Complete Blood Count (CBC)

Author: OncoLink Team
Last Reviewed: July 27, 2022

Blood tests can be done to see how well organs are working or to see how well treatments for a disease are working. Blood will be drawn and sent to a lab for testing. Your care provider will be able to talk to you about your results.

The complete blood count (CBC) is a blood test used to check a person’s general health. A CBC may be ordered as part of a routine exam or as part of a work-up when a person is having issues such as feeling tired, bleeding, or infection.

A CBC is a panel of tests that looks at the three main types of cells that make up blood. These are the white blood cells (WBC), red blood cells (RBC) and platelets. There are 5 types of WBCs. A ‘WBC differential’ counts the number of each type of WBC: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

Some therapies, such as chemotherapy and radiation, can stop the bone marrow from working like it should. The job of your bone marrow is to make blood cells. A CBC may be ordered on a routine basis to see the effects of cancer treatment on blood cells.

The range of values that is thought to be normal for most people is usually given with your lab test results.

There is nothing special you need to do before having a CBC. Some medications can affect the results of a CBC. Your provider should know about all medications and supplements you are taking.

Test done

What is it?

What does an abnormal value mean?



Cells that fight infection.

A low value (leukopenia) can be caused by a bone marrow issue, autoimmune issue, or infection. Radiation and chemotherapy can also lower values. A high value (leukocytosis) can be caused by leukemia, inflammation, allergy, or infection.


Act as first line of defense against infection. Neutrophils ingest and kill foreign particles.

A low value (neutropenia) can be caused by cancer treatments, bad infection (sepsis), autoimmune issues, and damage to bone marrow. A high value (neutrophilia) can be caused by bacterial infection, inflammation, physiological stress, and some types of leukemia.


Two types: “B” and “T”. B lymphocytes make antibodies. T lymphocytes defend against foreign cells (bacteria, cancer).

A low value (lymphocytopenia) can be caused by an autoimmune issue, infection, damaged bone marrow, and corticosteroids. A high count (lymphocytosis) can be caused by viral and bacterial infections, a chronic inflammatory disorder, stress, lymphocytic leukemia, and lymphoma.


Defend against infection by ingesting bacteria and foreign particles within the body.

A low value (monocytopenia) is not significant. But, if the value is low on a series of CBCs, it may be caused by damaged bone marrow or hairy cell leukemia. A high value (monocytosis) can be caused by chronic infection and certain types of leukemia such as monocytic or myelomonocytic.


Function in allergic responses and resist infection.

A low level of eosinophils is normal since the body is not always undergoing an allergic reaction. A high value (eosinophilia) can be caused by an allergic reaction, inflammatory disorders, and some leukemias and lymphomas.


Work also during allergic reactions by releasing histamine, which eases symptoms related to an allergic reaction.

A low level of basophils is normal since the body is not always undergoing an allergic reaction. A high value (basophilia) can be caused by the body’s response to inflammation, allergic reactions, and certain leukemias.

Red Blood Cell

(RBC, Erythrocyte)

Circulate in the blood and carry oxygen throughout the body.

A low value (anemia) can be caused by chemotherapy, radiation, bleeding, poor nutrition, bone marrow issue, and kidney failure. A high value (polycythemia) can be caused by dehydration, lung disease, and smoking.

Hemoglobin (Hgb)

The protein in RBCs that carries oxygen.

A low value can be caused by the destruction of blood cells within the body or the body not being able to make them. A high value can be caused by lung disease or overproduction by the body.

Hematocrit (Hct)

A measure of the percent of the total blood volume that is RBCs.

A low value can be caused by a decrease in the production of RBCs. A high value can be caused by dehydration.



Help normal blood clotting.

Low values (thrombocytopenia) can be caused by viral infection, sepsis, chemotherapy, radiation, certain medications, leukemia, and lymphoma. High values (thrombocytosis) can be caused by rheumatoid arthritis, anemia, or some types of cancer.

These values can be affected by the type of cancer that you have or the medications you are taking. Your health care team will help you to understand the results of this test.


Bennfleck-Shannon J, editor. Medical Tests Sourcebook. 2nd edition. Detroit (MI): Omnigraphics; 2004.

Fischbach FT. Manual of laboratory and diagnostic tests. 6th edition. Philadelphia: Lippincott Williams and Wilkins. 2000.

Lab tests online – Complete blood count. 2020.


September 1, 2023

Coming Soon: Medicare Drug Price Negotiations

by Christina Bach, MSW, LCSW, OSW-C

July 26, 2022


by Rodney Warner, JD

July 21, 2022

Five Questions With…Jessica, Katie, and Melanie

by OncoLink Team