What Is an Alkaline Phosphatase (ALP) Blood Test?

What to expect when undergoing this test

An alkaline phosphatase (ALP) blood test assesses a group of enzymes found in several parts of the body. Elevations in ALP may indicate a problem with the liver, gallbladder, bile ducts, bones, or some other organ systems.

Teenage African Medical Student Drawing Blood in Hospital
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Purpose of Test

The ALP test technically measures the activity of a group of related enzymes (alkaline phosphatases). ALP can be used as a type of medical marker of certain potential medical problems.

What Is Alkaline Phosphatase?

These enzymes make certain chemical reactions in the body go more quickly. The most common version of the ALP test assesses multiple versions of these enzymes given in a single value.

Scientists don’t fully understand the normal role of these enzymes, although they know ALP plays a role in bone mineralization. In medicine, alkaline phosphatases are important because their presence in the blood can give information about damage in specific parts of the body.

Alkaline phosphatase is active in many different types of tissues. The most important of these are the bones, liver, gallbladder, and bile ducts. However, ALP can also come from other sources, like the intestines, the kidney, or the placenta during pregnancy.

So it’s not a surprise that measuring ALP can sometimes give clues about problems in these body systems. Abnormalities in ALP can be used along with symptoms, additional tests, and other clinical factors to help diagnose many different medical conditions.

Medical Significance

A number of different medical conditions within the liver and throughout the bile duct system can lead to elevated ALP. High ALP levels can indicate a problem with an obstruction of the bile ducts. These ducts normally carry bile produced by the liver and stored in the gallbladder out to the intestines.

ALP also becomes elevated in certain diseases that affect the bone or the amount of calcium in the blood. Examples include Paget’s disease, hyperparathyroidism, or vitamin D deficiency.

Less commonly, other groups of medical disorders can decrease ALP, including malnutrition, hypothyroidism, and pernicious anemia.

Often, but not always, ALP is tested at the same time as different blood tests often used in diagnosing liver disease, such as aspartate aminotransferase (AST) and (alanine aminotransferase (ALT). These tests might be performed if you have a disease involving the liver, or if you have symptoms that are worrisome for liver or gallbladder disease.

For example, this might be needed if you have jaundice, abdominal pain, and nausea. Your clinician might also have you get an ALP if you have symptoms that might be due to a bone disorder, such as bone pain.

Risks and Contraindications

ALP is a simple blood test. As such, there are few contraindications. You might experience some pain and bruising at the site of the blood draw. Infection is also a minimal risk.

If you have a bleeding disorder or are taking a medication that inhibits blood clotting, like coumadin, you may be at increased risk of bleeding. Make sure your clinician knows about all your medical conditions and medications before your test.

Before the Test

Overnight fasting is usually recommended before taking an ALP test, because fatty meals can impact the results, temporarily increasing levels of ALP. You might want to wear a loose-fitting shirt, so it’s easy for the phlebotomist to assess a vein on your upper arm.

The test might be performed at a hospital or in an outpatient clinic setting. Usually the process only takes a few minutes.

During the Test

To perform the test, a healthcare professional needs to take a blood sample. First, they will clean the area. Next, a tourniquet will be applied above the vein to be used, usually in the upper arm.

You may be asked to squeeze your fist while your phlebotomist finds a good vein to use. The needle will be inserted into a vein in your arm. This usually only hurts for a moment or two.

The sample is drawn into a labeled tube to be sent to the lab. The needle is removed and the blood draw site is covered with a bandage or tape.

After the Test

The sample is promptly sent to a medical laboratory for analysis. Most of the time, you will be able to return to your normal activities right away.

If you are dizzy after the blood draw, you may need to sit for a while or have something to eat or drink before going about the rest of your day. You might have some soreness or bruising where your blood was taken.

Interpreting Results

Results of the test should come back fairly quickly, within a day or so. Different laboratories may have different reference ranges. However, roughly speaking, levels between 44 to 147 international units per liter (IU/L) are considered normal.

However, interpretation is sometimes challenging. Normal ALP levels vary based on factors such as one’s age, sex, and even blood type. Adolescents may have elevated levels compared to adults due to active bone growth. ALP may also be slightly elevated in smokers compared to non-smokers.

Pregnancy can temporarily increase ALP levels, even though no medical problem exists. ALP can also be temporarily elevated by bone fractures or by certain medications.

These factors must be considered during diagnosis. Your doctor can help you interpret your lab results and tell you whether there is a reason for concern.

High Levels of ALP

Clinicians consider the degree of elevation when evaluating ALP levels in a medical context. For example, ALP is often four times higher than normal, or even more than that, in medical conditions that block the flow of bile out the body. For example, this might be due to a cancer blocking the ducts, gallstones in the bile ducts, or other causes.

Very high levels can also be found if significant liver damage is causing the poor flow of bile out of the ducts. This might happen due to the autoimmune disease primary biliary cholangitis, liver injury from drug use, or from severe hepatitis from alcohol use. Diseases that infiltrate the liver can also cause this issue, such as cancer, sarcoidosis, or amyloidosis.

If elevation is not quite that high, other possibilities are more likely. For example, this might happen due to:

Low Levels of ALP

Although high levels of ALP are more common than low levels, low levels may be medically meaningful in some circumstances. For example, low levels of ALP are sometimes found in the following medical situations:

ALP can be used in diagnosis of these and related conditions. It is also sometimes used to monitor these conditions after treatment.

Follow-Up

You might not need extensive medical evaluation if your ALP is only a little elevated. Instead, your clinician may choose to monitor you with repeated blood tests. Somewhat increased ALP can occur in some individuals, even though no real problem is present. However, high levels need further medical investigation.

Follow-up tests will depend on the medical context: your symptoms and physical exam, your other past and present medical conditions, other test findings, etc. You may have gotten your ALP test in the context of other tests that help evaluate the liver, which can provide more information about what your test results mean. If not, other blood tests that are often given to evaluate the liver may be needed.

Sometimes it’s helpful to get a specific blood test called γ-glutamyl peptidase (GGTP) if one’s ALP is elevated. This test can help determine if the liver is the source of the increase or if it’s probably coming from another place.

It is also sometimes helpful to run a different type of ALP test (electrophoresis). This test gives information about the fraction of ALP that comes from different organs, indicating the probable organ of concern. Most of the time, these tests find the increase coming from bone disease or liver or gallbladder problems.

Other blood tests may also be needed, depending on the context. For example, if your clinician is concerned that hepatitis might be the cause of your symptoms and ALP results, you might need to test for hepatitis B and hepatitis C virus. Additional blood tests may also be necessary, such as antimitochondrial antibody (AMA). AMA is a blood test that helps diagnose primary biliary cholangitis, which is a disease which damages bile ducts in the liver and predominately affects women.

Abdominal ultrasound can also be useful as the first imaging modality to evaluate the liver, gallbladder, and related structures. If necessary, other techniques may be performed as well, like endoscopic retrograde cholangiopancreatography (ERCP) or magnetic resonance cholangiopancreatography (MRCP).

Other investigations, like liver biopsy, might be needed under specific circumstances. Your clinician will work with you to identify the underlying cause.

A Word From Verywell

ALP can be a helpful diagnostic test, especially when performed along with other related tests. But your ALP might be a little elevated even if there is no real reason for concern. Talk to your clinician about all your questions. Together, you can formulate the best possible diagnostic and treatment plan. 

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