Alkaline phosphatase (ALP) is an enzyme in a person's blood that helps break down proteins. The body uses ALP for a wide range of processes, and it plays a particularly important role in liver function and bone development. Using an ALP test, it is possible to measure how much of this enzyme is circulating in a person's blood.

Slightly irregular ALP levels are usually no cause for concern. However, severely abnormal levels can signify a severe underlying medical condition, typically one relating to the liver, bones, or gallbladder.

In this article, we look at the ALP test in more detail and explain what it involves, how to prepare for it, and what the results may mean.

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An ALP test checks the level of alkaline phosphatase in the blood.

An ALP test is a simple procedure that measures the amount of ALP in the blood. ALP is an enzyme that is present in most parts of the body but is most abundant in the bones, liver, kidneys, gallbladder, and intestines.

Researchers still do not understand the full range of ALP's functions, but it seems to contribute to a wide range of 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

Doctors will often request that a routine blood test includes an ALP test if a person has symptoms that may indicate liver problems. These include:

  • yellowing of the skin or eyes
  • painful or swollen tummy
  • feeling nauseous
  • vomiting
  • feeling tired or weak
  • unexplained weight loss
  • urine that is darker than usual

They will also specifically order an ALP test if they think a person has a condition affecting any of the following:

  • bones
  • gallbladder
  • kidney
  • intestines

The ALP level in healthy adults should be 20–140 units per liter (U/L).

Children tend to have significantly higher levels of ALP than adults because their bones are still growing. A person recovering from a bone injury may also have a raised ALP level in the 3 months after the injury while their bone heals.

It is also common to have higher ALP levels than usual during the second and third trimesters of pregnancy.

However, everyone's natural ALP level will be a bit different. After an ALP test, a doctor will be able to explain what it means if the ALP level is higher or lower than they would expect.

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Abnormal ALP levels indicate that a person may have gallstones.

As ALP is most abundant in the bones and liver, elevated ALP levels are generally a sign of a liver or bone condition. An obstruction of the liver or damage to it will cause ALP levels to rise. This will also occur if there is an increase in bone cell activity.

Abnormal ALP levels usually indicate one of the following conditions:

Conditions that abnormal ALP levels may indicate less regularly include:

  • rickets — a weakening of the bones in children, usually as a result of a vitamin D or calcium deficiency
  • osteomalacia — a weakening of the bones in adults, usually relating to a vitamin deficiency
  • Paget's disease — a condition that causes significant bone deformity and problems with bone regeneration

Unusual ALP levels may also signify one of the following conditions, although this is rare:

Someone having an ALP test will usually need to fast for 10–12 hours before the test. As such, most people prefer to have their test first thing in the morning. Most local clinics and doctor's offices can carry out these tests.

Several medications, such as birth control pills and anti-epileptic drugs, can interfere with ALP levels. Therefore, anyone having the test should tell the doctor about any medications and herbal supplements they are taking.

An ALP test only requires a blood sample.

A phlebotomist or nurse will tie a strap around the person's upper arm to expose the veins in the inner elbow. Once they find a suitable vein, the nurse will swab the area with alcohol then insert a small needle into the vein.

They will 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 will gently remove the needle and cover the area with a small bandage or dressing.

If the person needs other blood tests at the same time, the nurse may have to fill multiple collection tubes.

ALP tests have few complications, aside from some minor bleeding or bruising at the needle insertion site. Most people do not feel any pain or discomfort during or after the test, other than a slight prick when the needle enters the arm.

Occasionally drawing blood can lead to inflammation around the vein or tissues in the inner elbow. Applying a warm compress and gentle pressure to the site for 20–30 minutes can help to relieve this. If the inflammation remains for more than a few hours after the blood test or gets worse, it is best to seek medical attention.

Most people with abnormal ALP levels have high, rather than low, levels. The most common causes of high ALP levels include:

  • liver conditions, often bile duct obstructions
  • gallbladder conditions, usually gallstones
  • bone conditions, such as abnormal growths and occasionally cancers
  • pregnancy
  • young age, as children who are still growing tend to have much higher ALP levels

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 raised in the blood. Each part of the body makes a distinct type of ALP enzyme.

Further tests may also include the following liver tests:

  • bilirubin test
  • aspartate aminotransferase (AST) test
  • alanine aminotransferase (ALT) test

If a person has raised ALP levels but the results of other liver tests are normal, the problem may not be with their liver. It may be that their bones are affected. The doctor may use imaging tests to diagnose bone disorders.

Rarely, high ALP levels may indicate:

  • bacterial infection
  • cancer
  • heart failure
  • kidney failure

It is relatively rare for a person to have low ALP levels. These are most often the result of:

  • severe or long-term vitamin and mineral deficiencies
  • chronic conditions that can cause malnutrition, such as untreated celiac disease

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A malnourished person with low ALP levels should eat whole fruits and vegetables.

The treatment for abnormal ALP levels depends entirely on the cause of the condition.

Slightly abnormal ALP levels are generally no cause for concern as ALP levels naturally fluctuate during the day and vary from person to person.

People with malnutrition should adjust their diet accordingly or take supplements. They should take care to include the following foods in their diet:

  • whole fruits and vegetables, especially citrus fruits, berries, and dark, leafy greens
  • whole-grain cereals, bread, rice, and pasta
  • red meats and fatty fish
  • probiotic food products, such as yogurt, kefir, kimchi, and sauerkraut

For infections, a doctor may prescribe a course of antibiotics or other antimicrobial medications.

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

For chronic conditions that cause bone malformations or density issues, a person may need to have an X-ray.

People with cancer may need to undergo a combination of surgery, chemotherapy, and radiotherapy.