Liver Function Tests (LFTs): ALT, AST, ALP, Bilirubin & More

Liver function tests — often shortened to LFTs, and reported by the lab as a hepatic function panel — are a group of blood tests that check how your liver is doing. They are among the most commonly ordered blood tests in U.S. medicine, and a mildly high liver enzyme is one of the most common abnormal results a person will ever see. Here is the reassuring headline up front: a slightly elevated ALT or AST is usually not a sign of serious liver disease — the single most frequent explanation is fatty liver, and it is often reversible. This hub explains what a liver function test actually measures, gives the normal ranges in U.S. units, and — most importantly — teaches you to read the pattern: whether your results point to liver-cell (hepatocellular) injury or to a bile-flow (cholestatic) problem. Four of the markers have their own detailed guide: ALT, AST, GGT, and bilirubin. The interpretation always belongs to your clinician.

Key takeaways

• A liver function test (LFT), or hepatic function panel, bundles several markers: the enzymes ALT, AST, ALP, and GGT; the pigment bilirubin; and the synthetic proteins albumin and total protein.¹²
• The enzymes are not a measure of "function" at all — they leak from injured cells. True function is reflected by albumin and clotting, which fall when the liver is failing.³
• Typical adult benchmarks: ALT ~7–55 U/L, AST ~8–48 U/L, ALP ~40–130 U/L, GGT ~8–61 U/L, total bilirubin ~0.1–1.2 mg/dL, albumin ~3.5–5.0 g/dL — always confirmed against your report's range.²⁴
• The key skill is reading the pattern: ALT/AST up = hepatocellular injury; ALP up (with GGT) = cholestatic (bile-flow) problem.³
• The most common cause of mildly elevated enzymes is fatty liver disease (MASLD), tied to weight, blood sugar, and cholesterol — often reversible with lifestyle change.⁵⁶
• A single mildly high value is common and usually benign; a markedly high result, or one with jaundice and abnormal clotting, needs prompt evaluation.³
• No number is a diagnosis: LFTs are read together, in the context of your history, medications, and metabolic health.

What are liver function tests?

The liver is the body's central chemical plant: it builds proteins, stores and releases sugar, processes medications and toxins, makes bile to digest fat, and clears the waste pigment left when red blood cells break down. Liver function tests — also called a hepatic function panel, a liver panel, or simply LFTs — are the blood tests that reflect how well that plant is running and whether its cells are being damaged.¹

It helps to know that "liver function test" is a slightly misleading name. Most of the markers are enzymes that leak out of liver cells when those cells are injured — they signal damage, not "function." Only a couple of the values (albumin and the clotting time) actually measure what the liver is making, and therefore how well it is truly working.³ Keeping that distinction in mind is the first step to reading the panel correctly.

An LFT is not one test but a panel of complementary markers. In the U.S. it usually arrives as part of a comprehensive metabolic panel (CMP) or as a stand-alone hepatic function panel when the liver is specifically in question.¹² A doctor may order it to investigate symptoms like fatigue, abdominal pain, dark urine, or jaundice; to screen people with risk factors such as obesity, type 2 diabetes, or heavy alcohol use; to monitor a known liver condition; or to watch for medication side effects. Very often, an abnormal result is discovered by chance on a routine panel — the start of the work-up, not a reason to panic.

The normal ranges

Here are the usual adult benchmarks in U.S. units. Enzymes are reported in U/L (units per liter), bilirubin in mg/dL, and the proteins in g/dL. Ranges vary by laboratory, method, sex, and age — men typically run slightly higher on the transaminases and GGT.²⁴

Good to know: these intervals are indicative and differ from one lab to another. The reference range that counts is the one printed on your report. For ALT in particular, many experts now argue the traditional upper limits are too high and that a "truly healthy" ALT is lower — around ≤33 U/L for men and ≤25 U/L for women — which helps catch early fatty liver sooner.⁴⁵

The enzymes

This is the heart of the panel. Four enzymes are measured, and they split into two teams: the transaminases (ALT and AST), which mark liver-cell injury, and the cholestatic enzymes (ALP and GGT), which mark a problem with bile flow. Which team is elevated is what tells the story.

ALT

ALT (alanine aminotransferase, sometimes written SGPT) is an enzyme concentrated inside liver cells. Because it is the most liver-specific of the four, a high ALT points almost directly at the liver — the single most useful early flag of hepatocellular trouble. A mildly high ALT is extremely common, and the leading cause is fatty liver disease; a markedly high ALT (many times the upper limit) points instead to acute injury — viral hepatitis, a drug or toxin, or reduced blood flow.³ For normal ranges, causes of a high or low result, and how ALT pairs with AST, see the full ALT blood test guide.

