Alkaline phosphatase (ALP) in cats and dogs
ALP (alkaline phosphatase) is an enzyme bound to the membranes of liver and bile duct cells, bone-building cells and several other tissues. A high ALP makes a vet think about bile flow, but also about medicines, hormones and bone growth: in dogs and in young animals it has many sources besides the liver.
What it shows
Blood ALP is a mixture of forms (isoenzymes) made by different tissues. In healthy adults most of it comes from the liver. Bone-building cells add a bone fraction, which is larger in young, growing animals. Dogs alone have a third, corticosteroid-induced form: it is made in the liver when the body is exposed to more steroid hormones — from medicines, long stress or an overactive adrenal gland. Intestinal, kidney and placental forms vanish from the blood within minutes.
When bile cannot drain freely (cholestasis), liver cells make more ALP and release it into the blood, so the rise builds over days. In dogs liver ALP has a half-life of about 2–3 days, in cats only about 6 hours, and the feline liver contains less of the enzyme — so in cats rises are smaller, and any rise is considered significant.
Higher than reference — what it may be a sign of
- Bile flow problems inside or outside the liver: inflammation, a blocked bile duct, pancreatitis involving the bile duct, liver tumours. In dogs such disorders can push ALP to 100 times normal or more (Merck).
- Steroids in dogs: prednisone, prednisolone and similar medicines, and Cushing’s disease (hyperadrenocorticism), induce the steroid form; with Cushing’s the rise is moderate to marked, up to 100-fold (eClinPath). Chronic illness can add a smaller rise, up to 2–3 times.
- Anticonvulsants in dogs: phenobarbital, primidone and phenytoin usually raise ALP 2–6 times (Merck).
- Bone: growth in young animals, with values 2–10 times higher than in adults (eClinPath), healing fractures, bone tumours, overactive parathyroid glands.
- Cats: hyperthyroidism — Merck reports increased ALP in up to 75% of hyperthyroid cats, with a large share from bone. Fatty liver (hepatic lipidosis) causes the most dramatic ALP rises in cats (eClinPath).
- Breed: some Siberian husky puppies have a transient familial rise, and Scottish terriers with adrenal dysfunction can have very high ALP.
Lower than reference
A low ALP is rarely discussed as a problem. In cats ALP falls clearly once hyperthyroidism is brought under control. In young animals the value drops as growth slows and reaches the adult range by about 15 months (eClinPath). A technical point: ALP cannot be measured in plasma from an EDTA tube, because EDTA binds the magnesium and zinc the reaction needs.
Look at together with
- GGT — the second bile flow enzyme. In dogs ALP is more sensitive and GGT more specific. In cats GGT may rise more than ALP in inflammatory liver disease, while fatty liver usually raises ALP much more than GGT (Merck).
- ALT and AST — injury to liver cells. Corticosteroids and phenobarbital can raise ALT too, so the medicine list matters for both (VCA).
- Bilirubin and bile acids — show whether bile really is not draining and how well the liver works.
Units and conversion
ALP is an enzyme, and 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 nothing needs converting. Activity depends on the analyzer, so each result is compared with its own lab’s range.
Reference range
The reference range is on your lab’s form. Adult ranges do not suit a growing puppy or kitten, whose ALP can be several times higher. Merck notes that in liver disease dogs usually show rises above 4–5 times normal and cats only 2–3 times, so the same multiple means different things in the two species. Moderate hemolysis in the sample can raise the result; lipemia and jaundice do not interfere significantly (eClinPath).
Cats and dogs
Cats have little ALP in the liver and clear it within hours, so even a mild rise is taken seriously. The feline liver is relatively insensitive to glucocorticoids, and ALP is rarely affected by them; in a study phenobarbital did not raise feline ALP (Merck). Typical feline reasons are hyperthyroidism, fatty liver, inflammation of the bile ducts and a blocked bile duct; comparing ALP with GGT helps to tell them apart.
In dogs ALP is more sensitive but less specific than GGT: steroid medicines, Cushing’s disease, anticonvulsants, bone growth and breed can all raise it without liver disease. The steroid-induced rise appears within 5–7 days of starting prednisone or prednisolone (Merck). Separating the forms is possible, but Merck notes it adds little, because the steroid form eventually predominates even in dogs with liver disease.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.