Alkaline Phosphatase (ALP)
A blood sample drawn from a vein in your arm
Fasting overnight may be recommended (drinking only water permitted); follow any instructions from your healthcare practitioner and/or the lab performing the test.
Alkaline phosphatase (ALP) is an enzyme found in several tissues throughout the body. The ALP in blood samples of healthy adults comes mainly from the liver, with most of the rest coming from bones (skeleton). Elevated levels of ALP in the blood are most commonly caused by liver disease, bile duct obstruction, gallbladder disease, or bone disorders. This test measures the level of ALP in the blood.
In the liver, ALP is found on the edges of cells that join to form bile ducts, tiny tubes that drain bile from the liver to the bowels, where it is needed to help digest fat in the diet. ALP in bone is produced by special cells called osteoblasts that are involved in the formation of bone. Each of the various tissue types produces distinct forms of ALP called isoenzymes.
ALP blood levels can be greatly increased, for example, in cases where one or more bile ducts are blocked. This can occur as a result of inflammation of the gallbladder (cholecystitis) or gallstones. Smaller increases of blood ALP are seen in liver cancer and cirrhosis, with use of drugs toxic to the liver, and in hepatitis.
Any condition causing excessive bone formation, including bone disorders such as Paget's disease, can cause increased ALP levels. Children and adolescents typically have higher blood ALP levels because their bones are still growing. As a result, the ALP test must be interpreted with different reference (normal) values for children and for adults.
It is possible to distinguish between the different forms (isoenzymes) of ALP produced by different types of tissues in the body. If it is not apparent from clinical signs and symptoms whether the source of a high ALP test result is from liver or bone disease, then a test may be performed to determine which isoenzyme is increased in the blood.
How is the test used?
The alkaline phosphatase test (ALP) is used to help detect liver disease or bone disorders. It is often ordered along with other tests, such as a gamma-glutamyl transferase (GGT) test and/or as part of a liver panel.
- In conditions affecting the liver, damaged liver cells release increased amounts of ALP into the blood. This test is often used to detect blocked bile ducts because ALP is especially high in the edges of cells that join to form bile ducts. If one or more of them are obstructed, for example by gallstones or by a tumor, then blood levels of ALP will often be high.
- Any condition that affects bone growth or causes increased activity of bone cells can affect ALP levels in the blood. An ALP test may be used, for example, to detect cancers that have spread to the bones or to help diagnose Paget disease, a condition that causes malformed bones. This test may also sometimes be used to monitor treatment of Paget disease or other bone conditions, such as vitamin D deficiency.
If ALP results are increased but it is not clear whether this is due to liver or bone disease, tests for ALP isoenzyme may be done to determine the cause.
When is it ordered?
An ALP test may be ordered as part of routine laboratory testing, often with a group of other tests called a liver panel. It is also usually ordered along with several other tests when you have symptoms of a liver or bone disorder.
- Weakness, fatigue
- Loss of appetite
- Nausea, vomiting
- Abdominal swelling and/or pain
- Dark urine, light-colored stool
- Itching (pruritus)
Some examples of the signs and symptoms suggesting a bone disorder include:
- Bone and/or joint pain
- Increased frequency of fractures
- Deformed bones
An ALP test may be ordered when you have symptoms of a bile duct obstruction (e.g., gallstone), such as:
- Severe abdominal pain, especially at the top right side or center of the abdomen
- Pain that spreads to the back or right shoulder blade
- Nausea that occurs at the same time as abdominal pain
- Abdominal pain that is linked to eating a fatty meal (when the gallbladder contracts)
- Normally, the intense pain lasts at least 30 minutes but begins to subside within an hour. An attack of biliary colic usually last less than 6 hours.
What does the test result mean?
Results of the ALP test are typically interpreted along with gamma-glutamyl transferase (GGT) results and with other tests that may have been performed at the same time, such as a liver panel, as well as in the context of your medical history and physical exam.
High ALP usually means that either the liver has been damaged or a condition causing increased bone cell activity is present.
- If other liver tests such as bilirubin, aspartate aminotransferase (AST), or alanine aminotransferase (ALT) are also high, usually the increased ALP is caused by liver damage or disease. If GGT is also increased, then the high ALP is likely due to liver disease. Significantly elevated ALP, often more than 4 times what is normal, may indicate bile duct obstruction and/or gallbladder inflammation.
- If liver tests and GGT are normal, then the high ALP is likely caused by bone diseases. Likewise, if calcium and/or phosphorus measurements are abnormal, usually the ALP is coming from bone.
If it is not clear from signs and symptoms or from other routine tests whether the high ALP is from liver or bone, then a test for ALP isoenzymes may be necessary to distinguish between bone and liver ALP.
ALP in liver disease
ALP results are usually evaluated along with other tests for liver disease. In some forms of liver disease, such as hepatitis, ALP is usually much less elevated than AST and ALT. When the bile ducts are blocked (usually by gallstones, scars from previous gallstones or surgery, or by cancers), ALP and bilirubin may be increased much more than AST or ALT. ALP may also be increased in liver cancer.
ALP in bone disease
In some bone diseases, such as Paget disease, where bones become enlarged and deformed, or in certain cancers that spread to bone, ALP may be increased.
If you are being successfully treated for Paget disease, then ALP levels will decrease or return to normal over time. If you have bone or liver cancer that responds to treatment, ALP levels should decrease.
Low levels of ALP may be seen temporarily after blood transfusions or heart bypass surgery. A deficiency in zinc may cause decreased levels. Persistent low levels of ALP could indicate hypophosphatasia, a rare genetic disorder of bone metabolism. Malnutrition may lower serum ALP activity.
Is there anything else I should know?
Pregnancy can increase ALP levels. Temporary elevations are also seen with healing fractures.
Children and adolescents normally have higher ALP levels than adults because their bones are growing, and ALP is often very high during a growth spurt, which occurs at different ages in boys and girls.
Some drugs may affect ALP levels. For example, oral contraceptives may decrease levels while anti-epileptics may increase levels.
What other tests are used to evaluate liver disorders?
There are other commonly used liver tests that measure other enzymes found in liver cells, such as alanine aminotransferase (ALT) and aspartate aminotransferase (AST). A test for bilirubin, a substance produced by the breakdown of red blood cells and removed from the body by the liver, may also be performed. Sometimes these tests (along with albumin and total protein testing) are run together as a liver panel. Other tests that may be performed individually or as part of a liver panel to detect or monitor liver disease include gamma-glutamyl transferase (GGT), lactate dehydrogenase (LDH), and prothrombin time (PT).
Who is at risk for liver disease and/or damage?
Some of those who are at risk of liver disease include the following:
- People who have been exposed to hepatitis viruses
- Heavy drinkers
- People who take medication that can be toxic to the liver or who are exposed to other liver toxins
- Those who are obese, have metabolic syndrome or insulin resistance
- Those who suffer from congestive heart failure
- People with an inherited disorder affecting the liver such as Wilson disease or hemochromatosis
See the article on Liver Disease for more on these.
What other laboratory tests may be done if I have a bone disorder?