Alkaline phosphatase (ALP) is a group of related enzymes made primarily in the liver, bone, intestines, kidneys and by the placentas of pregnant women. The liver, bile ducts and bone are tissues with particularly high levels of ALP. A liver function test or blood chemistry panel may measure alkaline phosphate levels. Certain conditions may result in elevated or low alkaline phosphatase levels, but abnormal numbers are not diagnostic for any specific disease.
The Test
The alkaline phosphatase enzyme functions in the body by removing phosphorus, an acidic mineral, and creating an alkaline or basic pH. Because ALP works best in an alkaline environment, this enzyme becomes inactive when in enters the blood. When the liver, bile ducts or gallbladder system are malfunctioning or become blocked, the ALP enzyme is not excreted and released into the bloodstream. Therefore, the ALP blood test measures the amount of alkaline phosphatase released from the tissues into the blood and is a marker of the hepatobilary system function.
Why It Is Done
An alkaline phosphatase test is routine and may be ordered for a variety of reasons. Your doctor may wish to check for liver disease; this is especially true if you take medications known to cause liver damage. Jaundice, abdominal pain, vomiting and nausea are symptoms of liver disease. An ALP test is useful in diagnosing bone problems, which may be confirmed with an x-ray. These conditions include osteoporosis, rickets, bone tumors and Paget's disease, a condition affecting bone tissue.
Normal Levels
Alkaline phosphatase values may vary from one laboratory to another and with age or gender. The normal ALP range is 44 to 147 IU/L. This is a measure of the amount of enzyme ALP found in the blood. High levels of ALP enzymes may be seen in children during growth spurts and in pregnant women. Alkaline phosphatase may be altered by food and medication use, so you may be required to fast for 8 hours before the test is performed. This test is ordered routinely as part of a liver function test.
Low ALP
You may experience low ALP levels after a blood transfusion or heart bypass surgery. In addition, protein, vitamin B-6, zinc, folate and vitamin C deficiencies can cause a lower than normal alkaline phosphatase reading. Conditions that lead to malnutrition, such as celiac disease, low electrolytes and pernicious anemia, can cause a low ALP. Other factors that may result in decreased ALP levels include hypothyroidism, hypophosphatesia, excess vitamin D intake and insufficient parathyroid gland function.
Elevated ALP
While an elevated ALP level typically indicates liver damage or increased bone cell activity, there are other causes of a high alkaline phosphatase. Liver damage is evident if other liver function tests such as bilirubin, aspartate aminotransferase (AST), or alanine aminotransfere (ALT) are also high. If your serum calcium and phosphorus measurements are abnormal, elevated ALP is indicative of bone disorders. If your ALP levels are elevated, you may be suffering from biliary obstruction, hepatitis, hyperparathyroidism, leukemia, osteoblastic bone cancer, osteomalacia, gallstones, cirrhosis, heart failure or sepsis. Alkaline phosphatase is not a diagnostic for any specific disease; therefore, you will have to undergo further testing to determine the exact cause of your symptoms.
