Alkaline phosphatase, or ALP, is the collective term for a series of closely related enzymes synthesized in various parts of the body, primarily in the liver and in bone. Elevated serum levels of ALP thus often signify a disorder in the liver or in bony tissue. ALP levels considered normal in children range considerably higher than normal adult levels, because children's bones are growing, but a high ALP in this age group may still herald serious problems.
Background
ALP is responsible for removing phosphate groups from a variety of molecules, but its roles in the body are not completely understood; as the name implies, it functions best in an alkaline environment--that is, one with a pH above 7--and so while its activity in laboratory settings is easily studied at a pH of 10 or higher, this does not correspond to physiological conditions. According to research published in 1997 in the journal "Laboratory Investigation," ALP has been proposed to play a role in detoxifying harmful substances produced by bacteria.
Identification
A normal adult level of ALP lies within the range of 30 to 126 units per liter, or U/L. In children, however, anything in the range of 30 to 300 U/L is considered normal. These values vary slightly depending on the lab at which the test is conducted. Pregnant women, especially in the third trimester, also have high ALP levels because the growing placenta manufactures ALP.
Significance
A grossly elevated ALP in children may indicate a number of pathological conditions. It can result from liver congestion secondary to viral infections such as hepatitis, mononucleosis or CMV; parasitic infections; or a liver malignancy. Alternatively, increased ALP levels caused by heightened metabolic activity in bone can result from hyperthyroidism, hyperparathyroidism, vitamin D deficiency, bone cancers such osteogenic sarcoma, and osteomalacia. Other diseases and anomalies unrelated to either the liver or bone can also raise ALP, but these are rare in young people.
Effects
Elevated ALP itself is not known to cause organ or tissue damage. It is strictly a marker for other forms of pathology or normal biological activity. This is typical of enzyme assays in general; elevated levels of various enzymes manufactured in the liver may be indicative of intrinsic liver disease, but the increased enzyme levels themselves do not cause harm.
Considerations
Lab tests done in conjunction with ALP assays help point the way toward the source of high ALP levels. If bilirubin, aspartate aminotransferase, or alanine aminotransfere levels are also elevated, the excess ALP is most likely originating in the liver, whereas abnormal levels of the minerals calcium and phosphate suggest that the ALP is coming from bone.
