What is a leukocyte alkaline phosphatase (LAP) test?

A leukocyte alkaline phosphatase (LAP) test is a laboratory test that can be conducted on a sample of your blood. Your doctor can order it to measure the amount of alkaline phosphatase, a group of enzymes, in certain white blood cells.

Before the advent of more advanced tests, the LAP test was commonly use to diagnosechronic myeloid leukemia(CML). This is a type of cancer than affects white blood cells. If you have CML, the level of alkaline phosphatase in your white blood cells will be lower than normal. Some doctors still order the LAP test to check for signs of CML. It can also help them rule out other disorders. But it’s now generally accepted that a cytogenetic test (a test of your cells and chromosomes) is needed to confirm a CML diagnosis. As a result, the LAP test is used less often now than in the past.

Alkaline phosphatase is a group of enzymes that remove phosphate groups from many types of molecules in your body. They work best in environments that are alkaline, or basic, rather than acidic. They’re found throughout your body, but they tend to be especially concentrated in your liver, kidney, bone tissue, and bile duct. They’re also concentrated in the placenta of pregnant women.

Leukocyte alkaline phosphatase (LAP) is the term for alkaline phosphatase that’s found in leukocytes. Another name for leukocytes is white blood cells. These are several types of white bloods cells. Each one has a different role in defending your body against viruses, bacteria, and other germs. They’re a crucial part of your immune system.

When you have chronic myeloid leukemia (CML), you have less alkaline phosphatase in your white blood cells than normal. As a result, in the past, doctors ordered an LAP test to diagnose CML. Now, they usually order a cytogenetic test instead. In a cytogenetic test, laboratory technicians look at the chromosomes in your white blood cells to check for abnormalities that cause CML.

In some cases, your doctor may still order a LAP test to check for signs of CML or other conditions. For example, they may order the test to help rule out or diagnose:

  • leukemoid reaction, an elevated white blood cell count that’s not caused by infection or cancer
  • essential thrombocytosis, an overproduction of blood platelets
  • myelofibrosis, a disorder in which scarring of the bone marrow occurs
  • polycythemia vera, a disorder in which your bone marrow makes too many red blood cells
  • aplastic anemia, a disorder in which your bone marrow makes too few blood cells
  • pernicious anemia, a drop in red blood cells often caused by the stomach’s inability to absorb vitamin B12

To conduct a LAP test, your doctor will need to collect a sample of your blood to send to a laboratory for testing. Before your blood is drawn, your doctor may ask you to take certain steps to prepare. For example, they may advise you to stop eating or drinking for six hours before your blood draw. They may also ask you to stop taking certain medications beforehand, including medications that can interfere with your test results. Make sure your doctor knows what medications and supplements you take.

Your blood may be drawn in your doctor’s office or a nearby clinic or laboratory. A nurse or phlebotomist will insert a small needle into one of your veins, likely located in your arm. They will use the needle to draw a small amount of blood into a vial.

It should only take a few minutes for them to draw your blood. Afterward, they will likely ask you to put pressure on the injection site or apply a bandage to stop the bleeding. Then they will send your sample of blood to a laboratory for testing.

A laboratory technician will smear your blood onto a microscope slide. They will add a special staining agent, which helps them to see which white blood cells contain alkaline phosphatase. They will use a microscope to count the proportion of cells that contain alkaline phosphatase.

When your test results are available, your doctor will discuss them with you. They will help you understand what the results mean and discuss follow-up steps. Scores for the LAP test can range from zero to 400, with those between 20 and 100 being considered normal.

A score that’s higher than normal may be caused by:

  • leukemoid reaction
  • essential thrombocytosis
  • myelofibrosis
  • polycythemia vera

A score that’s lower than normal may indicate:

  • CML
  • aplastic anemia
  • pernicious anemia

If your doctor suspects you may have CML based on your test results, they will likely order a cytogenetic test. This will help them confirm their diagnosis.

Getting your blood drawn involves minor risks. If you fail to put pressure on the needle site after your blood is drawn, you may experience mild bruising. Although it’s rare, you may also experience phlebitis, inflammation within a segment of your vein.

Tell your doctor if you have a bleeding disorder before you get your blood drawn. Bleeding disorders raise the risk of complications.

For most people, the benefits of undergoing the LAP test likely outweigh the risks. It can help your doctor diagnose potentially serious diseases and prescribe appropriate treatment. Ask them for more information about the potential benefits and risks.