What Is Alkaline Phosphatase?

Enzyme serves a biomarker for liver, bone, and other diseases

In This Article

Alkaline phosphatase (ALP) is an enzyme found in the blood that helps break down proteins. ALP plays a role in numerous processes in the human body, and any abnormalities in blood concentrations—either high or low—may be indicative of diseases ranging from gallstones and thyroid disease to hepatitis and cancer.

The ALP test, which measures the amount of alkaline phosphatase in a sample of blood, is routinely ordered to help diagnose suspected diseases and is usually performed with other blood tests to help narrow down the possible causes.

Alkaline phosphatase test can be drawn in a serum separator tube
David Silverman / Getty Images

What Is Alkaline Phosphatase?

Alkaline phosphatase is an enzyme. An enzyme is a type of protein that catalyzes (causes or accelerates) chemical reactions in the body. ALP is produced mainly by the liver and bones but is also synthesized in smaller quantities by the intestines and kidneys. ALP is also secreted from the placenta during pregnancy.

Scientists have not yet identified the full range of biochemical reactions that ALP instigates, but the enzyme is known to contribute to the following processes:

  • Transporting nutrients and enzymes to and from the liver
  • Aiding in the development, growth, and maintenance of bone
  • Transporting calcium and phosphate from the intestines to bones, muscles, and nerve cells to ensure normal function
  • Transporting fatty acids to store energy in adipose tissues and help maintain the structural integrity of cells
  • Regulating cell growth in fetuses during pregnancy

ALP is found in all tissues of the body but, not surprisingly, is found in its highest concentration in the liver, bones, kidneys, and intestines as well as the bile ducts (that drain bile from the liver) and gallbladder (that stores bile).

Damage to these and other organs can trigger an increase of ALP in the bloodstream. By contrast, certain diseases or conditions can impair ALP synthesis and lead to a drop in the blood concentration.

ALP Testing

Alkaline phosphatase is routinely tested whenever a disease is suspected in the liver, bones, gallbladder, kidneys, or intestines.

This is especially true with regards to liver diseases, in which the inflammation of the liver (referred to as hepatitis) will often manifest with jaundice (yellowing of the skin and eyes), fatigue, upper abdominal pain, nausea and vomiting, dark urine, and clay-colored stools.

ALP tests play an important role in the diagnosis of liver disease. Similar symptoms can occur with gallbladder diseases.

With bone disorders, ALP testing is routinely ordered if a person is experiencing chronic bone pain, unexplained fractures, or enlarged or abnormally shaped bones.

The ALP test involves a simple blood draw performed by a nurse, doctor, or phlebotomist. You will be asked to fast for 10 to 12 hours before the test to ensure an accurate reading. Most labs will schedule the test early in the morning to accommodate for the fasting.

There are certain drugs that can interfere with the results. Advise your doctor or the lab if you take any of the following:

If your ALP levels are modestly raised, it may due to the drugs you are taking rather than an underlying disease or medical condition.

Interpretation

Alpha phosphatase is considered a biomarker for many disorders, with abnormally high and abnormally low ALP levels pointing the doctor in the direction of the possible causes.

The ALP test results are presented as a reference range of values measures in enzyme units per liter (U/L) or microkatals per liter (μkat/L). Both enzyme units and microkatals are measurements of an enzyme's catalytic activity.

The normal range of values of ALP differs by age and pregnancy status. ALP values are higher in children because their bones are actively growing, while the ALP values increase during pregnancy in tandem with the development of the placenta and the approach of childbirth.

  By U/L By μkat/L
Adults 33 to 96 U/L 0.55 to 1.6 μkat/L
Children Under 350 U/L Under 5.8 μkat/L
Pregnacy (first trimester) 17 to 88 U/L 0.28 to 1.47 μkat/L
Pregnacy (second trimester) 25 to 126 U/L 0.42 to 2.1 μkat/L
Pregnacy (third trimester) 38 to 229 U/L 0.63 to 3.8 μkat/L

ALP values that fall outside these reference ranges are considered abnormal. On their own, abnormal ALP values are not diagnostic but can provide a clue as to the underlying cause.

High Values

Abnormally high ALP values may be the result of:

Low Values

Abnormally low ALP values may be the result of:

Other Blood Tests

An ALP test is typically accompanied by a bilirubin, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) test. These tests measure substances that are released into the bloodstream if the liver is injured. If bilirubin, ALT, and AST values are normal, it would indicate that the liver is not involved and allow the doctor to look elsewhere for a cause.

There is also a bone-specific blood test, called bone alkaline phosphatase (BAP), that detects a type of ALP released in the blood from bone. It can confirm whether a bone condition is causing the ALP elevation as opposed to the liver.

To pinpoint the cause, the doctor may also order blood tests to look for hormone imbalances (such as occurs thyroid or parathyroid disease), generalized inflammation and increased white blood cell counts (signs of infection), kidney enzyme abnormalities, and various tumor markers in the blood that suggest the presence of cancer.

Based on the results of these blood tests, the doctor can expand the investigation and order the appropriate tests and procedures (including imaging studies and biopsies) to definitively diagnose the cause.

A Word From Verywell

Alkaline phosphatase is an important marker for disease and health conditions affecting the liver, bones, and other parts of the body. In order for an ALP test to be accurate, you need to ensure that you adhere to the fasting guidelines and advise your doctor of any medications you are taking, whether they are prescription, over-the-counter, herbal, nutritional, or recreational.

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