A liver function test is a set of blood measurements that shows whether liver cells are being damaged, whether bile is flowing, and whether the liver is still making the proteins it should. The usual panel is ALT, AST, alkaline phosphatase, bilirubin, albumin, and total protein, all of which sit on a comprehensive metabolic panel. Mild abnormalities are common, most often from fatty liver, alcohol, or medication, and the liver rarely causes symptoms until damage is advanced, which is why this panel is how most liver problems are found.

What the panel measures

The name is slightly misleading, and the American College of Gastroenterology prefers "liver chemistries," because most of these markers detect injury rather than measure function. Three groups answer three questions.

Are liver cells being damaged? ALT and AST are enzymes that leak out of injured liver cells. ALT lives almost entirely in the liver, so it is the more specific of the two. AST is also found in muscle and heart, which is why a hard workout raises it.

Is bile flowing? Alkaline phosphatase rises when bile ducts are blocked or irritated, and also with bone turnover. Bilirubin, the yellow pigment from old red cells, backs up when the liver cannot process it or the ducts are blocked, and causes jaundice when high enough. GGT, sometimes added, confirms that a high alkaline phosphatase is coming from the liver rather than bone.

Is the liver still doing its job? Albumin and total protein are made by the liver and fall when it has been damaged for a long time. In more advanced disease, clotting time is added. MedlinePlus lists each marker.

Who needs a liver function test

Almost everyone gets one without asking for it, because the six core markers are part of every comprehensive metabolic panel drawn at an annual physical.

Beyond routine work, there are four reasons to check deliberately. The first is medication. Statins, acetaminophen in regular use, methotrexate, antifungals, some antibiotics, and many supplements can injure the liver, and a baseline plus periodic checks are standard. The second is risk for fatty liver: excess weight, type 2 diabetes, high triglycerides, or high blood pressure, which together describe a large share of US adults. The third is alcohol above moderate levels. The fourth is symptoms: fatigue, nausea, right-sided abdominal discomfort, dark urine, pale stools, itching, or yellowing of the skin or eyes.

A family history of hemochromatosis or other inherited liver disease, and any exposure to hepatitis, are reasons too.

How to prepare

Fasting is not required for the liver markers themselves. They are usually drawn with a fasting metabolic panel because glucose on the same panel needs it, so follow your order.

Two things matter more. Skip hard exercise for a day or two before the draw, because it raises AST and ALT and is a common reason for an unexpected flag. And avoid alcohol for at least 24 hours, since even a moderate evening raises GGT and the transaminases.

Tell whoever reviews your result about every medication and supplement you take. Herbal and bodybuilding supplements are a frequent and often unsuspected cause of liver enzyme elevation.

Reading your result

Every lab prints its own reference range, and that is the one to use. What a clinician reads is the pattern.

Mildly elevated ALT or AST, under two to three times the upper limit, is common. The NCBI Bookshelf review puts mild elevations in 10 to 20 percent of the general population, and notes that up to a third of these in people without symptoms resolve on their own within a few weeks. The most frequent cause in the US is metabolic dysfunction-associated steatotic liver disease, the condition formerly called nonalcoholic fatty liver, followed by alcohol and medications. The standard response is to repeat the test after a few weeks of no alcohol and no heavy workouts; if it stays up, the search widens to fatty liver, viral hepatitis, iron overload, celiac disease, and thyroid disease. Our guide to elevated liver enzymes covers that workup.

Markedly elevated ALT and AST, many times the upper limit, is a different situation. Acute viral hepatitis, a drug reaction, severe fatty liver inflammation, or a blocked blood supply can all do this, and it needs prompt evaluation.

The ratio helps. ALT higher than AST points toward fatty liver or viral hepatitis. AST about twice ALT points toward alcohol.

High alkaline phosphatase and bilirubin with only modest enzyme changes is a bile-flow pattern: gallstones, a duct blockage, certain medications, or a primary biliary condition. GGT confirms the liver as the source.

Isolated mildly high bilirubin with everything else normal is often Gilbert's syndrome, an inherited and harmless variant that shows up when fasting or unwell.

Low albumin suggests long-standing liver disease, kidney protein loss, inflammation, or poor nutrition, and it is the one marker on the panel that actually reflects function.

What a normal panel does not rule out

A normal liver function test does not mean a healthy liver. Early fatty liver and even established scarring can sit behind normal enzymes, and the review above notes that normal chemistries do not exclude meaningful disease. That is why people with diabetes or obesity are increasingly offered a fibrosis score, calculated from age, AST, ALT, and platelets, or an ultrasound, even when the panel is normal.

The panel also does not identify a cause. A high ALT says look closer; it does not say why. Hepatitis serology, iron studies, celiac antibodies, and imaging are the next layer.

How to get tested

You can order one yourself. August's lab tests include the liver markers within every comprehensive metabolic panel and routine panel, and a dedicated liver panel that adds GGT, at one price that covers the draw. A phlebotomist comes to your home or you visit a nearby lab. Results post the next day in most cases, and a clinician reviews every one before you see it, so a flagged enzyme comes with context rather than a bare number. August Care members get up to 50 percent off next-day lab tests and a first consultation included.

Not sure whether your symptoms point to the liver? Describe them to August, a free AI health assistant, to help decide what to test. It is a starting point, not a diagnosis.

Frequently Asked Questions

ALT and AST, the enzymes that leak from injured liver cells; alkaline phosphatase and bilirubin, which rise when bile flow is blocked; and albumin and total protein, which the liver makes. Together they show whether the liver is being damaged, whether bile is moving, and whether the liver is still producing what it should.

Most often fatty liver disease, alcohol, medications and supplements, or hard exercise in the day or two before the draw. Less commonly, viral hepatitis, iron overload, celiac disease, thyroid disease, or autoimmune liver conditions. A mild rise is usually repeated after a few weeks before any further workup.

Not for the liver markers themselves, though they are usually drawn with a fasting metabolic panel. What matters more is avoiding alcohol for 24 hours and heavy exercise for a day or two beforehand, since both raise the enzymes and are common reasons for a flagged value in a healthy liver.

Yes. Early fatty liver and even scarring can hide behind normal enzymes, which is why people with diabetes, obesity, or heavy alcohol use are often offered a fibrosis score or an ultrasound regardless of the panel. A normal panel is reassuring but not conclusive.

There is no single number, but values many times the upper limit of the lab's range, or any elevation alongside jaundice, confusion, severe abdominal pain, or vomiting, need prompt evaluation. Mild elevations under two to three times the limit are common and usually followed with a repeat test.

Yes. Intense workouts, especially weight training, release AST and ALT from muscle and raise both for a day or two. A flagged enzyme in someone who trained hard the day before is often nothing more than that, and a repeat after rest usually normalizes it.

An inherited, harmless variant that reduces the liver's ability to process bilirubin. It shows up as a mildly high bilirubin with otherwise normal results, often when fasting or unwell, and sometimes as faint yellowing of the eyes. It needs no treatment and does not damage the liver.

Yes. An August phlebotomist draws the sample at home, and a certified lab runs the same markers a clinic would, usually as part of a metabolic or liver panel. Results post the next day in most cases, each value shown against the lab's range, with a clinician's review.