Lymphocyte counts help doctors diagnose, stage, and treat chronic lymphocytic leukemia (CLL). Other factors, such as the risk level of the type of CLL you have, also play a role.

CLL is a slowly progressing type of leukemia that affects stem cells that will eventually become white blood cells called lymphocytes. It’s the most common type of leukemia in adults.

Because CLL affects cells that become lymphocytes, the number of lymphocytes in your blood is important for many aspects of CLL.

In this article, we cover how doctors use lymphocyte counts to diagnose, stage, and treat CLL.

Lymphocytes are a type of white blood cell. All white blood cells, including lymphocytes, are made in the bone marrow. As a part of your immune system, lymphocytes play an important role in protecting you from infections.

Lymphocyte counts are determined using a complete blood count (CBC). A CBC is a blood test that calculates the number of different types of blood cells, including lymphocytes, in a blood sample drawn from a vein in your arm. A lymphocyte count is the number of lymphocytes you have per microliter (uL) of blood.

A peripheral blood smear can also assess the number of lymphocytes in your blood. In this test, a healthcare professional spreads a blood sample on a slide and analyzes it under a microscope. They can also review the size and shape of different blood cells using this test.

In CLL, lymphocytic cells begin to grow and divide uncontrollably. This leads to high levels of lymphocytes in the blood, a condition called lymphocytosis.

In a healthy adult, lymphocyte counts usually fall between 1,000 and 4,800 lymphocytes per microliter (uL) of blood.

When your lymphocyte count is above the upper part of this range, you have lymphocytosis. According to a 2017 review, many people with CLL present to their doctor with lymphocyte counts over 10,000 lymphocytes per uL of blood.

If your lymphocyte count is high, your doctor will most likely order a blood flow cytometry test. A flow cytometry test can identify whether your cancer is CLL and not another form of leukemia.

In addition to diagnosing CLL, a flow cytometry test can help doctors stage your cancer, look for markers that determine how aggressive your cancer might be, and choose your initial treatment.

For a doctor to diagnose CLL, you must have 5,000 clonal B-lymphocytes per uL or more in your blood for at least 3 months. “Clonal” means the cells in question are all descended from the same original cell.

Staging helps determine the extent of your cancer.

Doctors use two staging systems for CLL. One of these, the Rai system, is used more often in the United States.

According to the American Cancer Society, the Rai stages are as follows:

Rai stageDescription
stage 0has lymphocytosis but no other physical signs
stage 1• has lymphocytosis and enlarged lymph nodes
• does not have an enlarged liver or spleen, anemia, or low levels of platelets
stage 2• has lymphocytosis and an enlarged spleen, liver, or both
• may or may not have swollen lymph nodes
stage 3• has lymphocytosis and anemia
• may or may not have swollen lymph nodes and an enlarged liver or spleen
stage 4• has lymphocytosis and low levels of platelets
• may or may not have swollen lymph nodes, an enlarged liver or spleen, or anemia

Many people with CLL may never require treatment. In these cases, a doctor routinely monitors them and checks their blood counts every few months.

The need for treatment depends on a few factors:

  • if a person is experiencing significant symptoms from their CLL, such as fever, drenching night sweats, severe fatigue, and weight loss
  • if a person has swollen lymph nodes that are large and bothersome, or if their other blood counts (hemoglobin and platelets) are significantly low due to the lymphocytes crowding out other blood cells in the bone marrow
  • if a person has a rapid doubling time of their lymphocytes (number doubling in fewer than 6 months) and their doctor deems their CLL to be high risk

Some research shows the trajectory of lymphocyte levels before diagnosis can predict the time to treatment for CLL. People whose lymphocyte levels were growing faster before diagnosis had a higher risk of needing treatment sooner.

If you need treatment for CLL, your doctor will monitor your lymphocyte levels throughout your treatment. Dropping lymphocyte levels can indicate a particular treatment is effective against your CLL.

Generally speaking, having a high lymphocyte count and a rapidly progressive lymphocyte count are both factors associated with a poorer outlook for someone with CLL. A 2017 review notes this is typically greater than 50,000 lymphocytes per uL.

However, lymphocyte counts are only one piece of the puzzle. Other factors important for your CLL outlook are:

Your care team will take all these factors into account to help predict your outlook and whether treatment is needed.

According to the SEER database of the National Cancer Institute, the average 5-year survival rate for CLL is 87.9%. However, your care team can give you a better idea of your outlook based off of your specific situation.

In CLL, cells that will eventually become lymphocytes begin to grow and divide out of control. Because of this, people with CLL have high lymphocyte counts.

Lymphocyte counts play an important role in many aspects of CLL. These include diagnosis, staging, and treatment.

While higher lymphocyte counts are associated with a worse CLL outlook, many other factors play vital roles as well. As such, be sure to have an open conversation with your care team for a better idea of your individual outlook.