Enzyme leakage, bile flow, and liver function — three different questions.
The liver panel answers three separate questions, and keeping them apart is the key to reading it. Enzyme leakage — ALT and AST — tells you liver cells are being injured. Bile flow — ALP, GGT and bilirubin — tells you whether the bile system is obstructed or inflamed. And liver function — albumin, total protein and bile acids — tells you how well the liver is actually doing its job of making proteins and clearing waste.
A crucial point runs through all of it: the enzymes measure damage or leakage, not function. A pet can have dramatically raised enzymes with a liver that is working fine, or a quietly failing liver with near-normal enzymes. That is why no single number is read alone — the pattern across leakage, bile flow and function is what localises the problem and judges its severity.
Species matters here more than almost anywhere else: in dogs, ALP is very sensitive but non-specific (steroids, bone and hormones lift it), whereas in cats ALP rises reluctantly, so almost any elevation in a cat is meaningful. Tracked over time, this panel distinguishes a transient insult that settles from a chronic process that advances.
ALT is an enzyme that lives inside liver cells and leaks into the blood when those cells are stressed or damaged, so a raised ALT points towards the liver. A single mild elevation is common and often passes; a sustained or rising level across repeat tests is the more meaningful signal, and the height of the number does not reliably tell you the cause.
ALP is an enzyme that rises with liver and bile-system changes, but in dogs it also climbs with steroids, some medicines, bone growth and hormonal conditions — so it's quite non-specific and is read together with ALT and GGT rather than alone. In cats, by contrast, almost any ALP rise is meaningful.
GGT is an enzyme that points more specifically at the bile system than ALP, so a rise here helps separate liver-and-bile changes from the bone and steroid effects that also lift ALP. It's usually read as a partner to ALP and ALT rather than on its own.
Bilirubin is the pigment left over when old red blood cells are broken down; the liver processes it and sends it out in bile. A clear rise causes the yellow tinge of jaundice and points to one of three things — accelerated red-cell breakdown, liver disease, or blocked bile flow — and always warrants prompt veterinary attention.
Albumin is the main protein made by the liver; it holds water inside blood vessels and carries many substances around the body. A low albumin is the meaningful finding — it can reflect reduced liver production, loss through the gut or kidneys, or inflammation — which is why it helps tie several body systems together.
Total protein is albumin plus all the globulins (mostly antibodies) added together. Because two very different proteins are combined, the total is only the headline — it's the split between albumin and globulin that tells the story, so both unusually high and low values are worth understanding.
AST is an enzyme found in both liver and muscle, so it's read alongside ALT to judge whether a rise is coming from the liver or from muscle. When AST and ALT climb together it points to the liver; when AST rises with muscle markers, it points to muscle.
Bile acids test how well the liver is actually working — its function — rather than whether liver cells are leaking enzymes. That means they can flag reduced liver function or a portosystemic shunt even when ALT and ALP look normal, which makes them a powerful second-line liver test.
GLDH is a liver enzyme held inside the mitochondria, deep within liver cells — so it only escapes when injury is more substantial than the surface leakage that raises ALT. Common on UK and European panels, it is a more liver-specific marker and a useful complement to ALT.
Ammonia is a gut-derived toxin that a healthy liver converts to urea before it reaches the brain. A raised blood ammonia means that detoxification has failed — pointing to a portosystemic shunt or significant liver failure — and it correlates with the neurological signs of hepatic encephalopathy.