When your doctor orders liver function tests, it’s not because they think you have liver disease - it’s because something in your blood is out of place. Maybe your ALT is a little high. Maybe your bilirubin is creeping up. Or maybe you’re just feeling off, and they’re trying to rule things out. The truth? Most people don’t understand what these numbers actually mean. And that’s okay. But if you’ve ever stared at a lab report wondering if you’re in trouble, this is what you need to know.
What Liver Function Tests Actually Measure
Liver function tests - or LFTs - aren’t really about how well your liver is working. That’s a misnomer. They’re really about damage. These tests measure enzymes and proteins that leak into your blood when liver cells are injured. Think of it like a car engine: if a sensor starts reading high temperatures, it doesn’t mean the car isn’t running - it means something’s broken inside.
The most common tests include ALT (alanine aminotransferase), AST (aspartate aminotransferase), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), bilirubin, albumin, and prothrombin time. But not all of them are created equal. ALT and AST tell you about cell damage. Bilirubin tells you about bile flow. Albumin and prothrombin time tell you if your liver can still make the stuff it’s supposed to.
Normal ranges vary by lab, but here’s a general guide: ALT is 7-55 U/L, AST is 8-48 U/L, total bilirubin is 3-17 μmol/L, and albumin is 35-50 g/L. But here’s the catch - these numbers aren’t the same for everyone. Men tend to have higher ALT and AST than women. People with obesity (BMI over 30) can have levels 10-15% higher and still be perfectly healthy. So a number that looks alarming to you might be normal for your body.
ALT vs AST: The Key Difference
ALT is mostly found in the liver. If it’s high, it’s almost always a liver problem. AST, on the other hand, is also in your heart, muscles, and kidneys. So if AST is up but ALT isn’t, it could be a heart attack, intense workout, or muscle injury - not your liver.
But here’s where it gets interesting: the ratio between AST and ALT tells doctors more than either number alone. If AST is more than twice as high as ALT, it’s a red flag for alcohol-related liver damage. In fact, over 90% of people with alcoholic hepatitis have an AST:ALT ratio above 1, and often above 2. That’s not a coincidence. It’s a pattern.
On the flip side, if ALT is higher than AST - especially if it’s more than double - it’s more likely to be viral hepatitis, fatty liver disease (now called MASLD), or drug-induced injury. In acute hepatitis A or B, ALT can spike to 10 times the upper limit. That’s not just high - it’s dramatic. But here’s the thing: if your ALT is only 60 or 80 U/L, that’s not a crisis. About 10-15% of healthy people have mild elevations like that. No treatment needed. Just watch it.
Bilirubin: The Yellow Clue
Bilirubin is what turns your skin and eyes yellow when you have jaundice. It’s a waste product from broken-down red blood cells. Your liver normally processes it and sends it out in bile. If that process gets blocked - say, by a gallstone or tumor - bilirubin backs up in your blood.
There are two types: unconjugated (before the liver processes it) and conjugated (after). Total bilirubin includes both. If conjugated bilirubin is high, it means your liver can’t get it out. That’s cholestasis. You’ll usually see ALP and GGT up too. But if only unconjugated bilirubin is high, it’s likely a problem with red blood cell breakdown - not your liver.
Here’s a trick: if ALP is more than three times higher than ALT, you’re looking at a bile flow problem - not liver cell damage. But if ALP is up and GGT is normal? That’s a red flag for bone disease. ALP comes from your bones too. So if you’ve got a broken bone or osteoporosis, your ALP can rise without any liver issue at all.
Albumin and Prothrombin Time: The Real Liver Function Tests
Here’s what most people miss: albumin and prothrombin time are the only tests that actually measure how well your liver is working - not just how damaged it is.
Albumin is a protein your liver makes to keep fluid in your blood vessels. It has a half-life of 20 days. That means if your albumin drops, it’s not because you had a bad night out. It’s because your liver has been struggling for weeks or months. Low albumin is a sign of chronic liver disease - like cirrhosis.
Prothrombin time (PT) measures how long it takes your blood to clot. Your liver makes the clotting factors. If PT is prolonged, it means your liver can’t keep up. This is urgent. It can happen fast - within days - after severe liver injury, like acetaminophen overdose or acute viral hepatitis. A high PT is one of the first signs your liver is failing.
That’s why doctors don’t just look at ALT and AST. They look at the whole picture. Two people can have the same ALT level. One has normal albumin and PT - probably just fatty liver. The other has low albumin and high PT - that’s cirrhosis. Same number. Totally different story.
Patterns Matter More Than Numbers
Doctors don’t diagnose based on one number. They look at patterns. Here are the three main ones:
- Hepatocellular pattern: ALT and AST are high (often more than 10x normal), ALP and bilirubin are only mildly elevated. This means liver cells are dying - viral hepatitis, drug injury, or fatty liver.
- Cholestatic pattern: ALP and bilirubin are high (often more than 3x), ALT and AST are only slightly up. This means bile flow is blocked - gallstones, tumors, or medication side effects.
- Mixed pattern: Everything is up. This is common in drug-induced liver injury or autoimmune hepatitis.
There’s also a fourth pattern: normal or only slightly elevated transaminases with low albumin and high PT. That’s advanced cirrhosis. The liver is so scarred, it’s not leaking enzymes anymore - it’s just not working.
One study of over 12,000 patients found that using just LFTs to guess liver fibrosis was only 68% accurate. But when doctors added a simple score called FIB-4 (which uses age, AST, ALT, and platelet count), accuracy jumped to 89%. That’s huge. It means your doctor doesn’t need to order a biopsy right away. They can use blood tests to tell if you’re at risk.
