Alkaline phosphatase is an enzyme found in all of your tissues. It is produced in relatively large amounts by your liver, bones, intestine and kidneys, and by the placenta during pregnancy. Different tissues synthesize slightly different forms -- isozymes -- of this enzyme. Alkaline phosphatase is instrumental in the absorption and metabolism of carbohydrates and lipids, the synthesis of DNA and RNA and the mineralization of bone. Heightened metabolism or disease in tissues that contain alkaline phosphatase can increase its presence in your bloodstream.
Liver
Your liver is one of the most metabolically active organs in your body. It produces large quantities of enzymes, including alkaline phosphatase, to accomplish its many tasks. When your liver is injured by infection, toxins or cancer, it releases alkaline phosphatase into your bloodstream. According to "The Merck Manual of Diagnosis and Therapy," the bile ducts within your liver are particularly rich in alkaline phosphatase, so your blood levels can rise markedly in response to obstruction or inflammation of these structures.
Bone
Bone growth, remodeling and disease are common causes of high alkaline phosphatase levels. Actively growing children typically exhibit higher levels -- up to 6 times higher in infants -- than adults. Paget's disease, a fairly common bone disease that occurs in elderly persons, can cause elevated alkaline phosphatase levels, as can malignancies involving bone, such as breast or prostate cancer. Fractures will raise alkaline phosphatase for several weeks until healing and remodeling have occurred. Other conditions that increase bone "turnover" and elevate alkaline phosphatase include bone infections, hyperparathyroidism and vitamin D deficiency.
Kidney
Acute kidney injury is a relatively uncommon cause of alkaline phosphatase elevation. Shock due to excessive hemorrhage or trauma, infections and toxic insults are the most common causes of acute kidney injury, according to the 2010 issue of "Blood Purification." Renal cell carcinoma, a cancer of the kidney, can increase alkaline phosphatase by damaging the kidney itself, growing to the extent that it obstructs your bile ducts or spreading to your bones.
Sorting It Out
Alkaline phosphatase isozymes can be differentiated to some extent. For example, the bone isozyme can be distinguished from the liver isozyme with a test called heat fractionation. However, this test is technically difficult, so tissue-specific enzymes, such as liver-derived gamma-glutamyl transpeptidase, are usually measured to determine the source of an elevated alkaline phosphatase level. Even when your physician has ordered all available blood tests, sometimes the precise cause of a high alkaline phosphatase level cannot be determined. A more extensive search for an occult cause may need to be undertaken.
Considerations
Because alkaline phosphatase is produced by a variety of tissues, the cause of an elevated blood level may not be immediately apparent. When there is an obvious cause, such as pregnancy, active bone growth or a fractured bone, no further investigation is usually warranted. However, a persistently elevated alkaline phosphatase level may require extensive evaluation to rule out a serious hidden problem, such as lymphoma or lung cancer.
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
- "Taber's Cyclopedic Medical Dictionary, 18th Edition"; Clayton L. Thomas, M.D., Editor; 1997
- "Blood Purification"; Serum and Urinary Biomarkers of Acute Kidney Injury; B. Lisowska-Myjak; 2010
- "Cases Journal"; Stauffer's Syndrome as a Prominent Manifestation of Renal Cancer: A Case Report; G.P. Kranidiotis, et al.; January 2009
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