Alanine aminotransferase

Also known as ALT, SGPT, serum glutamic-pyruvic transaminase, alanine transaminase

enzyme activity · no single molecular formula

LOINC 1742-6 code verified serum U/L

unverified These explanations are written to be easy to read and have not yet been checked against a source. The ranges and measurement notes on this page show theirs.

definition unverified

An enzyme that lives inside liver cells. When those cells are irritated or damaged, some of it leaks into the blood, so a blood test can pick it up.

Cytosolic transaminase concentrated in hepatocytes, catalysing transfer of an amino group from alanine to alpha-ketoglutarate. Released into plasma when hepatocyte membrane integrity is lost.

function unverified

Inside the liver it helps recycle protein building blocks and helps make new sugar when you have not eaten.

Participates in the alanine cycle, shuttling nitrogen from muscle to liver for urea synthesis and providing a gluconeogenic substrate.

why measured unverified

Of the common enzymes, this one comes most purely from the liver. That makes it the usual first sign that something is bothering it.

No clinical version written yet.

if high unverified

Suggests liver cells are being irritated. Very common causes are fatty liver, alcohol, some medicines, and viral infections. Mild rises are frequent and often settle on their own, but a lasting rise is worth investigating.

Indicates hepatocellular injury. Common causes span fatty liver disease, alcohol, viral hepatitis, and drug effect including paracetamol and statins.

if low unverified

A low result is not usually meaningful and is rarely acted on.

No clinical version written yet.

caveats unverified

A single mildly raised result is common and often means little. What matters is whether it stays up when repeated a few weeks later.

No clinical version written yet.

What this substance is

ALT is reported in U/L -- units of catalytic activity, not concentration. The assay measures how fast the enzyme converts a substrate under defined conditions, not how much enzyme protein is present. Two samples with identical ALT protein can report different ALT if one is partly denatured. Its chemical identity is therefore the reaction it catalyses, not a drawable structure.

What it catalyses

EC 2.6.1.2

L-alanine + 2-oxoglutarate = pyruvate + L-glutamate

Requires pyridoxal 5'-phosphate.

UniProt P24298

Each molecule is drawn from ChEBI's curated 2D drawing, accepted only after its InChIKey, formula and charge matched the ChEBI release. Atom diagrams are checked against the NIST Atomic Spectra Database.

3D model

Predicted structure of one ALT chain (AlphaFold). The working enzyme is a pair of chains. Predicted by AlphaFold (model v6, average confidence 95 of 100); its sequence matches UniProt exactly. Colour is AlphaFold's own confidence for each part: dark blue very high (above 90), light blue confident (70-90), yellow low (50-70), orange very low (below 50). Drag to rotate, scroll to zoom.

Reference ranges

Checking your own result against this range needs the running application; this is a static copy. Nothing you type anywhere in Empeiria is ever sent to a server.

This open edition shows a range only where a public-domain or openly licensed source states one, and none does for this marker yet. Compare your result with the range printed on your own lab report.

How it is measured

In practice

A health worker takes a small blood sample from a vein in your arm with a needle. It usually takes less than five minutes. ALT is usually checked along with other blood tests, so you may need to skip food and drink for several hours first.

Serum, or EDTA- or heparin-anticoagulated plasma, from venipuncture; separated serum or plasma should be removed from cells within one hour. Usually ordered with other tests, for which patients usually fast for several hours. At therapeutic concentrations, calcium dobesilate and isoniazid can falsely lower and furosemide falsely raise ALT results.

Before the test

Fast (no food or drink) for several hours if your provider asks, since ALT is usually ordered with other blood tests.
Tell your provider about everything you take, but do not stop any medicine unless your provider tells you to.

Where: laboratory

Sources: ALT Blood Test, MedlinePlus (U.S. National Library of Medicine); NHANES 2017-2018 Laboratory Procedure Manual, Alanine Amino Transferase (ALT) (CDC National Center for Health Statistics) cited · not yet checked by a person

In the lab

kinetic enzyme

IFCC kinetic assay with P5P

The lab gives the enzyme something to work on and watches how fast it works. The speed is the result. This is why the temperature and the recipe have to be standard: change either and the same blood gives a different number.

ALT in the sample transaminates alanine; the pyruvate produced is reduced by lactate dehydrogenase with consumption of NADH, and the rate of NADH disappearance is proportional to enzyme activity.

Read by: Rate of absorbance decrease at 340 nm
Standardised to: IFCC reference procedure at 37 °C

What can throw it off

Haemolysis

Why labs disagree: Assays with and without added pyridoxal 5'-phosphate give different results. The enzyme needs this vitamin B6 cofactor to be active, and some ALT in blood circulates without it, so samples low in vitamin B6 can read falsely low on an assay that does not add it. ALT also varies with sex, age and body mass, and many studies recommend an upper limit of only about 29-33 U/L for men.

Sources: Low Alanine Aminotransferase Levels in Alcohol Consuming Male Subjects, Carriers of the Common ALDH2 Deficient Variant, ALDH2*2 (2025), PMC open access; NHANES 2017-2018 Laboratory Procedure Manual: Alanine Aminotransferase (ALT), CDC/NCHS cited · not yet checked by a person

What it reports on

hepatocyte released on injury
ALT is found mainly in the liver, and damaged liver cells release it into the blood.
liver released on injury
ALT is mostly found in the liver, so a high ALT level points to liver injury.

Associated symptoms

When high

high Feeling tired all the time · non specific · common
Reported in many forms of liver irritation, but far too common to point at the liver on its own.
high Pain under the right ribs · suggestive · occasional
Stretching of the liver capsule as the organ swells.
high Yellow skin or eyes · suggestive · rare
Appears only when injury is severe enough to impair bilirubin handling.
high Loss of appetite · non specific · occasional

Conditions this marker features in

When high

high A cluster of risks that travel together · supportive
Raised liver enzymes and fatty liver are among the effects linked to metabolic syndrome.
high Blocked or slowed bile flow · supportive
ALT can also rise in cholestasis, but less than ALP. In drug-induced cholestatic injury, the ALT-to-ALP ratio is below 2.
high Fatty liver · supportive
A raised ALT can make a doctor suspect it, but it is not part of the definition, and liver scarring can be present with normal enzymes.
high Iron overload · supportive
Raised liver enzymes can be a clue that leads to testing, and raised ALT is one reason to assess the liver for damage.
high Liver damage from alcohol · supportive
Often raised, but usually less than AST. In alcohol-related hepatitis the AST:ALT ratio is above 1.5.

Either direction

either A harmless inherited cause of high bilirubin · excludes
Normal in Gilbert syndrome. A raised ALT points away from it and towards liver injury.

Ordered as part of

cmp · hepatic