Liver Function Tests Explained: ALT, AST, Bilirubin, and What They Really Mean

When your doctor orders liver function tests, they’re not checking how well your liver is working like a machine. They’re looking for signs it’s been damaged. The term ‘liver function tests’ is misleading-most of these blood markers don’t measure function at all. They measure leakage. When liver cells get injured, enzymes and pigments spill into the bloodstream. That’s what shows up on your lab report.

What’s Actually Measured in a Liver Panel?

A standard liver panel includes five key markers: ALT, AST, ALP, bilirubin, and albumin. Sometimes GGT and prothrombin time are added. Each tells a different story.

• ALT (alanine aminotransferase): Almost entirely found in liver cells. When liver cells die, ALT pours into the blood. It’s the most specific marker for liver injury.
• AST (aspartate aminotransferase): Found in liver, heart, muscles, and kidneys. Less specific than ALT. A high AST can mean liver damage-but it could also mean a heart attack or intense workout.
• ALP (alkaline phosphatase): Elevated when bile flow is blocked. Comes from liver and bones. If ALP is high but GGT is normal, it might be a bone issue, not liver.
• Bilirubin: A yellow pigment made when red blood cells break down. The liver processes it and sends it out in bile. High levels cause jaundice. Total bilirubin is split into direct (conjugated) and indirect (unconjugated) to figure out why it’s up.
• Albumin: Made by the liver. Low levels mean the liver isn’t making proteins anymore-usually a sign of long-term damage, not sudden injury.

Reference ranges vary by lab, but general normal values are: ALT 7-55 U/L, AST 8-48 U/L, total bilirubin 3-17 μmol/L, albumin 35-50 g/L. But here’s the catch: normal doesn’t always mean healthy.

Why ALT and AST Don’t Tell the Whole Story

Many people panic when their ALT is 60 U/L. But 10-15% of healthy people have ALT levels slightly above normal. Obesity, intense exercise, or even a recent binge drinking episode can nudge it up without causing disease.

What matters is the pattern. Two numbers tell you more than five:

• ALT > AST: This is the classic sign of viral hepatitis, fatty liver (now called MASLD), or drug-induced injury. In acute hepatitis A or B, ALT can spike 10 times above normal.
• AST > ALT: Especially if AST is twice as high as ALT? That’s a red flag for alcohol-related liver damage. In alcoholic hepatitis, AST often runs 2-6 times above normal, while ALT only climbs 1-2 times. The ratio is so reliable that over 90% of alcoholic hepatitis cases show AST:ALT > 1.
• AST > 500 U/L: If you’re a heavy drinker and your AST is this high, it’s probably not just alcohol. Think acetaminophen overdose.

ALT has a half-life of about 47 hours. That means if you had an acute injury-like a viral infection or drug reaction-ALT will stay elevated for days, even after you feel better. AST clears faster. So if ALT is still high after a week, the liver is still under stress.

Bilirubin: The Yellow Clue

Bilirubin doesn’t rise in most liver injuries. It only goes up when bile flow is blocked or the liver can’t process it. That’s called cholestasis.

If bilirubin is high and ALP is also high (3x or more), you’re looking at a bile duct problem: gallstones, tumors, or medication side effects. If bilirubin is high but ALP is normal? That’s usually a problem with red blood cell breakdown (hemolysis) or inherited conditions like Gilbert’s syndrome.

Direct bilirubin (conjugated) being high means the liver processed it but couldn’t get it out. Indirect bilirubin high? The liver isn’t processing it fast enough. A simple split tells you where the problem lies.

Albumin and Prothrombin Time: The Real Function Tests

These are the only two markers that actually measure what the liver does: make proteins and help blood clot.

Albumin has a half-life of 20 days. So if your albumin is low, your liver has been struggling for weeks or months. It’s not a sign of a sudden problem. Low albumin shows up in advanced cirrhosis, chronic alcohol use, or severe malnutrition.

Prothrombin time (PT) is more sensitive. It measures how fast your blood clots. The liver makes clotting factors that need vitamin K. If PT is prolonged, it means the liver is failing to make those proteins quickly. This can happen in days during acute liver failure. A rising PT is one of the most serious signs a liver is shutting down.

When Do You Worry? When Do You Wait?

Not every elevated liver test needs a CT scan or biopsy.

• Mild elevations (ALT/AST under 2x normal): Watch and repeat in 3-6 months. This is common in people with obesity, prediabetes, or occasional alcohol use. Lifestyle changes often fix it.
• Moderate elevations (2-5x normal): Look for causes: alcohol, medications (including herbal supplements), viral hepatitis, or fatty liver. Get tested for hepatitis B and C. Check your BMI and blood sugar.
• Severe elevations (above 500 U/L): This is urgent. Could be acute viral hepatitis, drug toxicity (especially acetaminophen), or ischemic injury. Don’t wait. Go to the ER.
• Isolated ALP elevation: If GGT is normal, check for bone disease. Ask about bone pain or fractures.

A 2022 JAMA study found that nearly 4 out of 10 primary care doctors ordered unnecessary imaging for ALT levels between 41-80 U/L. That’s not just wasteful-it causes anxiety. Most of those people had no liver disease.

What’s Missing? Fibrosis, Not Just Inflammation

Two people can have the same ALT level. One has mild fatty liver. The other has advanced scarring (fibrosis). The test can’t tell the difference.

That’s why experts now combine LFTs with non-invasive scores like FIB-4. FIB-4 uses age, ALT, AST, and platelet count to estimate liver scarring. A 2021 Hepatology study showed that adding FIB-4 to LFTs raised the accuracy of detecting advanced fibrosis from 68% to 89%.

Future tools like the ELF test (enhanced liver fibrosis) measure three proteins linked to scarring. In 2024, a Lancet study showed it detects severe fibrosis with 92% accuracy when combined with AST/ALT ratio.

What to Do Next

If your liver tests are off:

1. Stop alcohol completely for at least 4 weeks. Retest.
2. Review all medications and supplements-even ‘natural’ ones. Some herbs like kava, green tea extract, and high-dose niacin can hurt the liver.
3. Check your weight and blood sugar. Fatty liver is the #1 cause of elevated ALT in adults today.
4. Get tested for hepatitis B and C. Many people don’t know they have it.
5. Don’t panic over mild elevations. But don’t ignore them either. Track them over time.

The liver is quiet until it’s almost broken. But if you catch problems early-before scarring sets in-you can often reverse the damage. Diet, exercise, and avoiding alcohol do more than any pill.