ALT in cats and dogs

What it shows

In dogs and cats ALT is found mostly in the liver, where its activity is thousands of times higher than in blood. Healthy cells keep it inside; when their membranes are damaged or cells die, the enzyme escapes into the bloodstream. That makes ALT a sensitive alarm for injury to liver cells.

ALT measures damage, not function: a liver can have a high ALT and still do its job, and a liver that has lost much of its working tissue can show only a modest ALT. It also does not stay in the blood for long in cats — its half-life is a few hours, against about two and a half days in dogs.

Higher than reference — what it may be a sign of

• Liver cell injury: inflammation, infection, toxins or reactions to medicines, tumours. After sudden severe injury ALT can rise more than 100 times above the upper limit within the first days.
• Bile flow problems (cholestasis): when bile cannot drain, nearby liver cells are damaged too.
• Medicines in dogs: corticosteroids (prednisolone and similar) and anticonvulsants such as phenobarbital can raise ALT. With steroids the rise is usually mild and fades after the drug is withdrawn.
• Hormonal conditions in cats: hyperthyroidism usually gives a mild rise in ALT and ALP (alkaline phosphatase) that settles once the thyroid is controlled.
• Not the liver: severe muscle injury, and in dogs even vigorous exercise. In cats a hemolysed sample can raise the result, because feline red cells contain ALT.

Lower than reference

A low ALT is rarely a problem on its own — a vet reads it next to the other liver values. It matters in one situation: when the liver has lost most of its working cells, as at the end stage of chronic disease, a falling ALT is not a sign of recovery. Some medicines (cephalosporins, cyclosporine) can lower the measured value by interfering with vitamin B6, which the enzyme needs to work.

Look at together with

• ALP and GGT (gamma-glutamyl transferase) — enzymes linked to bile flow. ALT high with ALP in range points more to cell injury; both high also raises the question of bile flow problems.
• AST (aspartate aminotransferase) — another cell enzyme, but not liver-specific: muscle damage raises it more than ALT.
• Bilirubin, albumin, glucose, urea, cholesterol — indirect clues to how well the liver works; ALT and bilirubin rising together point more firmly to the liver.
• Bile acids and ultrasound — bile acids test liver function more directly; imaging, and sometimes a biopsy, show what is wrong.

Units and conversion

ALT is an enzyme, so labs report its activity in units per litre: U/L or IU/L, on Russian forms Ед/л or МЕ/л. These are the same unit, and no conversion is needed. Some European forms print µkat/L: U/L × 0.0167 = µkat/L. Ranges still differ between analyzers, so compare trends within one lab where possible.

Reference range

The reference range is on your lab’s form. Vets often describe a rise by how many times it exceeds the upper limit: roughly 2–3 times is called mild, 4–5 times moderate, more than 10 times marked. The size of the rise does not always match how severe the disease is: a sudden toxic injury can push ALT very high and then settle, while a slow chronic process can keep it only modestly raised. The trend over repeated tests tells more, and a normal ALT does not rule out liver disease.

Cats and dogs

Sources

1. eClinPath (Cornell University) — ALT
2. Center SA. Enzyme Activity in Hepatic Disease in Small Animals. Merck Veterinary Manual
3. Center SA. Metabolic Diseases Affecting the Liver in Small Animals. Merck Veterinary Manual
4. VCA Animal Hospitals — Testing for Liver Disease
5. VCA Animal Hospitals — Chronic Hepatitis in Dogs