ALT (alanine aminotransferase) is a liver enzyme measured in routine cat blood work. When your cat’s liver cells are healthy, ALT stays inside them. When those cells are damaged or inflamed, ALT leaks into the bloodstream, and the blood test picks it up. A high ALT result tells your vet that something is irritating or injuring your cat’s liver cells, though it doesn’t pinpoint the exact cause on its own.
What ALT Actually Measures
ALT is an enzyme concentrated in the cytoplasm of liver cells (hepatocytes). Its normal job is helping metabolize amino acids, specifically converting alanine into a molecule the body can use for energy. Of all the enzymes vets can measure, ALT is the most liver-specific in cats. It exists in small amounts in muscle tissue too, but the vast majority sits inside liver cells.
Healthy liver cells hold onto their ALT. When cells become damaged, whether from infection, inflammation, toxins, or reduced blood flow, their membranes break down and ALT spills into the surrounding fluid, then into the bloodstream. Once released, ALT clears slowly from the blood, with a half-life of roughly 42 hours. That means a single episode of liver injury can keep ALT elevated for several days after the damage has stopped.
What a Normal Result Looks Like
There’s no single universal number for “normal” ALT in cats because reference ranges vary between laboratories and the specific equipment they use. Your vet’s lab report will list its own reference range alongside your cat’s result, making it easy to see whether the value falls inside or outside the expected window. If the number is within range, it generally means there’s no significant liver cell damage happening at that moment.
What Elevated ALT Means
Vets classify elevations by how far above the reference range they go. A result less than three times the upper limit is considered mild. Three to nine times the upper limit is moderate. Anything above ten times the upper limit is marked, suggesting substantial liver cell injury. The higher the number, the more liver cells are leaking enzyme, though even a very high ALT doesn’t always mean permanent damage. Liver cells can regenerate, and temporary insults sometimes cause dramatic spikes that resolve once the underlying problem is treated.
A mild bump in ALT can show up for relatively minor reasons, including a recent illness that temporarily stressed the liver or even vigorous muscle activity. Moderate to marked elevations point more strongly toward active liver disease that needs investigation.
Common Causes of High ALT in Cats
Hepatic lipidosis (fatty liver disease) is one of the most common liver conditions in cats, often triggered when a cat stops eating for several days. It causes dramatic weight loss, sometimes more than 25% of body weight, along with lethargy, vomiting, and jaundice. Cholangiohepatitis, an inflammation of the liver and bile ducts, is another frequent culprit. The acute form tends to hit suddenly, with fever, abdominal pain, vomiting, and a visibly yellow tinge to the gums or whites of the eyes.
Other causes include hyperthyroidism (which increases blood flow through the liver and can stress liver cells), infections, reduced oxygen delivery from anemia or heart disease, and certain viral infections. Bile duct obstruction, liver flukes, and toxoplasmosis can also drive ALT up, though these are less common.
Several medications used in cats are known to elevate ALT. Methimazole, widely prescribed for hyperthyroidism, occasionally causes immune-related liver inflammation. Benzodiazepines like diazepam, oxazepam, and alprazolam can trigger severe, sometimes fatal, liver failure in cats. The antifungal griseofulvin has also been linked to increased ALT. If your cat takes any of these medications and shows elevated ALT, your vet may need to reassess the treatment plan.
Signs You Might Notice at Home
Mildly elevated ALT often produces no visible symptoms at all, which is why routine blood work catches problems you wouldn’t otherwise know about. As liver involvement worsens, you may notice your cat eating less or refusing food entirely, losing weight, becoming unusually tired, or vomiting. Jaundice, a yellowish discoloration visible in the gums, inner ears, or whites of the eyes, is a hallmark sign of more significant liver trouble. Some cats drink and urinate more than usual, develop a swollen abdomen from fluid buildup, or show changes in stool color.
How ALT Differs From Other Liver Values
Your cat’s blood panel likely includes several liver-related enzymes, and each one tells a different part of the story. ALT and AST are “leakage enzymes,” released when liver cells are physically damaged. ALP and GGT, on the other hand, tend to rise when bile flow is obstructed or the bile ducts are under pressure. Vets look at the pattern across all of these values to narrow down what’s going on.
A cat with high ALT and AST but normal ALP and GGT likely has direct liver cell damage. A cat with high ALP and GGT but normal ALT probably has a bile duct or gallbladder issue instead. When everything is elevated, it suggests a more widespread process affecting both the liver tissue and the biliary system. One complication worth knowing: AST also lives in muscle cells, so a high AST with a normal ALT might reflect muscle injury rather than liver disease. ALT’s tighter link to the liver makes it the more reliable marker of the two.
What Happens After a High ALT Result
A single elevated ALT reading doesn’t always lead to an immediate deep dive. If the elevation is mild and your cat seems healthy, your vet may recommend rechecking the value in a few weeks to see if it resolves on its own. Persistent or significant elevations call for further investigation.
Bile acid testing is a common next step. Your cat fasts for 12 hours, a blood sample is drawn, then another sample is taken two hours after eating. Comparing the two values helps confirm whether the liver is functioning properly, not just whether cells are damaged. A simpler alternative is a urine bile acid test, which only requires a single urine sample. A ratio above 4.4 on this test is evidence of liver disease.
Imaging usually follows. Abdominal X-rays can reveal changes in liver size or shape, while ultrasound gives a more detailed look at liver texture, the gallbladder, and bile ducts. If imaging raises concerns but doesn’t provide a clear answer, your vet may recommend a liver biopsy, taken either with an ultrasound-guided needle, through a small laparoscopic incision, or during open surgery. Biopsy is the most definitive way to identify the specific type of liver disease.
Is Low ALT a Problem?
Low ALT values in cats are not a recognized clinical concern. A result at the low end of the reference range, or even slightly below it, simply means very little ALT is leaking from liver cells, which is the normal state of affairs. Vets focus on elevations, not dips.