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
Before the advent of more advanced tests, the LAP test was commonly
use to diagnose chronic
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.
Why is a leukocyte alkaline phosphatase test performed?
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
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
- 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
How should I prepare for the test?
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.
How is the test administered?
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.
What do the results of the test mean?
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
- polycythemia vera
A score that’s lower than normal may indicate:
- 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
What are the risks associated with the test?
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.