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

When your doctor orders a liver function test, it’s not because they think you have liver disease - it’s usually because something else is off. Maybe your blood work showed a slight bump in enzymes, or you’ve been feeling unusually tired. Maybe you drink alcohol occasionally, or you’re taking a new medication. The truth is, most people who get these tests done don’t have serious liver problems. But that doesn’t mean you should ignore the results. Understanding what ALT, AST, and bilirubin actually tell you can save you from unnecessary worry - or help catch something serious early.

What Liver Function Tests Actually Measure

The name ‘liver function tests’ is misleading. These aren’t tests that measure how well your liver is working, like filtering toxins or making bile. They measure damage. Think of them like smoke alarms: they don’t tell you if the house is burning down, but they do tell you something’s wrong inside the walls.

The most common tests are ALT (alanine aminotransferase), AST (aspartate aminotransferase), ALP (alkaline phosphatase), GGT (gamma-glutamyl transferase), bilirubin, albumin, and prothrombin time. Of these, ALT and AST are the most talked about. They’re enzymes found mostly in liver cells. When those cells get damaged - from alcohol, viruses, fat buildup, or drugs - these enzymes leak into the bloodstream. That’s what shows up on your blood test.

But here’s the catch: AST isn’t just in the liver. It’s also in your heart, muscles, and kidneys. So if you’ve had a heart attack or done an intense workout, your AST might be high - even if your liver is fine. ALT, on the other hand, is almost exclusively in the liver. That’s why ALT is a better signal of liver-specific damage.

Normal Ranges Are Not One-Size-Fits-All

You might see a lab report that says your ALT is 58 U/L and your AST is 49 U/L. The reference range says 7-55 for ALT and 8-48 for AST. So you’re over. But does that mean you have liver disease? Not necessarily.

Normal ranges vary by lab, sex, and body weight. Men typically have higher normal levels than women. People with a BMI over 30 - that’s overweight or obese - can have ALT and AST levels 10-15% higher than someone with a normal weight, and still be completely healthy. A 2022 study in JAMA Internal Medicine found that 10-15% of healthy people have mildly elevated liver enzymes with no underlying disease.

For bilirubin, normal levels are between 3-17 μmol/L. Anything above that can mean your liver isn’t processing bile properly. But mild elevations - say, 20-25 μmol/L - can happen after fasting, during dehydration, or in people with Gilbert’s syndrome, a harmless genetic condition that affects about 5-10% of the population.

Patterns Matter More Than Numbers

Here’s where most people get confused. It’s not about one number being high. It’s about the pattern. Three patterns tell doctors what’s likely going on:

• Hepatocellular pattern: ALT and AST are much higher than ALP and bilirubin. This means liver cells are damaged. ALT is usually more than twice AST. This is classic for viral hepatitis (like hepatitis A or B), drug reactions (like paracetamol overdose), or fatty liver disease (MASLD).
• Cholestatic pattern: ALP and bilirubin are high, but ALT and AST are only mildly raised. This suggests bile flow is blocked - maybe by gallstones, a tumor, or medication. If ALP is high but GGT is normal, it might not be liver-related at all. ALP is also made in bones, so a bone fracture or bone disease can raise it too.
• Mixed pattern: All enzymes are elevated. This often happens with drug-induced liver injury or autoimmune hepatitis.

The AST-to-ALT ratio is one of the most useful clues. If AST is more than twice as high as ALT - say, AST 120 and ALT 50 - it strongly points to alcohol-related liver damage. In fact, over 90% of people with alcoholic hepatitis have an AST:ALT ratio above 1, and often above 2. If the ratio is less than 1 - ALT higher than AST - it’s more likely non-alcoholic fatty liver disease (now called MASLD), viral hepatitis, or medication-induced injury.

But here’s the exception: if AST or ALT shoots above 500 U/L, alcohol alone is unlikely to be the cause. That kind of spike usually means acute viral hepatitis or a paracetamol overdose. In these cases, the damage happens fast - within hours - and the enzymes rise quickly.

What Bilirubin Tells You

Bilirubin is the yellow pigment your liver breaks down from old red blood cells. When the liver can’t process it, bilirubin builds up. That’s what causes jaundice - yellow skin and eyes.

There are two types: unconjugated (indirect) and conjugated (direct). If total bilirubin is high but direct bilirubin is normal, it’s likely due to too many red blood cells breaking down (hemolysis) or Gilbert’s syndrome. If direct bilirubin is high, it means the liver isn’t excreting bile properly - that’s cholestasis.

