Alkaline phosphatase, or ALP, is an enzyme that removes phosphate groups from other biological molecules. It is present in all of your cells, but it is found in particularly high concentrations in your liver, bones, kidneys and intestines. During pregnancy, the placenta also produces large amounts of ALP. Different tissues produce slightly different types of ALP called "isoenzymes." Injury, inflammation or increased metabolic activity in any of the organs that produce ALP usually prompts an increase in the levels of this enzyme in your bloodstream.
Isoenzymes
If a blood test reveals an elevation in your ALP, your doctor may order additional examinations to see what triggered the elevation. The two most common sources of increased ALP levels are liver and bone, which produce different isoenzymes of ALP. According to "The Merck Manual of Diagnosis and Therapy," bone ALP can be differentiated from liver ALP by performing a heat fractionation test. However, this test is technically difficult, so other tests to check for liver problems are usually performed in lieu of heat fractionation. If these evaluations indicate that your liver is damaged, it is assumed that the ALP arose from your liver.
Liver
Your liver, particularly its bile ducts, is a rich source of ALP. When the cells in your liver are injured by infection, toxins or cancer, they release ALP and a variety of other enzymes into your blood. Obstruction of your bile ducts, which can occur with gallstones or tumors, often triggers striking elevations in your ALP levels. Additional blood tests, such as gamma-glutamyl transpeptidase or 5´-nucleotidase levels, help to determine whether ALP comes from your liver or from other tissues.
Bone
Any condition that increases the rate of bone metabolism or turnover will increase ALP blood levels. Rapid growth, healing fractures, Paget's disease, vitamin D deficiency, bone infections, hyperparathyroidism and bone cancers all cause the release of ALP into your bloodstream, where it can be measured. Sometimes the process responsible for a high ALP is obvious, so no further testing is done. However, because ALP is generated by so many tissues, an abnormally high level may mandate an exhaustive search for the cause when no obvious reason is present.
Considerations
A normal ALP level varies from 20 to 140 IU/L, but normal ranges may vary among different laboratories. Although liver and bone are the most common sources of high ALP levels, other tissues and organs can cause elevations that may trigger an exhaustive and expensive medical evaluation. For example, your kidneys can release ALP into your bloodstream in response to injury, infection or cancer, but other signs of kidney damage may be absent. Occasionally, the cause of an abnormal ALP level eludes diagnosis.
References
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Testing for Hepatic and Biliary Disorders; Mark H. Beers, M.D., Editor-in-Chief; 2006
- MedlinePlus: ALP Isoenzyme Test
- "Cases Journal"; Stauffer's Syndrome as a Prominent Manifestation of Renal Cancer: A Case Report; G.P. Kranidiotis, et al.; January 2009
