A complete blood count (CBC) is one of several blood tests for leukemia. A CBC measures the main types of blood cells: platelets (thrombocytes), red blood cells (erythrocytes), and white blood cells (WBCs, also called leukocytes). While leukemia typically causes high WBC counts, it can also cause your levels to drop. Here’s why.

The Role of White Blood Cells

The bone marrow manufactures WBCs. Bone marrow is the spongy tissue in the center of the body’s larger bones. Under normal conditions, the bone marrow produces plenty of WBCs of various types. There are five primary WBC types:

• Basophils
• Eosinophils
• Lymphocytes
• Monocytes
• Neutrophils

Each type plays a specific role, responding to potential threats in specific ways. In general, normal WBCs protect against infection from invading bacteria, viruses, and other pathogens (infection-causing organisms). WBCs are the soldiers of the immune system.

Normal White Blood Cell Counts

The normal concentration of WBCs differs for adults and children. According to the Leukemia & Lymphoma Society, normal blood WBC counts fall within the following ranges:

• 5,000 to 10,000 per microliter of blood for men
• 4,500 to 11,000 per microliter of blood for women
• 5,000 to 10,000 per microliter of blood for children

The different types are counted by a measure called the WBC differential. The differential is the ratio of the various WBC types. In a normal blood count, the WBC differential includes:

• 55 percent to 70 percent neutrophils
• 20 percent to 40 percent lymphocytes
• 2 percent to 8 percent monocytes

The rest of the WBCs are basophils and eosinophils.

How Does Leukemia Affect White Blood Cells?

When someone has leukemia, their body overproduces WBCs. These extra “leukemia cells” don’t function properly. Unlike their normal counterparts, they can’t fight infection from viruses and bacteria, leaving the immune system vulnerable to attack.

Cancer cells crowd out normal cells in the bone marrow and interfere with the production of red blood cells and platelets. Additionally, leukemia and cancer treatments often affect blood counts.

White Blood Cells in Chronic Lymphocytic Leukemia

One type of leukemia, chronic lymphocytic leukemia (CLL), can cause very high WBC counts. Elevated WBCs may be one of the main signs of CLL. Doctors closely monitor blood counts in CLL because these numbers help guide treatment decisions.

People with CLL may be given an intense treatment combination over the course of a year that may include:

• Venetoclax (Venclexta)
• Obinutuzumab (Gazyva)
• Ibrutinib (Imbruvica)

Some of these therapy options list a low WBC count — also called leukopenia — as a potential side effect.

High vs. Low White Blood Cell Counts in Leukemia

Doctors monitor WBC counts in people with leukemia to track how the disease is responding to treatment or progressing.

Low White Blood Cell Count

Certain types of leukemia are especially associated with a low WBC count. These include:

• Acute promyelocytic leukemia
• Hairy cell leukemia
• Acute megakaryoblastic leukemia

Any type of leukemia can eventually show low WBC counts due to bone marrow failure, a condition in which the bone marrow is unable to produce normal amounts of healthy blood cells. Additionally, a low WBC count could signal an infection. It can also mean the person is not getting good nutrition or enough iron.

Both leukemia and some therapies to treat it can tax the immune system and reduce WBC levels. For example, chemotherapy is very effective at killing fast-growing cells, but it attacks both healthy blood cells and leukemia cells. That’s why low WBC counts are common during cancer treatment. A high blood cell count usually returns to normal on its own once treatment is over.

High White Blood Cell Count

A high WBC count may indicate that the body is fighting an infection. It may also be an indicator of leukemia. High WBC counts can also be a side effect of certain medications used in leukemia treatments, including chemotherapy and radiation therapy. WBC counts can temporarily increase after recovery from treatment. Health care providers may use CBC results to decide if they should adjust treatment regimens.

Other Conditions That May Cause Low White Blood Cell Counts

Having a low or high WBC count isn’t always an indication of leukemia. Five percent of people will experience a high or low WBC count in their lifetime. In fact, several noncancerous conditions can lead to an abnormal WBC count. For example, autoimmune diseases like rheumatoid arthritis can cause high WBC counts. Certain racial and ethnic groups may naturally have a lower normal range of circulating WBCs.

How To Avoid Infections

A person with leukemia is susceptible to foreign pathogens (viruses, bacteria, and allergens) and less equipped to fight off and heal from infections. This is especially true while undergoing chemotherapy. Having a low WBC count means that some infections may become life-threatening, so it’s important to take steps to help cut your risk of getting sick when your immune system isn’t in its strongest fighting form.

Avoid Situations That May Put You at Risk

You can pick up a bacterial infection (such as pneumonia or an ear infection) from the air, water, soil, or food. You can catch a viral infection (like a cold or the flu) from being around another person with the infection. Remain aware of your surroundings, the activities you get involved in, and the number of people you interact with. When you’re at increased risk of infection, it’s best to avoid high-risk situations.

If you do get sick or suspect that you’re not well, promptly addressing the illness is vital. If you think you may have an infection, reach out to a health care provider on your leukemia treatment team immediately.

Wash Your Hands

Keeping your hands clean is another effective way to help avoid catching a secondary infection. Wash your hands thoroughly and often — ideally with antibacterial soap and hot water — especially after using the toilet, being in a public space, and before eating or preparing food. You can also use antiseptic hand sanitizer when you’re away from a sink or bathroom.

Learn About Your Diagnosis and Treatment Side Effects

Understanding your diagnosis and how it affects your immune system may help you avoid secondary infections. Your risk may vary depending on your leukemia’s progression and treatment phase. Knowing the risks to your health can better inform your choices and the level of risk you feel comfortable taking. It’s also important to know the symptoms of a possible infection, such as fever, so that you can contact your health care provider at the first sign of illness. Be sure to also ask your doctor about potential side effects of treatment, such as fatigue.

Lower Stress Levels

Being under a great deal of stress can take a toll on the immune system. Learning of a cancer diagnosis, undergoing leukemia treatment, and making necessary life adjustments can be stressful. Consider calling on your support system, seeing a therapist, or trying mindfulness and meditation to help reduce your stress where possible.

Read about ​​8 Ways To Live Better With CLL, including how to stay positive and manage stress.

Connect With Others Who Understand

If you have leukemia, it can help to have the support of others who understand. MyLeukemiaTeam is the social network for people with leukemia and their loved ones. More than 17,000 members come together to ask questions, give advice, and share their experiences with others who understand life with leukemia.

Have your blood tests shown low white blood cell counts? At what points in the diagnosis or treatment of your leukemia did you get these results? Share your thoughts in the comments below, or start a conversation by posting on your Activities page.