A complete blood count (CBC) is a common medical test that your doctor may recommend to monitor your health. In cancer care, this blood test can be used to help diagnose a cancer or monitor how cancer or its treatment is affecting your body. For example, people undergoing chemotherapy often receive regular CBCs.
In cancer care, a CBC is used to:
Help diagnose some blood cancers, such as leukemia and lymphoma
Find out if cancer has spread to bone marrow
See how your body is being affected by cancer treatment
Diagnose and monitor noncancerous conditions during cancer treatment
A CBC is one of many ways that your doctor can monitor your health. Based on the results, your doctor may alter your treatment or prescribe new medications.
How is a CBC performed?
First, your doctor will order the CBC test be done as part of your medical care. Then, your blood will be drawn for a blood sample. Sometimes you will need to avoid eating or taking certain medications before getting a CBC to get the most accurate results. Your health care provider will let you know if this is the case.
After your blood is drawn, the sample is sent to a laboratory for analysis. Your health care provider will let you the results, such as through a phone call or your online patient portal. Learn more about how to manage anxiety when you get online medical test results.
What does a CBC measure?
A CBC test measures different substances in your blood, including your white blood cells, red blood cells, and platelets.
On your CBC, you may see the following measurements:
White blood cell count
White blood cell differential
Red blood cell count
Hemoglobin (Hg or Hgb)
Mean corpuscular volume (MCV)
Your doctor may measure other factors besides the ones listed above as part of a CBC. All of these factors have a "normal range." Your health care team will note this range on your CBC lab results. A range is used instead of a specific number because amounts vary from person to person. Your results may also indicate if your levels fall outside of these ranges.
What white blood cell tests are included in a CBC?
White blood cells protect your body from infection by attacking invading bacteria, viruses, and other foreign materials. Some white blood cells can also attack cancer cells.
There are usually 2 white blood count measurements in a CBC:
White blood cell count measures the number of white blood cells in your blood sample. It is also called a leukocyte count.
White blood cell differential measures the number of each type of the five major white blood cell types. Each type plays a role in protecting your body. Your doctor can learn valuable information about your health by measuring the levels of these cells.
These types of white blood cells are:
What red blood cell tests are included in a CBC?
Red blood cells are produced in the bone marrow and carry oxygen throughout your body. They also remove carbon dioxide from your body.
There are usually 4 red blood cell count measurements in a CBC:
Red blood cell count measures the number of red blood cells in a sample. It is also called an erythrocyte count.
Hematocrit measures the percentage of your blood that is made up of red blood cells.
Hemoglobin measures the amount of protein carrying oxygen in red blood cells
Mean corpuscular volume (MCV) measures the size of your red blood cells.
What platelet tests are included in a CBC?
Platelets are blood cells that form clots to stop bleeding. On your CBC test results, the platelet count measures the total number of platelets in your blood sample.
What do CBC results mean?
Your health care team will carefully read your CBC test results. Keep in mind that there are many factors, including noncancerous conditions, that can lead to results falling outside of the normal range. Ask your doctor to help you understand what your results mean. Common test results falling outside of normal ranges on a CBC are explained below.
Low white blood cell count. Some cancer treatments, mainly chemotherapy, can lower your white blood cell count.
Cancers that affect the blood can also lower white blood cell count. These types of cancers include leukemia, lymphoma, and multiple myeloma.
High and low results in the white blood cell differential. Having results that show higher or lower than normal numbers of certain white blood cells can mean different things.
Higher-than-normal numbers of lymphocytes or monocytes can indicate the possibility of certain types of cancer. Some cancers and their treatment may cause low numbers of neutrophils, a condition called neutropenia. Neutropenia can increase your chance of a bacterial infection.
If the CBC shows low numbers of neutrophils, the doctor may consider lowering the chemotherapy dose being given. This can help reduce the risk of developing neutropenia. You may also be prescribed medication, such as white blood cell growth factors, to increase your body's production of neutrophils, especially if you develop a fever.
Low red blood cell count. Some cancer treatments, such as chemotherapy and radiation therapy, can lower your red blood cell count. This condition is known as anemia.
Blood loss, either from surgery or certain types of cancer, and cancers that affect the bone marrow can also cause or worsen anemia.
People whose red blood cell count falls too low may need a blood transfusion or medication to help bring the level up.
Low platelet count. Some cancer treatments, such as chemotherapy or radiation therapy, can lower platelet count. Cancers that affect the bone marrow can also lower the platelet count. An unusually low number of platelets is called thrombocytopenia.
People with a low platelet level have a greater risk of serious bleeding or bruising. If your platelet count falls to very low levels, your doctor may recommend platelet transfusions.
Questions to ask your health care team
Your CBC test gives a snapshot of your health and your body's reaction to some cancer treatments. Ask your health care team if you have questions about the results or how to interpret them. Discuss how the results compare to previous tests and how they will be used to design or tailor your cancer treatments.
Questions to ask about your CBC test:
Why am I having this test?
How and where is this test done?
Do I need to avoid eating and drinking before the test? If so, for how long?
Do I need to stop taking any of my medications ahead of time for this test? If so, which medications? For how long?
How will I receive the results of my CBC test? When?
What do my CBC test results mean? Will someone explain them to me?
If my results are within a normal range, what are the next steps?
If my results are outside of a normal range, what are the next steps?
Am I at an increased risk for a problem, such as infections or bleeding, that I should watch out for based on these results?
How do these test results compare with my previous results?
- Will I need additional tests? If so, when?