Liver function tests (LFTs) are a panel of blood tests that measure enzymes, proteins, and waste products to reveal how well your liver is working and whether it’s been damaged. The panel typically includes five core markers: ALT, AST, alkaline phosphatase (ALP), bilirubin, and albumin. Each one tells a different part of the story, and interpreting them means looking at which values are off, how far off they are, and what pattern they form together.
The Five Core Markers and Their Ranges
Standard reference ranges vary slightly between laboratories, but the commonly used values are:
- ALT (alanine transaminase): 4 to 36 IU/L
- AST (aspartate transaminase): 5 to 30 IU/L
- ALP (alkaline phosphatase): 30 to 120 IU/L
- Total bilirubin: 2 to 17 µmol/L
- Albumin: 35 to 50 g/L
These markers fall into two categories. ALT, AST, ALP, and bilirubin are damage indicators. When liver cells are injured or bile flow is blocked, these values rise. Albumin, on the other hand, measures the liver’s ability to produce proteins, so a low albumin signals that the liver isn’t functioning well. Understanding which category is affected helps narrow down what’s going on.
One important note: the ranges printed on your lab report may be wider than what current guidelines recommend, especially for ALT. The American College of Gastroenterology sets the true healthy upper limit at 33 IU/L for men and 25 IU/L for women. A result of 30 IU/L might fall within your lab’s “normal” range but could still warrant attention in a woman.
ALT and AST: Signs of Liver Cell Damage
ALT and AST are enzymes that sit inside liver cells. When those cells are injured, the enzymes leak into your bloodstream. ALT is more specific to the liver, making it the primary marker clinicians focus on. AST also exists in heart muscle, skeletal muscle, and other tissues, so an isolated AST elevation doesn’t automatically point to the liver.
Mild elevations (less than three times the upper limit) are common and can result from fatty liver disease, medications, or even strenuous exercise. Moderate elevations (three to ten times the upper limit) typically suggest active inflammation from conditions like viral hepatitis or autoimmune hepatitis. Massive elevations, sometimes reaching into the thousands, point to acute events like acetaminophen toxicity, sudden loss of blood flow to the liver, or severe viral hepatitis.
Non-liver causes of elevated ALT and AST include muscle injury, heart attack, and an underactive thyroid. If your AST is elevated but your ALT is normal, the source is more likely muscle or heart tissue than the liver itself.
What the AST-to-ALT Ratio Tells You
The relationship between AST and ALT adds another layer of information. In most chronic liver conditions, ALT runs higher than AST, giving a ratio below 1.0. This pattern is typical of non-alcoholic fatty liver disease, particularly in people who are significantly overweight.
When AST is higher than ALT (ratio above 1.0), it raises the possibility of alcohol-related liver damage. A ratio above 2.0 is highly suggestive of alcoholic hepatitis. That said, about one-third of people with alcohol-related cirrhosis still have ratios below 1.0, so the ratio is a clue rather than a definitive answer. A high ratio can also reflect advanced scarring (cirrhosis) from any cause, because a scarred liver releases proportionally more AST.
ALP and GGT: Markers of Bile Flow Problems
Alkaline phosphatase rises when bile ducts inside or outside the liver become blocked or damaged. This pattern is called cholestasis. Common causes include gallstones blocking the bile duct, certain medications, and conditions where the immune system attacks the bile ducts.
The tricky part is that ALP isn’t exclusive to the liver. It’s also produced by bone, the placenta during pregnancy, and the intestines. A high ALP in a teenager going through a growth spurt, for example, is perfectly normal and comes from bone turnover, not liver disease.
To confirm that ALP is coming from the liver, a second enzyme called GGT (gamma-glutamyl transpeptidase) is tested. If both ALP and GGT are elevated, the source is almost certainly the liver or bile ducts, and an ultrasound is the typical next step to check whether the bile ducts are physically dilated. If ALP is high but GGT is normal, the elevation is likely from bone or placenta. GGT also rises with alcohol use, so an isolated GGT elevation in someone who drinks regularly may reflect that rather than bile duct disease.
Bilirubin: Why It Matters Whether It’s Direct or Indirect
Bilirubin is a yellow pigment produced when old red blood cells break down. The liver processes it, converts it to a water-soluble form, and excretes it in bile. When bilirubin builds up in the blood, you may notice yellowing of the skin and eyes (jaundice).
Your lab report may break bilirubin into two fractions: direct (conjugated) and indirect (unconjugated). This distinction matters because each points to a different problem. Elevated indirect bilirubin suggests the liver is being overwhelmed by too much bilirubin production, often from conditions that destroy red blood cells faster than normal. It can also reflect Gilbert syndrome, a harmless genetic condition affecting roughly 5% to 10% of the population that causes mild, intermittent jaundice.
