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

When your doctor orders liver function tests, it’s not because they think you’re drinking too much or eating too much fried food. It’s because something in your blood is sending a quiet alarm. These tests don’t measure how well your liver is working like a machine. They measure the leakage of its parts - and that’s where the real story begins.

What You’re Actually Measuring

Liver function tests - or LFTs - are a misnomer. They don’t test liver function. They test liver damage. Think of your liver like a factory. When the walls crack, chemicals spill out into the bloodstream. ALT, AST, ALP, GGT, bilirubin, albumin, and PT are those spilled chemicals. The goal isn’t to see if the factory is making products - it’s to find out if it’s falling apart.

ALT (alanine aminotransferase) is mostly found in liver cells. When those cells die - from virus, fat, toxin, or lack of blood flow - ALT pours into your blood. AST (aspartate aminotransferase) is also in the liver, but it’s everywhere else too: heart, muscles, kidneys. That’s why a high AST could mean a heart attack, not liver trouble. Bilirubin is the yellow waste from broken-down red blood cells. Your liver normally processes it and flushes it out in bile. If it builds up, your skin turns yellow - jaundice.

The Numbers That Matter

Reference ranges vary by lab, but here’s what’s generally accepted in Australia and the US:

• ALT: 7-55 U/L (higher in men, higher in people with obesity)
• AST: 8-48 U/L
• Bilirubin (total): 3-17 μmol/L
• ALP: 40-129 U/L
• Albumin: 35-50 g/L

But here’s the catch: 10-15% of healthy people have ALT or AST levels just above the normal range - and nothing’s wrong. A level of 60 U/L doesn’t mean cirrhosis. It might mean you’re overweight, had a hard workout, or took ibuprofen for a week. That’s why context matters more than the number.

Patterns Tell the Real Story

Doctors don’t look at one number. They look at the pattern.

Pattern 1: ALT and AST high, bilirubin and ALP normal or only slightly up
This is hepatocellular injury - liver cells are dying. The bigger the ALT compared to AST, the more likely it’s viral hepatitis or fatty liver. In acute hepatitis A or B, ALT can spike to 1,000 U/L or more. In fatty liver disease (now called MASLD), ALT is usually 1.5 to 2 times higher than AST. If ALT is over 10 times the upper limit, think: viral infection, drug overdose (like paracetamol), or lack of blood flow to the liver.

Pattern 2: ALP and bilirubin high, ALT and AST only mildly up
This is cholestasis - bile isn’t flowing. Something’s blocking the ducts: gallstones, tumors, or medication. ALP can rise 3-10 times normal. But if ALP is high and GGT is normal, look elsewhere - maybe your bones. ALP comes from bone too. A broken rib or bone cancer can trick you.

Pattern 3: AST higher than ALT
This is the alcohol clue. If AST is twice as high as ALT - or more - alcohol is the likely culprit. In alcoholic hepatitis, AST is often 2-6 times normal, while ALT is only 1-2 times. That ratio is more than 90% accurate for alcohol-related damage. But if AST is above 500 U/L, don’t assume it’s just alcohol. Think acetaminophen overdose - especially if the person drank heavily.

Albumin and PT: The True Function Test

Here’s where you see if the liver is still doing its job. Albumin is a protein made by the liver. It keeps fluid in your blood vessels. If albumin drops below 35 g/L, your liver has been struggling for months - maybe years. It’s not a quick signal. Albumin’s half-life is 20 days. A low level means chronic damage: cirrhosis, long-term alcohol use, or severe malnutrition.

Prothrombin time (PT) measures how fast your blood clots. The liver makes clotting factors. If PT is prolonged, your liver can’t make them fast enough. This is urgent. A rising PT means acute failure - maybe from drug toxicity or severe hepatitis. It’s a red flag. A normal PT with low albumin? That’s chronic. A high PT with normal albumin? That’s sudden and dangerous.

What Doesn’t Show Up

Many people think if their LFTs are normal, their liver is fine. That’s not true. In early cirrhosis, ALT and AST can be normal or only slightly raised. The liver is scarred, but not leaking much. The real damage is hidden. That’s why doctors now use FIB-4 or APRI scores - blood tests that combine age, platelets, ALT, and AST to estimate scarring. A 2021 study showed combining these with LFTs boosted accuracy for advanced liver disease from 68% to 89%.

And then there’s GGT. It’s not part of the classic panel, but it’s often checked. GGT rises with alcohol, bile duct problems, or even just being overweight. Isolated GGT elevation? It’s common in metabolic syndrome. But it’s not specific. A high GGT with normal ALT? Could be nothing. Could be early fatty liver. Could be a sign to cut back on wine.

When to Worry - And When to Wait

Not every elevated LFT needs a scan or a specialist. Here’s what most doctors do:

• ALT or AST under 80 U/L? Wait 3 months. Repeat. Check weight, alcohol, meds, diabetes. No need for ultrasound yet.
• ALT or AST over 500 U/L? Refer immediately. Could be drug overdose, acute viral hepatitis, or ischemic injury.
• ALT rising more than 100 U/L per week? That’s fast. Needs urgent review.
• Albumin below 35 g/L or PT prolonged? Refer to liver specialist. This isn’t mild.
• Isolated bilirubin high? Check for Gilbert’s syndrome - a harmless genetic condition that causes mild jaundice under stress.

And here’s a hard truth: 37% of GPs order ultrasounds or CT scans for ALT levels between 41-80 U/L - even when there’s no other sign of disease. That’s over-testing. It adds cost, anxiety, and radiation - for no benefit.

What You Can Do

If your LFTs are off:

• Stop alcohol completely - even if you think it’s “just a few drinks.”
• Check your medications. Some painkillers, antibiotics, and supplements (like green tea extract or high-dose niacin) can damage the liver.
• Manage your weight. Losing 5-10% of body weight can reverse fatty liver.
• Control blood sugar. Type 2 diabetes and fatty liver go hand-in-hand.
• Don’t panic. Most mild elevations improve with lifestyle changes.

And if your numbers are normal? Don’t assume you’re safe. Fatty liver can be silent for years. Get checked if you’re overweight, diabetic, or drink regularly - even if your tests are fine now.

What’s Next?

The future of liver testing isn’t just ALT and AST. New blood tests like the ELF test (Enhanced Liver Fibrosis) measure collagen fragments to detect scarring before it’s visible on ultrasound. In 2024, studies showed these tests catch advanced fibrosis with 92% accuracy when combined with AST/ALT ratios.

But the best tool is still you - paying attention to your body, your habits, and your numbers. The liver doesn’t shout. It whispers. And if you listen - with the right tests and the right questions - you can hear it before it breaks.