When to Worry - and When to Chill
Let’s be clear: a single mildly elevated ALT doesn’t mean you have liver disease. A 2022 JAMA Internal Medicine study found that 37% of primary care doctors ordered unnecessary imaging for ALT levels between 41-80 U/L. That’s overtesting. That’s anxiety. That’s wasted money.
Here’s when you should pay attention:
- ALT or AST over 500 U/L - that’s a red flag. Could be acetaminophen overdose, acute hepatitis, or ischemic injury.
- ALT or AST rising more than 100 U/L per week - that’s fast. Needs urgent evaluation.
- AST:ALT ratio above 2 - think alcohol.
- Albumin below 3.5 g/dL or PT prolonged - think cirrhosis.
- Bilirubin above 30 μmol/L with jaundice - needs immediate attention.
Here’s when you can wait:
- ALT under 80 U/L with no symptoms - monitor in 3-6 months.
- Isolated GGT elevation - common in fatty liver, especially in obese people. No action needed unless other markers rise.
- Normal ALT but high bilirubin - check for Gilbert’s syndrome. It’s harmless. Affects up to 10% of people.
What Happens Next?
If your tests are abnormal, your doctor won’t jump to a scan or biopsy. They’ll ask questions: Do you drink? Are you taking any supplements? Do you have diabetes or high cholesterol? Have you lost weight? Are you on any new meds?
They might order an ultrasound to check for fatty liver or gallstones. Or a FibroScan to measure liver stiffness. Or they might just tell you to come back in three months.
Most people with mild elevations don’t need treatment. They need lifestyle changes. Lose 5-10% of your body weight. Cut out alcohol. Stop taking unnecessary supplements. Control your blood sugar. That’s it. Fatty liver reverses. Viral hepatitis can be treated. Drug damage stops when you stop the drug.
But if you wait too long - if you ignore it - you risk cirrhosis. And cirrhosis can lead to liver cancer. That’s why early detection matters. Not because your ALT is 72. But because if it keeps climbing, and you do nothing, you might not have a second chance.
What’s New in Liver Testing?
The name “NAFLD” is gone. It’s now called MASLD - Metabolic dysfunction-Associated Steatotic Liver Disease. That’s because it’s not just about fat. It’s about metabolism. Insulin resistance. High triglycerides. Belly fat. The old name made people think it was harmless. It’s not.
And new tests are coming. The ELF test (Enhanced Liver Fibrosis) measures three proteins that show up when scarring begins. In 2024, a Lancet study showed it detects advanced fibrosis with 92% accuracy when combined with AST/ALT ratios. That means you might never need a biopsy. Just a blood test.
But for now, the old tests still work. ALT. AST. Bilirubin. Albumin. PT. The patterns haven’t changed. The science is solid. You just need to know what they mean.
Can ALT be high without liver disease?
Yes. ALT can be mildly elevated due to obesity, intense exercise, certain medications, or even normal variation. About 10-15% of healthy people have ALT levels slightly above the lab’s normal range. If it’s under 80 U/L and you feel fine, it’s usually not a problem. But it should be monitored.
What does a high AST:ALT ratio mean?
An AST:ALT ratio above 1, especially above 2, strongly suggests alcohol-related liver damage. In alcoholic hepatitis, AST is often twice as high as ALT. A ratio below 1 is more typical in fatty liver disease or viral hepatitis. But this isn’t a diagnosis on its own - it’s a clue that needs context.
Is bilirubin always high in liver disease?
No. Bilirubin only rises when bile flow is blocked or when the liver can’t process it. In early fatty liver or mild hepatitis, bilirubin is often normal. It’s one of the last markers to change. So normal bilirubin doesn’t mean your liver is healthy - it just means bile flow isn’t blocked yet.
Can a normal ALT rule out liver damage?
No. In advanced cirrhosis, ALT and AST can be normal or only slightly elevated because there are few healthy liver cells left to leak enzymes. That’s why doctors also check albumin and prothrombin time - they show if the liver can still function, even if it’s scarred.
How long does it take for ALT to return to normal?
ALT has a half-life of about 47 hours. That means if you stop drinking or stop a damaging medication, your ALT will drop by half in just under two days. It usually returns to normal within 2-4 weeks after removing the cause. But if it doesn’t, something else is going on.
Do I need a liver biopsy if my tests are abnormal?
Not usually. Most doctors start with blood tests, ultrasound, and non-invasive scores like FIB-4 or ELF. Biopsies are only done if the cause is unclear or if there’s a high risk of advanced fibrosis. Many people avoid biopsies entirely with today’s tools.
What to Do Next
If your liver tests are abnormal, don’t panic. Don’t Google symptoms. Don’t start taking supplements labeled “liver cleanse.” Those don’t work. And some can make it worse.
Do this instead:
- Ask your doctor: Is this a one-time spike or something persistent?
- Review your meds, supplements, and alcohol use.
- Get a fasting lipid panel and HbA1c - check for metabolic issues.
- Ask about a FibroScan or FIB-4 score - it tells you more than ALT alone.
- If you’re overweight, aim to lose 5-10% of your body weight. That’s the most effective treatment for fatty liver.
- Follow up in 3-6 months. Don’t wait a year.
Your liver is tough. It can heal - if you give it a chance. But it won’t heal if you ignore the signs. The numbers aren’t scary. They’re a message. Listen to them before it’s too late.
Skye Kooyman
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