ALP and GGT are often checked alongside bilirubin. If ALP is more than three times higher than ALT, and bilirubin is elevated, it’s almost always a bile duct issue. But if GGT is normal while ALP is high, look elsewhere - maybe your bones.

Albumin and Prothrombin Time: The Real Liver Function Tests

Here’s the part most people miss. Albumin and prothrombin time are the only true measures of liver function - not damage.

Albumin is a protein your liver makes. It helps keep fluid in your blood vessels and carries hormones and drugs. If your albumin is low (below 35 g/L), it means your liver has been struggling for weeks or months. It’s a sign of chronic liver disease - like cirrhosis. But albumin takes 20 days to drop. So if you had a sudden liver injury yesterday, your albumin will still look normal.

Prothrombin time (PT) measures how long it takes your blood to clot. Your liver makes clotting factors. If PT is prolonged, it means your liver isn’t making enough of them. This can happen quickly - within days - after severe liver damage. It’s one of the first signs your liver is failing.

That’s why doctors don’t rely on ALT and AST alone to judge how bad your liver disease is. A person with cirrhosis might have normal ALT and AST - because there’s little liver tissue left to damage. But their albumin is low and their PT is long. That’s the real picture.

When to Worry - And When to Wait

Not every abnormal result needs a scan or a specialist. Here’s what experts recommend:

• If ALT or AST is less than 2 times the upper limit (say, under 110 U/L) and you feel fine - wait. Repeat the test in 3-6 months. Watch for weight gain, alcohol use, or new medications.
• If ALT or AST is above 500 U/L, or if it’s rising fast (more than 100 U/L per week) - get evaluated immediately. This could be acute hepatitis or drug toxicity.
• If bilirubin is high and you’re yellow, or if you have dark urine, pale stools, or itching - see a doctor right away.
• If you have risk factors - heavy drinking, obesity, type 2 diabetes, or hepatitis B/C - even mild elevations should be followed up.

Studies show that combining liver enzyme patterns with simple non-invasive scores like FIB-4 (which uses age, platelets, ALT, and AST) can predict advanced liver scarring with 89% accuracy - far better than enzymes alone.

Common Mistakes and Misinterpretations

Doctors get it wrong more often than you think. A 2022 study found that 37% of primary care doctors ordered ultrasounds or CT scans for ALT levels between 41-80 U/L - even when the patient had no symptoms, no alcohol use, and no risk factors. Most of those people had no liver disease. The elevation was due to obesity, exercise, or even a recent illness.

Another mistake: blaming alcohol for everything. Yes, high AST:ALT ratio suggests alcohol. But if you’re a non-drinker with the same ratio, it could be MASLD - which is now the leading cause of liver disease worldwide. And if your AST is over 500 U/L, alcohol is probably not the culprit - you need to check for paracetamol overdose, especially if you’ve been taking cold medicine or painkillers.

And don’t forget: muscle injury can raise AST. A tough workout, a car accident, or even a seizure can cause temporary spikes. That’s why doctors look at the whole picture - not just the numbers.

What Comes Next?

If your tests are abnormal, your doctor might order more tests: hepatitis B and C screening, ultrasound, FIB-4 score, or even a FibroScan to check for liver stiffness. In some cases, a biopsy is needed - but that’s rare now. Most liver diseases can be diagnosed with blood tests and imaging.

For fatty liver (MASLD), the treatment isn’t a pill - it’s lifestyle. Losing 5-10% of your body weight can reverse fat buildup and lower ALT. Cutting back on sugar and alcohol helps. Exercise matters more than you think - even without weight loss, regular activity reduces liver fat.

For viral hepatitis, antiviral drugs can cure hepatitis C and control hepatitis B. For autoimmune liver disease, steroids or immunosuppressants can stop the attack. But none of that matters if you don’t know what you’re dealing with.

Don’t panic over a single high number. But don’t ignore it either. Liver disease often has no symptoms until it’s advanced. The best defense is awareness - and knowing what your numbers really mean.

What to Do Next

If your liver enzymes are slightly high and you’re otherwise healthy, start with lifestyle changes: lose weight if you’re overweight, cut out sugary drinks and alcohol, get moving. Re-test in 3 months. If they’re still up, ask for a FIB-4 score or ultrasound. Don’t rush to a specialist unless you have jaundice, swelling, or rapid enzyme rises.

If you’re diagnosed with MASLD or fatty liver, know this: it’s reversible. Most people who lose weight and improve their diet see their ALT drop back to normal within a year. The liver is one of the few organs that can regenerate. But only if you give it a chance.