High ALT in dogs: liver damage, not liver failure

What this marker measures

ALT, also written ALAT or SGPT, is an enzyme held inside liver cells. As long as those cells are intact, the enzyme stays in them. When a cell is damaged, or its membrane becomes leaky, the enzyme passes into the blood, where the laboratory measures it. So the result measures a leak.

This is the distinction almost nobody explains: ALT is a marker of damage, not a marker of function. It says that cells have suffered recently. It says nothing about what the liver can still do, which is to build proteins, handle sugars, neutralise waste and produce bile. Two situations show this clearly. A dog can have a very high ALT after a sudden insult and a liver that is still working properly. The other way round, an old, fibrosed liver, where few cells are left to leak, can give a barely disturbed ALT while function is clearly impaired.

What vets look into when the value sits outside the range

By rough frequency, not by severity.

• The liver reacting to a problem somewhere else. This is the most common cause. The liver receives all the blood coming from the intestine and reacts to pancreatitis, to digestive disease, to infection, to a dental focus, to heart failure, to endocrine disease.
• Current or recent medicines. Phenobarbital, corticosteroids, non-steroidal anti-inflammatories, certain antifungals and antiparasitics, and dietary supplements.
• Toxic substances swallowed. Xylitol, mushrooms, blue-green algae in standing water, certain plants, human medicines including paracetamol, mouldy food.
• Chronic liver disease in dogs, including chronic hepatitis and copper storage disease, more frequent in some breeds such as the Bedlington terrier, the Labrador, the Dobermann, the English cocker spaniel and the West Highland white terrier.
• Endocrine and metabolic disease: hyperadrenocorticism, diabetes mellitus, lipid disorders.
• A muscle origin. Significant muscle damage also pushes ALT up; CK is what picks that up.
• Vascular anomalies of the liver, shunts in particular, often with a moderately raised ALT in a young dog.
• Liver masses, benign or otherwise.

Context counts as much as the list. The dog's age, its breed, what it might have swallowed, the products given over the past few weeks and the existence of an earlier panel often point the way faster than the figure itself. A two-year-old dog and a twelve-year-old dog with the same ALT do not lead to the same reasoning.

What this value does not tell you on its own

An ALT on its own cannot name a cause, date the problem or grade its severity. What is read alongside it:

• Albumin, urea, glucose, cholesterol, bilirubin. These are the markers that come closest to function, and they are the ones that worry when they drift.
• Bile acids before and after a meal, or blood ammonia. The real function tests, requested when the question genuinely arises.
• ALP and GGT, which point towards the bile ducts rather than the cells.
• CK, to rule out a muscle origin.
• Full blood count and platelets, plus clotting times when a procedure is being considered.
• Abdominal ultrasound, which shows the structure of the liver, the gall bladder, the vessels, the pancreas.

The enzyme clears from the blood within a few days once the insult stops. That means one measurement mostly reflects what has happened recently, and that a figure staying high week after week describes something still active. Kinetics therefore count for more than the value. Repeating the test a few weeks later says whether the process is settling, holding or progressing. And the range printed on your report depends on the laboratory and the analyser used, so always compare results obtained by the same method.

Questions to ask your vet

1. Does this ALT reflect damage to the liver itself, or a reaction to something else you are looking for elsewhere?
2. Which liver function parameters do we have, and which are missing?
3. Among everything my dog is given, medicines and supplements included, could anything explain this value?
4. How soon should we recheck, and what change would make you think differently?
5. Would an ultrasound scan or a more detailed look at the liver give information the blood test cannot?

Does a high ALT mean my dog's liver is failing?

No, those are two separate questions. ALT says liver cells have been damaged; the liver's working capacity is judged on albumin, urea, cholesterol, glucose, bilirubin and function tests such as bile acids. A badly damaged liver can still function, and a liver that has been struggling for years can show only a modestly raised ALT.

The higher the number, the worse it is?

Not in any proportional way. A very high value during a sudden insult can fall back within days, whereas a moderately raised value that persists for months is more concerning. It is repeating the test that gives the figure its meaning.

My dog has swallowed something suspect and the ALT is high, what should I do?

Contact a vet or an animal poisons helpline straight away, even if the dog seems normal. Some poisonings, xylitol, certain mushrooms and certain plants among them, cause liver damage that shows up with a delay. Take the packaging, or a photo of what was eaten, with you.

Should the test be repeated, and when?

A recheck is nearly always worth doing, because the trend guides better than a single figure. The vet chooses the interval according to the dog's condition, the rest of the panel and the possibilities they want to rule out.