ALT

A raised ALT tells you liver cells are leaking; it does not, on its own, tell you why. The list of possibilities is broad — reactive change from an unrelated illness elsewhere in the body, inflammation (hepatitis), fatty change, toxins or medicines, reduced blood flow, endocrine disease such as Cushing's, and, less often, primary liver disease or tumours (Webster et al., 2019). Crucially, the magnitude of the rise does not reliably indicate the cause or the severity: a big spike can follow a self-limiting insult, while a modest, persistent elevation can accompany a smouldering chronic process.

Two features make a high ALT more significant. The first is persistence — a value that stays up, or climbs, across repeat samples several weeks apart is more concerning than a one-off. The second is company — an ALT rise accompanied by raised bilirubin, falling albumin, or clinical signs (poor appetite, vomiting, jaundice, drinking more) moves the liver up the priority list and usually prompts imaging and further testing.

ALT that sits within the reference range is reassuring about active hepatocyte injury, and a low ALT is not something to worry about. As with all liver markers, "normal ALT" does not completely exclude liver disease — some chronic and end-stage conditions run with normal or even low enzyme levels because there are fewer cells left to leak — so it is always interpreted alongside the rest of the panel and the clinical picture.

The liver has a large functional reserve and a real capacity to repair, so many liver conditions are slow and quiet in their early stages. This is where serial ALT measurements earn their keep. A single value is a snapshot; a trend line reveals whether a process is settling, stable, or advancing. An ALT that drifts steadily upward over months — even within or just above the range — can be the first objective sign of a chronic hepatopathy that has not yet produced symptoms, and it is often what prompts the investigations (imaging, bile acids, sometimes biopsy) that reach a diagnosis while treatment is still most effective. Conversely, watching an elevated ALT fall back towards normal is a reassuring, measurable sign that a liver insult is resolving.

ALT is a useful liver marker in both species, but the emphasis differs. In dogs ALT is joined by ALP, which is very sensitive but famously non-specific — canine ALP rises with steroids, some medicines, bone growth and endocrine disease as readily as with liver problems — so ALT helps anchor the interpretation. In cats ALP has a much shorter half-life and rises less readily, which paradoxically makes any elevation of ALP or ALT in a cat more clinically meaningful, and raises the index of suspicion for conditions such as hepatic lipidosis and cholangitis. GGT helps refine the picture in both species. The direction of these markers together, not any single one, is what localises the problem.

ALT is measured from a routine blood chemistry panel and reported in U/L. No special preparation is needed, though a fasting sample reduces the chance of lipaemia (fat in the sample) interfering with the reading. Reference intervals differ between laboratories and analysers, so read the result against the range printed on your own report and, where possible, follow the trend on the same lab's assay rather than comparing an absolute number between labs.

A mildly raised ALT on an otherwise well pet is frequently rechecked in a few weeks before anything more is done — many resolve. A markedly raised, persistent, or rising ALT, or one keeping company with other abnormal liver markers or clinical signs, warrants a fuller work-up with your veterinary surgeon: bile acids to assess function, imaging to look at the liver's structure, and a review of any medicines. As ever, the number opens the question; the trend and the surrounding tests answer it.