What Is an Alkaline Phosphatase Test?

Medically Reviewed by Jabeen Begum, MD on November 01, 2023
Written by Tammy Worth
6 min read

Alkaline phosphatase is an 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.

Alkaline phosphatase (ALP) is mainly found in your liver and bones, but small amounts are also in your digestive system, kidneys, and the placenta during pregnancy. Scientists don't completely understand what alkaline phosphatase does for your body, but it may help build bones and hard tissue (like your teeth) and transport phosphates and other molecules in your intestines.  

 

An alkaline phosphatase (ALP) test measures the amount of this enzyme in your blood to help diagnose certain health problems. If you show signs of liver disease or a bone disorder, your doctor may order an ALP test. Sometimes it’s part of a group of tests called a routine liver panel or hepatic panel, which check how your liver is working.

 

 

 

When your doctor orders a comprehensive metabolic panel (CMP) during an annual health checkup, it may include a general ALP blood test to make sure your alkaline phosphatase is within the normal range.

You may need regular ALP blood tests if you take vitamins or medications that affect your liver, like acetaminophen, or statins, which help lower cholesterol. Your doctor can let you know if that’s an issue with any of the drugs or herbal supplements you take.  

If your doctor thinks your liver isn’t working well or you have a problem with your bones, an ALP blood test can help them find out what’s going on. It can also help them keep an eye on any existing health problems you have, including diseases that affect your bones and liver like cancer, hepatitis, and cirrhosis. 

 

 

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

The person in charge of taking your blood will place 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 (other than water) for 10-12 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.

Don’t stop taking your medication without talking to your doctor first. They’ll let you know how to manage your drug treatment before your ALP blood test. 

Tell your doctor if you're pregnant because ALP in the placenta can increase the amount of the enzyme in your blood.

It generally takes 1-2 days for results to come back from the lab. Your ALP test report will show you:

  • What was measured
  • The levels in your blood 
  • The normal range
  • If your levels were high, low, or normal

Normal alkaline phosphatase

The normal range for alkaline phosphatase depends on the lab where you have your blood work performed. Normal ALP ranges can be 44-147 international units per liter (IU/L), or 30-120 IU/L. Your lab report will let you know what it considers normal. 

High alkaline phosphatase

Higher-than-normal ALP levels for your age and sex may not necessarily mean you have a health problem that needs treatment, especially if your levels are only slightly elevated. 

Elevated alkaline phosphatase can be related to several things: 

  • Children and teens have elevated levels because their bones are still developing.
  • Males (and those identified as male at birth) ages 15-50 usually have slightly higher ALP levels than people identified as female at birth.
  • ALP levels may go up in older age.
  • A bone break can temporarily increase ALP. 
  • For some people, eating a fatty meal before an ALP blood test can cause higher-than-normal levels.
  • Alkaline phosphatase may go up by as much as three times when you’re pregnant. If your blood test shows levels that are higher than expected, your doctor may check your levels more often to make sure your placenta is healthy. 

If your ALP level is very 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. Elevated alkaline phosphatase levels can be caused by conditions such as:

An elevated ALP isoenzyme test can indicate issues like:

Low alkaline phosphatase

This is less common than having high levels of ALP. Low levels of alkaline phosphatase may mean a deficiency in zinc, magnesium, or other nutrients. Low alkaline phosphatase can also indicate a rare genetic disease called hypophosphatasia (HPP), which affects bones and teeth.

You may have low levels of ALP if you have:

  • An underactive thyroid gland (hypothyroidism)
  • Low red blood cells due to a vitamin B12 deficiency (pernicious anemia)
  • Wilson’s disease, a rare genetic disorder that causes the body to make too much copper

Some medications, including hormonal birth control, can sometimes lower your levels of ALP. 

High or low alkaline phosphatase levels don’t always mean you have a health problem. Your doctor will consider several things before they order more tests, including: 

  • Past health problems or current symptoms
  • Any medications, supplements, or hormones you take
  • If you’ve ever had abnormal ALP levels before
  • What your other blood tests show

If your doctor decides to look deeper into your liver levels, you may need an ALP isoenzyme test or more detailed liver function tests. 

 

When should I worry about alkaline phosphatase?

Your doctor will let you know what high or low alkaline phosphatase levels mean. But you should seek medical help if you have symptoms like yellowing skin or pain in your bones or joints. Those could be a sign of liver or bone disease.

What causes alkaline phosphatase levels to change?

Medical conditions that affect the liver and bones can change your ALP levels. Factors such as recent bone fractures, pregnancy, dietary choices, certain medications, and hormonal birth control can also cause your ALP levels to go up or down.

What is alkaline phosphatase in liver damage?

Elevated alkaline phosphatase can be a sign your liver isn’t working the right way. High levels of ALP may indicate blocked bile ducts (tubes that connect your liver and gallbladder to the rest of your gut), liver inflammation or scarring, or an infection that causes liver swelling.