AST (and the AST/ALT ratio)

AST (aspartate aminotransferase, also reported as SGOT) is the other transaminase — but with a twist: it is not specific to the liver. AST also lives in skeletal muscle, the heart, and red blood cells, so a high AST with a normal ALT often points to a muscle source (intense exercise, an injury) rather than the liver.³ When the liver is the cause, ALT and AST usually rise together, and their AST/ALT (De Ritis) ratio helps point toward it: below 1 is typical of fatty liver disease, while above 2 classically suggests alcohol-related disease or advanced fibrosis. The full AST blood test guide covers the muscle trap and the ratio in detail.

ALP (alkaline phosphatase)

ALP (alkaline phosphatase) is the first of the two cholestatic enzymes — it rises when bile flow is obstructed. The catch is that ALP is not made only in the liver: a large share comes from bone, so a high ALP can be entirely skeletal — a healing fracture, Paget's disease, or normal bone growth in children and adolescents, where high ALP is expected and harmless (pregnancy also raises it, via the placenta). This is exactly why ALP is read together with GGT: a high ALP with a high GGT points to the liver or bile ducts; a high ALP with a normal GGT is almost certainly bone, and the liver is likely fine.³ ALP has no separate guide here — but its meaning is simple once you pair it with GGT.

GGT

GGT (gamma-glutamyl transferase) is the second cholestatic enzyme — the most sensitive liver marker on the panel, and the least specific. It climbs with bile-duct obstruction, but also with alcohol, fatty liver disease, medications, and even heart failure, so on its own it rarely names the cause. Its most valuable job is settling the ALP question: because GGT is in the liver but not in bone, it tells you whether a high ALP is hepatic or skeletal. One honest point: a high GGT does not prove you drink — its specificity as an alcohol marker is poor, and fatty liver and medications are far more common explanations. The full GGT blood test guide covers the normal ranges (which genuinely differ between sources), the alcohol question, and how to lower it.

Bilirubin

Bilirubin is the yellow-orange pigment left over when the body breaks down old red blood cells. The liver processes it and clears it into bile; when it backs up, it stains the skin and the whites of the eyes — jaundice. On the panel, bilirubin is reported as total, and can be split into two fractions: indirect (unconjugated), not yet processed by the liver, and direct (conjugated), already processed. It is that split, far more than the total, that points to the cause.³

A mildly high, isolated, indirect bilirubin with an otherwise normal liver panel is usually harmless — most often Gilbert syndrome, a benign and common genetic quirk that needs no treatment. A high direct bilirubin is the more meaningful result: it signals liver injury or blocked bile flow, and typically travels with a rising ALP and GGT. Visible jaundice generally appears only above about 3 mg/dL — nearly three times the upper limit of normal. For direct-versus-indirect, Gilbert syndrome, hemolysis, and newborn jaundice, see the full bilirubin blood test guide.

Albumin and total protein (the liver's synthetic function)

If the enzymes measure damage, albumin and total protein are the closest thing on the panel to measuring true function — what the liver is actually making.³

Albumin is the main protein in your blood, produced by the liver. Crucially, it behaves the opposite way to the enzymes: it doesn't spike with injury — it falls when the liver is chronically failing and can no longer produce enough. A low albumin can therefore point to advanced, long-standing liver disease (cirrhosis) — though it is not specific to the liver, since malnutrition, kidney disease, and inflammation also lower it.

Total protein measures albumin plus the globulins (largely antibodies); globulins often rise in chronic liver disease even as albumin falls. Because both reflect the liver's slow, cumulative work rather than acute injury, they are most useful for staging and following chronic disease, not for catching a sudden problem. Neither has its own guide here. (A related synthetic test, the prothrombin time / INR, measures clotting factors the liver makes; it is not part of the basic panel, but a prolonged INR alongside abnormal enzymes is a red flag for serious injury.)³

Reading the pattern: hepatocellular vs cholestatic

This is the single most important idea on the page, and the one that turns a confusing list of numbers into a story. Rather than reacting to any one value, clinicians ask which group of markers is leading — and the answer sorts almost every abnormal LFT into one of two patterns.³

• Hepatocellular pattern — the transaminases (ALT and AST) are elevated out of proportion to ALP. This means the liver cells themselves are being injured. Typical culprits: fatty liver disease, alcohol, viral hepatitis, and medications.
• Cholestatic pattern — ALP (confirmed hepatic by a high GGT), often with bilirubin, is elevated out of proportion to the transaminases. This means bile is not flowing properly — a gallstone in the bile duct, a stricture, a tumor, or certain intrinsic liver diseases.

