Alkaline phophatase (ALP) is widely distributed in tissue and intepretation of a serum elevation is difficult due to age and species differences and lack of information. It is part of a standard biochemistry profile in mammals but its significance in reptiles is unknown and its interpretation unknown. It may be a constituent of some laboratories' reptilian profiles.
ALP isoenzymes are found in a variety of tissues including intestine, liver, bone, placenta, kidney and leucocytes. Levels are high (approximately twice adult levels) in young growing animals due to increases in bone ALP. Unlike ALT, AST and GLDH, increased serum levels of ALP are due to increased synthesis of the enzyme.
The hepatic isoenzyme has a half life of 3 days in the dog and only 5-8 hours in the cat. Increases are usually of greater magnitude in dogs, but in the cat are more specific for hepatic and biliary disease. Increases in the hepatic isoenzyme are commonly associated with cholestatic disease which may be intra- or extra- hepatic. After an acute hepatic insult the activity of ALP increases within 2-4 days with a peak at 1-2 weeks. After recovery, ALP is often the last enzyme to return to the reference interval. Other ALP isoenzymes have a half life of only minutes and can effectively be ignored. Steroid-induced ALP can be determined in dogs but is of questionable value as a diagnostic aid. Steroid induction of ALP is recognised in dogs only.
Causes of increased ALP activity
- Cholestasis, either intrahepatic or posthepatic
- Cholangitis-cholangiohepatitis (cat)
- Hepatic lipidosis
- Other hepatopathies (see ALT)
- Glucocorticoid (dog), anticonvulsants, barbiturates
- Hyperthyroidism (cat)
- Diabetes mellitus
- Hyperadrenocorticism (dog)
- Healing bone fractures and bone disease
ALP is present in bone, liver, biliary tract and intestine. Elevated ALP is a less useful indicator of cholestasis because activity has a wide reference range making this less specific. In the horse a significant amount of ALP intestinal isoenzyme is released from the intestinal mucosa and this can be distinguished from total ALP by performing an intestinal ALP assay. However the half-life of intestinal AP in horses is short (8 minutes) and thus its use is probably minimal in many cases.
Common causes of increased ALP activity
- Chronic biliary obstructive liver pathology including chronic pyrrolizidine alkaloid toxicity (ragwort poisoning)
- Gastrointestinal disease
- Abnormalities of bone metabolism
- Systemic disease or inflammation as can be released from monocytes
ALP activity is usually determined in conjunction with other tests of hepatocellular damage or hepatic function, specifically AST, GGT, GLDH and bile acids. Intestinal ALP is useful where intestinal pathology is suspected or the ALP activity is increased without other biochemical evidence of hepatic disease. Interpretation should take into consideration the age related reference ranges for serum and intestinal ALP, particularly high results in immature horses and foals.
IAP (Intestinal alkaline phosphatase)
In horses, the intestinal isoenzyme of ALP may be released as a consequence of intestinal pathology. Increased IAP activity is associated with conditions such as intestinal parasitism, scouring and localised intestinal hypoxia following surgery. This test is of limited utility due to its short half life.