AST in cats and dogs

What it shows

AST works inside cells, in the cell fluid and in the mitochondria. Skeletal muscle holds the most, followed by the liver and heart muscle; smaller amounts are found in kidney cells and the brain. Red blood cells contain enough AST that their breakdown can raise the result.

Because so many tissues contain it, AST is not a liver-specific test. It is most useful as a partner: with ALT it tells about liver cell injury, with CK about muscle. AST also leaves the blood at different speeds in different species: in cats its half-life was measured at roughly an hour and a half, while in dogs it stays considerably longer.

Higher than reference — what it may be a sign of

• Liver cell injury: inflammation, toxins, reactions to medicines, tumours. After sudden severe liver necrosis AST climbs over the first three days to 10–30 times above normal in dogs and up to 50 times in cats; if the damage resolves, it falls over 2–3 weeks (Merck).
• Muscle injury: trauma, long lying down, muscle inflammation (myositis), inherited muscle disease. AST rises here together with CK. CK falls faster once the injury settles, so AST can stay raised while CK is already back in range.
• Broken-down red blood cells: moderate or marked hemolysis, in the body or in the tube, can falsely raise AST. Mild hemolysis is not expected to push it above the range (eClinPath).
• Hormones and medicines: in hyperthyroid cats AST may rise, usually mildly, together with ALT and ALP. In dogs anticonvulsants can raise AST, probably through liver cell injury.

Lower than reference

A low AST on its own is not considered a sign of disease. One technical reason can lower the measured result: the enzyme needs a form of vitamin B6 to work, and when the body is short of it, activity reads lower — unless the lab adds this cofactor to the test, as the Cornell laboratory does.

Look at together with

• ALT — the more liver-focused aminotransferase. AST rising without ALT suggests a source outside the liver, especially muscle (Merck). In dogs with severe muscle injury AST rises proportionally more than ALT.
• CK (creatine kinase) — a muscle enzyme. Liver disease itself does not raise CK; CK and AST high together point to muscle.
• ALP, GGT, bilirubin — markers of bile flow; with AST and ALT they help a vet tell injury to liver cells from problems with bile drainage.

Units and conversion

AST is an enzyme, so labs report its activity, not its amount: 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 between them. Test conditions differ between analyzers, so ranges differ too, and trends are best followed within one lab.

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: eClinPath calls about 2–3 times mild, 4–5 times moderate and more than 10 times marked, while noting that clinicians draw these lines differently. A mildly raised AST in a hemolysed sample means less than a steady rise over several tests. The trend and the companion values ALT and CK say more than one number.

Cats and dogs

Sources

1. eClinPath (Cornell University) — AST
2. Center SA. Enzyme Activity in Hepatic Disease in Small Animals. Merck Veterinary Manual
3. eClinPath (Cornell University) — Creatine kinase
4. VCA Animal Hospitals — Testing for Liver Disease