Liver Function Tests: ALT, AST, Bilirubin, and How to Interpret Them

When your doctor orders a liver function test, it’s not really about checking how well your liver is working. That’s the common misunderstanding. These tests don’t measure liver function like a machine’s output. Instead, they look for signs that liver cells are damaged or bile flow is blocked. Think of them as early warning signals - not a full diagnostic report. The most common markers are ALT, AST, and bilirubin. But knowing the numbers alone won’t tell you what’s going on. You need to understand the patterns.

What ALT and AST Really Tell You

ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are enzymes inside liver cells. When those cells get hurt, the enzymes leak into your blood. That’s what raises the numbers. But here’s the catch: AST isn’t just in the liver. It’s also in your heart, muscles, and kidneys. ALT? Almost all of it comes from the liver. That’s why ALT is a more specific marker for liver injury.

Normal ranges vary by lab, but generally:

• ALT: 7-55 U/L
• AST: 8-48 U/L

Men tend to have slightly higher levels than women. People with higher body mass index (BMI) also often show elevated levels - even if their liver is perfectly healthy. A BMI over 30 can raise normal ALT and AST by 10-15%. So if your numbers are just a little high, don’t panic. That’s not always liver disease.

What matters more is the ratio between AST and ALT. If AST is more than twice as high as ALT, that’s a red flag for alcohol-related liver damage. In fact, in 90% of people with alcoholic hepatitis, the AST:ALT ratio is over 2. If the ratio is close to 1 - meaning both are equally elevated - it’s more likely viral hepatitis, fatty liver, or medication injury.

And here’s something surprising: if your AST is over 500 U/L, alcohol alone is probably not the cause. That kind of spike usually points to something like acetaminophen overdose or severe ischemic injury. In acute viral hepatitis - like hepatitis A or B - ALT can jump to 10 times the upper limit. That’s a dramatic rise, and it often comes with nausea, fatigue, and yellowing skin.

Why Bilirubin Matters

Bilirubin is a yellow pigment made when old red blood cells break down. Your liver processes it and sends it out in bile. If that process breaks down, bilirubin builds up. That’s what causes jaundice - the yellowing of the skin and eyes.

There are two types:

• Unconjugated (indirect): Before the liver processes it. High levels suggest too much breakdown of red blood cells.
• Conjugated (direct): After the liver has processed it. High levels mean bile flow is blocked.

Total bilirubin normal range is 3-17 μmol/L. If it’s above 34 μmol/L, you’ll likely start to look yellow. But even mild elevations - say, 20-30 μmol/L - can mean something. If direct bilirubin is high, look for issues with bile ducts: gallstones, tumors, or inflammation. If indirect is high, think hemolysis (red blood cells breaking down too fast) or Gilbert’s syndrome - a harmless genetic condition that affects about 5-10% of people.

Cholestasis: When Bile Flow Stops

When bile can’t flow out of the liver, you get cholestasis. This isn’t about liver cell damage - it’s about plumbing. The enzymes that signal this are ALP (alkaline phosphatase) and GGT (gamma-glutamyl transferase). ALP is tricky because it’s also made in bones. So if ALP is high but GGT is normal, don’t assume it’s the liver. It could be a bone issue.

True cholestasis shows up when:

• ALP is more than 3 times higher than normal
• GGT is also elevated
• ALT and AST are only mildly raised (less than 3 times normal)

This pattern suggests blocked bile ducts - maybe from gallstones, primary biliary cholangitis, or a tumor. If bilirubin is rising at the same time, it confirms the blockage is getting worse.

Albumin and Clotting: The Real Function Test

Here’s where most people get confused. ALT and AST tell you about damage. But they don’t tell you if your liver can still do its job. That’s where albumin and prothrombin time (PT) come in.

Albumin is a protein your liver makes. Its half-life is 20 days. So if your albumin is low, it means your liver has been struggling for weeks or months. Low albumin is a sign of advanced cirrhosis or long-term liver failure.

Prothrombin time measures how fast your blood clots. Your liver makes clotting factors that need vitamin K. If PT is prolonged, your liver isn’t making enough of these proteins. This is a serious sign. It can happen quickly - within days - if the liver is severely injured.

These two markers are the real indicators of liver function. Two people can have the same ALT level. One might have a normal albumin and PT - meaning their liver is still working. The other might have low albumin and long PT - meaning their liver is failing. Same numbers. Totally different outcomes.

Patterns, Not Numbers

The key to reading liver tests is not the individual values - it’s the pattern. Here are the three main patterns you’ll see:

1. Hepatocellular injury: ALT and AST are high (often >10x normal), ALP and bilirubin are only slightly up. This is viral hepatitis, drug injury, or fatty liver.
2. Cholestatic injury: ALP and bilirubin are high (often >3x normal), ALT and AST are only mildly elevated. This is bile duct blockage.
3. Mixed pattern: All markers are elevated. This happens in drug-induced liver injury, advanced cirrhosis, or severe infections.

For example, someone with nonalcoholic fatty liver disease (now called MASLD) often has ALT slightly higher than AST - maybe 1.5 times. Their bilirubin and ALP are normal. No jaundice. No symptoms. Just a routine blood test that raises an eyebrow.

On the other hand, a heavy drinker with alcoholic hepatitis might have AST at 300 U/L and ALT at 120 U/L. Their bilirubin might be 50 μmol/L. That’s a classic pattern. And if their PT is prolonged and albumin is low? That’s a warning - their liver is starting to fail.

False Alarms and When to Worry

Not every high number means disease. About 10-15% of healthy people have ALT or AST slightly above normal. That’s just their baseline. A 2022 study in JAMA Internal Medicine found that 37% of primary care doctors ordered unnecessary imaging for ALT levels between 41-80 U/L. That’s overtesting.

What should you do if your ALT is 60 U/L? If you’re healthy, don’t drink, don’t take medications, and have no symptoms - wait. Repeat the test in 3-6 months. If it’s still up, then look deeper. But if your ALT jumps from 40 to 150 in a week? That’s urgent. It could be a viral infection or drug reaction.

Also, don’t forget non-liver causes. Strenuous exercise can raise AST. A heart attack can spike AST. Muscle damage from trauma or steroids can raise both. That’s why context matters. A 2021 study of over 12,000 patients showed that combining LFT patterns with a fibrosis score (like FIB-4) increased accuracy for detecting serious liver damage from 68% to 89%.

What Comes Next

If your tests show a clear pattern - say, high ALT with normal ALP - your doctor might check for hepatitis B and C, order an ultrasound for fatty liver, or ask about medications and supplements. If there’s cholestasis, they’ll look for gallstones or autoimmune conditions. If albumin is low and PT is long? That’s a referral to a liver specialist.

There’s no magic number that says “you have liver disease.” It’s always a combination: your numbers, your history, your symptoms, and your risk factors. A 45-year-old with obesity, high blood sugar, and ALT of 70 U/L? That’s likely MASLD. A 25-year-old with fever, dark urine, and ALT of 1,200 U/L? That’s acute hepatitis. Same test. Totally different story.

And remember: liver damage often happens quietly. By the time you feel sick, it’s often advanced. That’s why regular checkups matter - even if you feel fine.