Hairy cell leukemia (HCL) is a rare form of blood cancer. It affects your B cells, which are a type of infection-fighting white blood cell called lymphocytes. HCL causes B cells to grow thin projections on their outer surfaces that look like hair. These abnormal B cells grow out of control and crowd out other normal blood cells. HCL accounts for about 2 percent of all adult cases of leukemia.

If your doctor suspects that you have HCL, they may have you undergo several tests. One of these tests may be to check whether any of your lymphocytes contain a protein called tartrate-resistant acid phosphatase (TRAP). When TRAP is found in B cells, it usually means a person has HCL. Testing for TRAP may be one step toward an HCL diagnosis.

What Is the TRAP Protein?

Every cell in your body contains many proteins, each of which has a different role to help the cell survive, grow, or stay healthy. When B cells become cancerous, they often start making different proteins compared to normal B cells. By testing for these proteins, scientists can determine which type of leukemia a person has.

TRAP is a type of protein called an enzyme. TRAP is found in specific immune cells called macrophages and dendritic cells. Within these cells, TRAP helps the cells fight off germs such as bacteria and viruses. TRAP is also found in osteoclasts, cells in the bone that help get rid of old bone so that it can be replaced with new bone.

TRAP in Hairy Cell Leukemia

Normal B cells don’t make the TRAP protein. However, about 95 percent of people with HCL have cancerous B cells that make high levels of TRAP. If your doctor suspects you have HCL, they may have you undergo a TRAP test.

When Do You Need a TRAP Test?

Your doctor may want you to get a TRAP test if you have symptoms of HCL. These symptoms might include:

• Feeling tired
• Weight loss
• Getting a lot of infections
• Fever or chills
• Pale skin
• Bruising
• Enlarged spleen
• Breathing problems

Many people with HCL also have an enlarged spleen (splenomegaly). The spleen is an organ that fights infections and supports healthy blood cells. It sits on the upper left side of your stomach. When the spleen becomes enlarged, it may cause abdominal pain or make you feel unusually full after eating.

How Is TRAP Testing Performed?

A TRAP test is performed on a sample of your blood cells. There are a few different ways in which your doctor may get this sample, including a blood draw or a bone marrow biopsy.

Blood Test

To look for TRAP in your blood cells, your doctor may want you to have your blood drawn. In this case, a health care professional will insert a needle into a vein on your arm or hand. This needle is connected to a tube, which then collects and stores the blood. This procedure may cause a little bit of pain or bruising but comes with very few risks.

Bone Marrow Tests

Bone marrow is tissue found inside certain bones that helps make all of your blood cells. If you have HCL, you will probably have leukemia cells inside your bone marrow. A biopsy helps your doctor take a better look at these cells to confirm a diagnosis and better determine your cancer’s type, grade, and stage.

Bone marrow testing involves taking a small sample of bone marrow, usually from your pelvic bone. To get this sample, a doctor will first numb your lower back and hip area and then use a needle to draw out some of the liquid and cells inside.

There are usually two parts to bone marrow testing. During a bone marrow aspiration, the doctor will remove some of the liquid part of the bone marrow. During a bone marrow biopsy, the doctor takes out some of the solid portions of this tissue.

Bone marrow tests can cause pain, discomfort, and bruising. It may take a few days for the wound to heal and for the pain to completely go away. However, having this test is often an important step in getting the correct diagnosis and better understanding your treatment options.

Laboratory Tests

Once a sample of your blood or bone marrow has been obtained, it will be sent to a laboratory. A doctor called a pathologist will study your cells under a microscope and may test your cells for different features, including the presence of TRAP.

TRAP is typically identified using a test called immunohistochemistry. This test involves the use of a monoclonal antibody (a substance that can recognize and attach to a specific protein) that is attached to a dye or a stain. When an anti-TRAP antibody is added to cells that contain the TRAP protein, the cells will change color. This procedure allows the pathologist to see whether TRAP is present or absent.

Other Tests to Diagnose Hairy Cell Leukemia

TRAP tests are not used in the diagnosis of HCL as often as they used to be. One reason for this is that other types of cancer cells can also make the TRAP protein. In particular, cells in some other types of B-cell lymphomas and leukemias, such as marginal zone lymphoma, also make TRAP. Your doctor will probably also perform other tests in addition to TRAP testing to help with your diagnosis, or they may not have you undergo a TRAP test.

Other Blood Tests

If your doctor thinks you may have leukemia, they will probably want to do a complete blood count. This test, which also involves a blood sample, counts how many of each type of blood cell you have. HCL often causes pancytopenia, which means that you will have low levels of normal blood cells.

Another blood test you may undergo is a peripheral blood smear. In this test, a small number of cells are put onto a glass slide. A pathologist closely studies your white blood cells to see if any of them appear to have hair-like projections.

Immunophenotyping

HCL cells also make other distinctive proteins in addition to TRAP. Some of these other proteins include CD20 and DBA.44. If your doctor tests your cells for TRAP, they may also test for these or other proteins using different immunohistochemical stains. Immunophenotyping helps determine what type of leukemia you have.

Genetic Tests

Cancer cells often undergo many gene changes. Finding out which changes your cancer cells have can help your doctor know for sure whether you have cancer. Knowing the genetic changes can also help the doctor determine the cancer type and know which treatments may work best. Genetic tests for HCL include cytogenetic analysis (a test that counts the number of chromosomes in your cells and determines whether any of them are damaged) or BRAF gene testing (a test that looks for mutations in a gene called BRAF, which is often damaged in HCL cells).

What Comes Next After TRAP Testing?

Talk to your doctor about when you can expect results from your diagnostic tests. It may take several days before you find out what your tests showed. If you have questions about your results, you can always talk to members of your care team about what your tests mean. The good news is that HCL is imminently curable.

Connect With Others Who Understand

MyLeukemiaTeam is the social network for people with leukemia. On MyLeukemiaTeam, more than 10,000 members come together to ask questions, give advice, and share their stories with others who understand life with leukemia. Waiting to get tests done and then waiting for the results of those tests can be stressful. Feeling worried or scared during this time is normal. If you want to talk with others who understand what it’s like to be on this journey, you can find new connections at MyLeukemiaTeam.

Comment below, or post on MyLeukemiaTeam, if you want to talk to others about testing for hairy cell leukemia.