How to Read Liver Function Tests

The short version#

A "liver function panel" answers two different questions, and most of it is not about function at all. First, is the liver injured, and if so, in what way? That is the job of ALT, AST, alkaline phosphatase, and bilirubin. Second, is the liver still doing its work? Only albumin, the clotting time (INR), and to a degree bilirubin speak to that. The single most practical step when the numbers look off is to decide whether the injury pattern is hepatocellular or cholestatic, because that one distinction reorganizes the entire list of possible causes. The 2017 American College of Gastroenterology (ACG) guideline on abnormal liver chemistries is built around exactly this logic.

Key points#

• ALT and AST rise when liver cells are damaged and leak their contents, so they flag injury, not lost function.
• Alkaline phosphatase (ALP) and bilirubin rise when bile flow is blocked or clearance fails.
• Albumin and the INR are the values that actually track how well the liver is working.
• The R ratio sorts the panel into hepatocellular (mostly ALT) or cholestatic (mostly ALP), which points the workup in a specific direction.
• A result printed as "normal" can still sit above the truly healthy range, so context matters.

Start with the pattern, not the individual numbers#

It is tempting to scan a panel value by value and react to whatever is flagged high. A more reliable habit is to ask which of two patterns you are looking at before you worry about any single result.

The ACG guideline gives a formula for this. The R ratio compares how far ALT has risen against how far ALP has risen, each measured relative to its own upper limit of normal:

• R equals (ALT / upper limit of normal for ALT) divided by (ALP / upper limit of normal for ALP).
• R above 5 is a hepatocellular pattern, driven by the transaminases.
• R below 2 is a cholestatic pattern, driven by ALP.
• R between 2 and 5 is mixed.

A hepatocellular pattern turns attention toward viral hepatitis, medications and supplements, alcohol, autoimmune hepatitis, and metabolic (fatty) liver disease. A cholestatic pattern turns attention toward gallstones, strictures, and bile-duct disease, and it usually earns an early ultrasound. Two panels with the same handful of abnormal numbers can lead down completely different roads depending on where the ratio lands, which is why the pattern comes first.

What the injury markers are telling you#

ALT and AST: leakage, not failure#

Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) sit inside liver cells. When those cells are stressed or dying, the enzymes spill into the bloodstream, so a rise is a leak from damaged tissue rather than a sign the organ has stopped working. Think of them as a smoke detector: they tell you something is burning, not how much of the house is still standing.

ALT is fairly specific to the liver. AST is not; it also lives in muscle, heart, and red blood cells. That difference matters in practice. A high AST with a normal ALT frequently points away from the liver altogether and toward muscle injury or breakdown of red cells. It is one reason a single flagged enzyme is rarely the whole story.

Alkaline phosphatase and bilirubin: the bile-flow markers#

Alkaline phosphatase (ALP) lines the small bile ducts and climbs when bile cannot drain freely. It is not liver-exclusive either; bone, placenta, and intestine also produce it. So an isolated ALP elevation is usually confirmed as coming from the liver, using a gamma-glutamyl transferase (GGT) level or a fractionated ALP, before anyone assumes a biliary problem.

Bilirubin is a clearance measure. The liver conjugates it and sends it out in bile, so how much builds up says something about that pathway. The ACG guideline highlights a useful split: a predominantly conjugated (direct) rise points toward liver-cell disease or biliary obstruction, while a predominantly unconjugated rise is often benign, as in Gilbert syndrome or ordinary red-cell turnover. That one fork can separate a result that needs imaging from one that needs only reassurance.

How high and how fast both count#

Beyond the pattern, the size and speed of a change carry information. The guideline grades transaminase elevations from borderline (under twice the upper limit) through mild, moderate, and severe, up to massive (above 10,000 IU/L). Numbers in the thousands that climb quickly narrow the field toward acute injury: reduced blood flow to the liver, acetaminophen toxicity, or acute viral hepatitis.

The relationship between the two transaminases adds another clue. An AST running more than twice the ALT is a classic, though not exclusive, signature of alcohol-related injury. Most other chronic liver-cell processes keep ALT at or above AST until scarring becomes advanced, at which point the pattern can shift again.

Why "normal" is a moving target#

A result printed inside the reference range is not automatically a clean bill of health. The ACG guideline argues that a genuinely healthy ALT sits near 29 to 33 IU/L for men and 19 to 25 IU/L for women, yet many laboratories report upper limits well above those figures. A value labeled normal can therefore still fall above the truly healthy band, which is easy to miss and worth a second look when metabolic risk factors are also present. Reference ranges are population conveniences, not biological verdicts.

The numbers that actually measure function#

If you want to know how the liver is performing rather than whether it is irritated, three values do the real work: albumin, the prothrombin time or INR, and bilirubin. The liver manufactures albumin and clotting factors, so a low albumin or a prolonged INR reflects lost synthetic capacity. A rising bilirubin reflects failing clearance. These are the markers that track severity in established disease, and they explain a scenario that surprises many patients: a person with advanced cirrhosis can have nearly normal transaminases and still be seriously ill. Injury enzymes and function markers answer different questions, and reading one as if it were the other is a common trap.

A pattern is a starting point, not a diagnosis#

Put together, these values sort a person into hepatocellular, cholestatic, or mixed injury, indicate how severe and how sudden the process looks, and separate irritation from true dysfunction. None of that names a cause on its own. The ACG guideline treats an abnormal panel as a prompt for a structured evaluation built around history, risk factors, the R ratio, and targeted follow-up testing, not as a verdict to act on directly.