Alkaline phosphatase (ALP) is an enzyme in blood that helps break down proteins. An ALP test measures how much ALP is circulating in the bloodstream. Having high or low ALP levels can indicate an underlying condition.

Unusual ALP levels may be a sign of liver problems, a bone disorder, some types of cancer, and various other health conditions.

Slightly abnormal ALP levels can occur naturally and without any underlying illness. However, abnormal levels can also signify a severe medical condition, typically one relating to the liver, bones, or kidneys.

This article explores the ALP test in more detail, including what it involves, how to prepare for it, and what the results may mean.

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An ALP test is a blood test. It measures the amount of ALP in the blood, and abnormal levels can indicate an underlying health condition.

Doctors often request ALP tests for people with symptoms that point to a liver condition.

Symptoms of a liver problem can include:

A doctor may also order an ALP test if a person has symptoms of a bone disorder, such as:

The ALP test can be part of screening. It may be one of many tests in a liver panel, for example. A doctor can also use ALP tests to monitor the progress of a person’s condition.

In addition to an ALP test, a doctor may order an ALP isoenzyme test. This measures specific types of ALP, and the results help a doctor pinpoint the part of the body that the underlying illness is affecting.

ALP is an enzyme present in many parts of the body. It is most abundant in the bones, liver, and intestines.

Researchers still do not know the full range of ALP’s functions, but it seems to contribute to many processes, such as:

  • transporting nutrients and other enzymes in the liver
  • aiding the formation and growth of bones
  • transporting fatty acids, phosphates, and calcium in the intestines
  • digesting fat in the intestines
  • regulating cell growth, death, and migration during fetal development

Healthcare professionals may measure ALP levels in international units per liter (IU/l). The reference range for normal levels of ALP can vary from laboratory to laboratory. The United Kingdom’s National Health Service (NHS) lists the reference range for adults aged 18 years and older as: 30–130 IU/l.

However, other labs may consider the normal range to be different and research suggests that normal upper limits of ALP levels may vary depending on multiple factors, such as a person’s age, sex assigned at birth, and ethnicity.

A person should check the reference report that comes with their test results to fully understand the findings. They may also wish to discuss the results with their doctor for further guidance.

Most people with abnormal ALP levels have high, rather than low, levels.

Causes of high ALP levels

The ALP level reference range for adults aged 18 years and older is 30–130 IU/l, according to the NHS. Therefore, some healthcare professionals may consider ALP levels over 130 IU/l as high.

However, reference ranges vary between labs and can depend on individual factors, so a person should speak with a doctor before interpreting their results. A doctor may not consider ALP levels to be high unless they are significantly more than the upper limit of the reference range.

Elevated ALP levels are generally a sign of a liver or bone condition. An obstruction of the liver or damage to it causes ALP levels to rise. Elevated levels can also result from an increase in bone cell activity.

High ALP levels may stem from:

Elevated ALP levels can also occur in people with:

  • a bacterial infection
  • osteomyelitis, an infection that causes bone pain
  • bile duct damage
  • a decrease in bile flow
  • overactive parathyroid or thyroid glands
  • osteomalacia, which is softening of the bones
  • Paget’s disease, which can cause bones to become unusually large, weak, or prone to fracture
  • myeloid metaplasia, which can cause bone marrow to be replaced by fibrous tissue and blood to be made in the organs
  • heart failure
  • AIDS
  • Hodgkin lymphoma, a cancer of the lymphatic system
  • osteogenic sarcoma, a bone cancer
  • certain other cancers
  • bone metastasis, which involves cancer cells spreading to bone

Malnutrition can also lead to high ALP levels.

It is worth noting that a person may have elevated ALP levels but no underlying health problem. ALP levels may be naturally higher in:

  • pregnant people
  • growing children and adolescents
  • older adults
  • people with healing fractures

Intestinal ALP levels may also increase in people with blood types O and B after they have eaten a fatty meal. In addition, certain drugs can cause ALP levels to vary.

Causes of low ALP levels

According to the NHS, the ALP level reference range for adults aged 18 years and older is 30–130 IU/l. Therefore, healthcare professionals may consider ALP levels under 30 IU/l to be low. However, these ranges vary between labs and can depend on individual factors, so a person should discuss their results with a doctor.

There are various causes of low ALP levels, including:

In addition, some medications, such as birth control pills, can lower ALP levels.

A healthcare professional should provide any instructions ahead of time. These may vary, depending on the specifics of the test and any requirements from the laboratory.

A person may need to fast for about 8 hours before an ALP test. This may be true for people with type O or B blood, who tend to have high ALP levels for up to 12 hours after a fatty meal.

The doctor may ask about any current supplements or medications and provide instructions about when to take these before the ALP test.

A person may be able to conduct the ALP test at home. In this case, following all the instructions carefully is key.

An ALP test requires a blood sample from a vein. Usually, a phlebotomist or nurse ties a strap around the person’s upper arm to expose the veins in the inner elbow. Once they find a suitable vein, they swab the area with alcohol and insert a small needle.

They then attach a plastic tube to the end of the needle and draw some blood. Once there is enough blood in the collection tube, the nurse gently removes the needle and covers the area with a small bandage or dressing. An ALP test typically takes less than 5 minutes.

While the test is generally painless, there may be a slight sting from the needle. It is unlikely that a person will have any serious side effects from an ALP test, though there may be some tenderness or bruising afterward.

Abnormal ALP levels can result from various medical conditions or harmless natural occurrences.

When they interpret the results, a doctor will take the following into consideration:

  • how high or low the ALP levels are
  • the person’s overall health
  • any additional symptoms
  • the results of additional tests

If related symptoms do not help the doctor reach a diagnosis, they may carry out further tests. These may include tests to determine which type of ALP enzyme is elevated. Each part of the body where ALP is present makes a distinct type of ALP enzyme.

Further testing may also include these liver tests:

  • a bilirubin test
  • an aspartate aminotransferase, or AST, test
  • an alanine aminotransferase, or ALT, test

If a person has high ALP levels, but the results of other liver tests are normal, the underlying cause may involve the bones rather than the liver. In this case, the doctor may request imaging tests to check the bones.

The treatment for abnormal ALP levels depends on the cause. If this is pregnancy or bone development, no treatment is necessary.

If malnutrition is responsible, the person may need supplements and should have a healthy diet rich in:

  • fruits
  • vegetables
  • whole grains
  • lean sources of protein, such as chicken or fish
  • beans
  • low-fat dairy products
  • nuts
  • seeds

If an infection is responsible for abnormal ALP levels, the doctor may prescribe antibiotics or other antimicrobial medications. It is crucial to take these exactly as directed.

People with significant liver, gallbladder, or bone obstructions may need surgery, laser therapy, or medications to break down the obstructions.

Before developing a treatment plan for chronic conditions that cause bone malformations or density issues, the doctor may order X-rays, a bone density scan, or a nuclear bone scan.

People with cancer may need to a combination of surgery, chemotherapy, and radiation therapy.

ALP is an enzyme present in various parts of the body. Abnormal ALP levels can indicate an underlying health problem. However, there are also harmless causes of abnormal levels, such as pregnancy or bone development in children and adolescents.

ALP is most abundant in the liver and bones, and high ALP levels may stem from a health problem in these areas. A doctor may order an ALP test if a person has symptoms of bone or liver problems or as part of routine chemistry testing.

ALP tests are quick and usually have no adverse effects. Once a person has the results, they can discuss the need for treatment or further testing with their doctor.