What Is an Alkaline Phosphatase Test?

Medically Reviewed by Minesh Khatri, MD on May 28, 2023
3 min read

Alkaline phosphatase is one kind of enzyme found in your body. Enzymes are proteins that help chemical reactions happen. For instance, they can break big molecules down into smaller parts, or they can help smaller molecules join together to form bigger structures.

You have alkaline phosphatase throughout your body, including your liver, digestive system, kidneys, and bones.

If you show signs of liver disease or a bone disorder, your doctor may order an alkaline phosphatase (ALP) test to measure the amount of the enzyme in your blood and help in diagnosing the problem. Sometimes it’s part of a broader group of tests called a routine liver or hepatic panel, which checks how your liver is working.

If your liver isn’t working right, the amount of ALP in your blood may be high. Doctors often use the test to look for blocked bile ducts. Other conditions that can cause problems with your liver include:

The test can also spot problems with your bones, including:

  • Cancers that have spread to your bones
  • Paget’s disease, which affects how bones grow
  • Issues caused by vitamin D deficiency

The lab will need a small amount of blood to perform the test.

The person in charge of taking your blood will start by placing a tight elastic band, called a tourniquet, around your upper arm. This makes your veins swell with blood.

The lab tech will clean an area of your skin with a germ-killing solution. (It might be a spot inside your elbow or the back of your hand). You’ll feel a small stick when the needle goes into your vein. The blood flows into a small vial attached to the needle.

When the test is done, the lab tech will take the tourniquet off, and you’ll get a bandage on the spot where the needle went in. It takes only a few minutes.

Taking blood samples is usually very safe. Some things that might happen after the test include a bruise at the spot where the needle went in, and a little dizziness. There’s also a slight chance of infection.

You may have to limit food and liquids for several hours before the test. Some medicines interfere with the results, so make sure your doctor knows about all drugs you take, including over-the-counter medicines, vitamins, and supplements.

Be sure to let your doctor know if you are pregnant, because that will increase the level of ALP in your blood.

It generally takes 1-2 days for results to come back from the lab.

Higher-than-normal ALP levels for your age and sex may not necessarily mean you have a problem. (Children and teens naturally have higher levels than adults because their bones are still developing).

If your ALP level is high, your doctor may have you take another test, called an ALP isoenzyme test, to determine whether the alkaline phosphatase in your blood is coming from your liver or your bones.

Low levels of ALP indicate a deficiency in zinc and magnesium but can also be an indicator of a rare genetic disease called hypophosphatasia (HPP), which affects bones and teeth.