Doctors formalize this comparison with the "R-factor," the relative size of the ALT rise versus the ALP rise (each as a multiple of its upper-normal limit). You don't need the math — the plain-language version is enough: transaminases leading = hepatocellular; ALP leading = cholestatic. The two patterns often overlap (a "mixed" picture), which is precisely why the panel is read as a whole: the relationship between the markers orients the diagnosis far more than any value alone.³

Common causes of abnormal LFTs

Most abnormal LFTs — especially mild elevations found on a routine panel — trace back to a short list of common causes.³⁷

1. Fatty liver disease (MASLD). By far the most common cause of mildly elevated transaminases in the U.S. Once called NAFLD, it was renamed MASLD (metabolic dysfunction-associated steatotic liver disease) in 2023 to reflect that it is fundamentally a metabolic problem — liver fat plus a cardiometabolic risk factor such as excess weight, high blood sugar, or high triglycerides.⁸⁹ The scale is enormous: NIDDK estimates about 24% of U.S. adults have it, rising to two-thirds of people with type 2 diabetes.⁵ Early on it is often reversible with weight loss and physical activity.⁶
2. Alcohol. Regular heavy drinking raises the enzymes — classically with an AST above the ALT (ratio > 2) and a high GGT.³ Cutting back lowers them over weeks.
3. Viral hepatitis. Hepatitis A, B, and C should be checked whenever a hepatocellular pattern appears; B and C can smolder for years with few symptoms.³
4. Medications and supplements. Many drugs raise liver enzymes — statins, some antibiotics, anti-seizure medications, high-dose acetaminophen — as can herbal and "detox" supplements. This is drug-induced liver injury (DILI).³ Never stop a prescribed medication on your own; raise it with your clinician.
5. Gallstones and bile-duct problems. A stone in the bile duct is a classic cause of a cholestatic pattern — a high ALP and GGT, often with a rising bilirubin, dark urine, and pale stools.³
6. Less common causes. Hemochromatosis (iron overload — worth a ferritin check), autoimmune hepatitis, Wilson disease, celiac disease, thyroid disorders, and — for an isolated high AST — a muscle source.³

When to worry

Most abnormal LFTs are mild and silent, which is exactly why they turn up by chance. Two things separate a routine finding from an urgent one: the size of the elevation and the company it keeps.³

A mild elevation (transaminases up to about 5× the upper limit), with no symptoms, is common and usually explored without urgency: a repeat test, a review of alcohol, weight, and medications, and screening for fatty liver and viral hepatitis. A markedly high result (10× or more) points to acute injury and warrants a prompt work-up. The most concerning combination is high transaminases plus a rising bilirubin and a prolonged INR — a signal of serious injury that needs immediate attention.³

Regardless of the numbers, certain symptoms mean you should see a clinician soon: jaundice (yellow skin or eyes), dark urine, pale stools, itching, persistent right-upper-abdominal pain, abdominal or leg swelling, unusual fatigue, or confusion. These do not diagnose anything on their own, but they help set the urgency.⁷

Recent research

According to recent publications indexed on PubMed, the framework for interpreting LFTs has been consolidated and modernized:

• A new name for fatty liver: MASLD. In 2023, a multisociety Delphi consensus replaced NAFLD with MASLD (metabolic dysfunction-associated steatotic liver disease), and NASH with MASH — reframing abnormal enzymes as a metabolic finding and removing stigmatizing language.⁸ The AASLD practice guidance was updated to match.⁹
• Updated management guidelines. The 2024 EASL-EASD-EASO guidelines confirm weight loss and physical activity as the foundation of MASLD care and endorse noninvasive fibrosis scores (such as FIB-4) to find who needs referral.¹⁰ A meta-analysis of randomized trials found combined diet and exercise significantly reduce ALT and AST — more than either alone.⁶
• A standardized, modern approach. A 2026 Gastroenterology review of abnormal liver biochemical tests reaffirms the discipline that runs through this page: classify the pattern (hepatocellular vs cholestatic), weigh risk factors, and use noninvasive tools before biopsy.¹¹
• The enzymes as metabolic markers. Large studies confirm that elevated transaminases and GGT track closely with metabolic syndrome and with fatty liver in type 2 diabetes — a reason to read LFTs alongside your triglycerides, A1c, and weight, not in isolation.¹²¹³

These findings concern interpretation and management; they do not authorize self-medication, and interpretation thresholds must be confirmed with your physician.

Get your liver panel interpreted by AI DiagMe

A liver panel is never read one number at a time: ALT, AST, ALP, GGT, bilirubin, and albumin are interpreted together. It is the pattern — hepatocellular or cholestatic — and your context (weight, alcohol, medications, blood sugar) that orients the result, not any single value.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

Bottom line

Liver function tests (LFTs) — the hepatic function panel — bundle the enzymes ALT, AST, ALP, and GGT with bilirubin and the synthetic proteins albumin and total protein. Remember two things above all. First, the enzymes measure damage, not function; true function shows in albumin and clotting, which fall when the liver is failing. Second, the panel is read by its pattern: ALT/AST up = hepatocellular injury, ALP (with GGT) up = cholestatic bile-flow problem. A mildly high enzyme is common and usually reflects fatty liver disease, which is often reversible; a markedly high result — or one with jaundice and abnormal clotting — needs prompt care. No value is a verdict, ranges vary by lab, and it is the whole set of markers, read in your context, that gives a result meaning — which is what AI DiagMe provides, alongside your physician.

Sources

Official sources and peer-reviewed publications (PubMed) used for this guide: