see if levels return to normal on their own. Persistent or significantly elevated results typically lead to further investigation: imaging like an ultrasound, additional blood work for specific liver diseases, or in some cases a referral to a liver specialist.