Elevated direct bilirubin, on the other hand, signals that the liver has processed the bilirubin but can’t excrete it properly. This points to liver disease or a blockage in the bile ducts. One practical clue: dark-colored urine occurs when excess direct bilirubin spills into urine, since it’s water-soluble. Indirect bilirubin binds to a protein in your blood and can’t pass through the kidneys, so urine stays its normal color even when indirect levels are high.
Albumin and Clotting Time: How Well the Liver Actually Works
The markers discussed so far measure damage. Albumin and prothrombin time (PT/INR) measure something different: how well the liver is doing its job of manufacturing proteins. This is called synthetic function, and it’s one of the most important things LFTs can reveal.
Albumin is produced exclusively by the liver. It keeps fluid inside your blood vessels and carries hormones, vitamins, and medications through the bloodstream. Low albumin (below 35 g/L) can indicate chronic liver disease, though it also drops in kidney disease, malnutrition, and severe inflammation. Because albumin has a long half-life of about 20 days, it reflects liver function over weeks rather than hours. A sudden liver injury won’t immediately lower albumin.
Prothrombin time measures how quickly your blood clots. The liver produces most of the clotting factors involved, so when liver function declines, clotting slows and INR rises. Unlike albumin, clotting factors have short half-lives, so PT/INR can change within hours during acute liver failure. This makes it a sensitive early warning sign. INR is included in several scoring systems used to assess the severity of liver disease for exactly this reason.
When both albumin is low and INR is elevated, it generally means the liver’s manufacturing capacity is significantly compromised.
Common Medications That Affect Results
Many widely used medications can cause liver enzymes to rise, sometimes without any actual liver damage occurring. Acetaminophen (Tylenol) is the most common culprit, particularly at doses above the recommended limit. NSAIDs like ibuprofen and naproxen can also trigger elevations. Statins, commonly prescribed for cholesterol, cause mild enzyme bumps in some people, which often stabilize without needing to stop the medication.
Other drugs known to affect LFTs include certain antibiotics (amoxicillin-clavulanate, erythromycin), anti-seizure medications, birth control pills, and the tuberculosis drug isoniazid. Drug-induced liver injury most often resolves within days or weeks after stopping the medication. If your LFTs are mildly abnormal and you recently started a new drug, that connection is worth flagging.
What Happens After an Abnormal Result
A common instinct, both for patients and sometimes for clinicians, is to simply repeat the test in a few weeks. Current guidelines push back on this approach. Repeating the same panel without investigating the cause is only justified when there’s strong reason to believe the abnormality is temporary, like a known acute illness or a medication that’s been stopped. In all other cases, the first abnormal result should trigger further testing to identify the underlying cause.
A standard workup for unexplained abnormal LFTs typically includes an abdominal ultrasound to look at liver structure and bile ducts, along with blood tests for hepatitis B and C, autoimmune antibodies, iron levels, and immune protein levels. If fatty liver disease is suspected, additional testing to assess the degree of liver scarring (fibrosis) may follow, using specialized blood panels or an imaging technique called elastography that measures liver stiffness.
Even when all of these tests come back negative, persistently abnormal liver enzymes still warrant specialist evaluation. Some liver conditions don’t show up on standard screening panels and require more targeted investigation.
Putting the Pattern Together
No single LFT value tells the full story. Interpretation depends on recognizing patterns across the panel:
- High ALT and AST with normal ALP: suggests direct liver cell injury (hepatocellular pattern). Think viral hepatitis, fatty liver disease, medications, or toxins.
- High ALP and GGT with normal or near-normal ALT/AST: suggests bile flow obstruction (cholestatic pattern). Think gallstones, bile duct disease, or infiltrative liver conditions.
- Everything mildly elevated: a mixed pattern that can occur with medications, advanced liver disease, or conditions affecting both liver cells and bile ducts.
- Low albumin and/or elevated INR: indicates the liver’s production capacity is impaired, pointing to more advanced or chronic disease.
- Isolated elevated bilirubin with everything else normal: often Gilbert syndrome, especially if the indirect fraction predominates and the person is otherwise healthy.
The degree of elevation also matters. Mild bumps in ALT (less than twice normal) are extremely common in the general population and frequently trace back to fatty liver, alcohol, or body weight. Dramatic spikes tell a more urgent story. Context is everything: the same ALT of 60 IU/L means something very different in a 25-year-old marathon runner who just raced than in a 55-year-old with diabetes and obesity.