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What is Alkaline Phosphatase?
Alkaline phosphatase is an enzyme commonly found in many human tissues, predominantly in the liver, bones, placenta, intestines and kidneys. Its function is to remove the phosphate group from proteins and other bio-molecules, and is called ‘alkaline’ phosphatase as it required a basic pH for optimal activity. Its function is vital for our body’s health. There are many isoforms of this enzyme, with intestinal, placental, liver, and bone ALP being the important ones.
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Why is the ALP Test Performed?
The ALP test may be suggested by your physician in any of the following conditions:
- Symptoms of liver diseases like pain or swelling in the belly, jaundice, and weakness. It may also be done if certain medications known to cause liver damage are being prescribed for you.
- To check for bone damage or to check for progress of bone healing following treatment
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How is the ALP Test Performed?
ALP levels are usually measured in blood, and the procedure is that of a routine blood test. No special preparation is necessary, though it is recommended to give blood after 8-10 hours of fasting, as food can cause transient increase in ALP levels. About 5 mL blood is sufficient for this test, though if multiple tests have been prescribed, the amount of blood drawn may be higher. Certain medications may interfere with test results, and hence it is recommended to inform your physician about every other medication being consumed before the test.
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How to Interpret Results?
Normal levels of ALP are in the range of 25-100 units per litre (U/L) for persons aged 15 or more, < 350 U/L for children aged 1-12, and < 500 U/L for boys 12-15. Children have higher ALP levels as there is a lot of bone growth taking place in them. A low level of ALP in children is indicative of malnutrition.
Elevated levels of ALP in adults should be diagnosed with care, as elevations can arise due to multiple causes, some indicative of disease, others perfectly normal.
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What causes Elevation in ALP levels?
Levels of ALP can increase due to both normal physiological conditions or in certain disease states.
In normal physiology:
- ALP levels are high in the first three months of life. It again increases during puberty and reaches normal levels only by the age of 18-20 years.
- Men generally have a higher ALP levels till around the age of 50. After menopause, the situation reverses and women tend to have higher ALP levels.
- Apart from puberty and post-menopause, pregnancy also causes high placental ALP levels in women.
- Higher intestinal ALP levels are known to occur in persons having blood group B and O.
- Use of tobacco or an increase in blood sugar causes transient increase in ALP levels.
In disease states:
- Liver and bile diseases: Elevated ALP levels are known to occur in conditions like liver cirrhosis, stone or tumor in the gall bladder, tumor in the liver, cholestasis, cholecystitis, cholangitis, hepatitis and fatty liver syndrome.
- Bone diseases: Diseases affecting the bones, like osteomalacia, fractures, Paget’s disease, Vitamin D deficiency or hyperparathyroidism can cause elevated ALP levels.
- Other syndromes: ALP levels get elevated during the healing phase of cardiac or splenic infarctions, or in cancers of the breast, colon, ovary, and cervix or prostrate.
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The bottom-line is that elevated ALP levels per se are not conclusive of disease; it can only be used as an index of liver or bone dysfunction when co-related with other clinical test results. Hence additional tests to confirm the underlying cause are necessary for accurate diagnosis. Consultation with an experienced physician is highly recommended.
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- Pagana KD, Pagana TJ: Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. , 2006
- Fischbach FT, Dunning MB: A Manual of Laboratory and Diagnostic Tests, 8th ed., 2009
- Lippincott Williams & Wilkins: Deciphering Diagnostic Tests, 2008