The leukocyte alkaline phosphatase (LAP) test is a laboratory test that measures the amount of a certain enzyme (alkaline phosphatase) in white blood cells. White blood cells (WBC) are also called leukocytes. This test is not often used today. In the past, before the advent of more advanced tests, the LAP test was used as a critical indicator of chronic myeloid leukemia (CML), a cancer of the white blood cells. The test is still ordered by some doctors to give a first indication of CML and to rule out or diagnose other disorders. However, it is now generally accepted that a cytogenetic test is needed to confirm a CML diagnosis.
To get a score from the test, a laboratory technician must look at your blood in a microscope and count the amount of staining, a process that reveals the enzyme, in 100 white blood cells.
Alkaline phosphatase is group of enzymes that remove phosphate groups from many types of molecules in the body. They work best in an environment that is alkaline, or basic (rather than acidic). These enzymes are found throughout the body, but are especially concentrated in the liver, kidney, bone tissue, bile duct, and in the placenta in pregnant women.
Members of this enzyme group that are found in white blood cells are called leukocyte alkaline phosphatase (LAP). Leukocytes are the cells in your blood that are responsible for defending you against viruses, bacteria, and other germs. They are a crucial part of your immune system.
One of the most important reasons that doctors used LAP tests in the past was to diagnose CML. Levels of alkaline phosphatase are low to absent in malignant white blood cells. The test, however, being subjective, has largely been replaced with newer technology. When CML is suspected now, doctors will usually order a cytogenetic test. In a cytogenetic test, technologists look at the chromosomes in the cells to find abnormalities that indicate the cancer is present.
The LAP test can still be a starting test for CML and can be useful for other diagnoses as well. Your doctor may order the test to rule out or to confirm certain medical conditions, including:
- leukemoid reaction: an elevated white blood cell count that is not due to infection or cancer
- myelofibrosis: a bone marrow disorder in which scarring occurs
- polycythemia vera: a bone marrow disorder in which the marrow makes too many red blood cells
- aplastic anemia: a bone marrow disorder in which the marrow makes too few blood cells
- pernicious anemia: a decrease in red blood cells caused by a vitamin B12 absorption problem in the intestines
- essential thrombocytosis: an overproduction of blood platelets
Your doctor will explain anything that you need to do to prepare for a LAP test. This could include not eating or drinking for six hours before your blood is drawn. You may also be asked to stop taking some medications, which can interfere with the results. Make sure your doctor knows all of the medications and supplements that you take.
A LAP test is administered by drawing a small sample of blood from you. It will probably be taken in your doctor’s office or a lab in the building. A nurse or a phlebotomist will draw a small amount of blood into a vial using a needle inserted into a vein in your arm.
The withdrawal will take only a few minutes and will cause minimal pain. After the blood is withdrawn, you will put pressure on the needle site or apply a bandage to stop the bleeding.
There are not many risks involved with taking a sample of blood. If you fail to put pressure on the needle site after the withdrawal, you may experience mild bruising. Although unlikely, you may also experience phlebitis, an inflammation in your vein. Your doctor will tell you to put a warm compress on it throughout the day until the inflammation goes down. You should tell your doctor before the withdrawal if you have a bleeding disorder.
When the results are available, your doctor will discuss them with you. You will talk about what the results mean and what you should do next. Scores for a LAP test can range from zero to 400, with scores between 20 and 100 being normal.
A score that is lower than normal may be caused by:
- essential thrombosis
- leukemoid reaction
- polycythemia vera
A score that is higher than normal may indicate aplastic or pernicious anemia, or CML. If the result indicates CML, your doctor will likely order a cytogenetic test to confirm the diagnosis. A LAP test is no longer a definite indicator of this type of cancer.