A complete blood count (CBC) is a blood test that measures the number and quality of red blood cellsred blood cellsA type of blood cell that carries oxygen to and carbon dioxide from tissues in the body. Red blood cells contain hemoglobin (a protein that carries oxygen and gives blood its red colour) and are made in the bone marrow. (RBCs or erythrocytes), white blood cellswhite blood cellsA type of blood cell that helps the body fight infection and diseases. (WBCs or leukocytes) and plateletsplateletsA type of blood cell that helps blood to clot. Platelets are made in the bone marrow. (thrombocytes).
A CBC includes the following information:
- RBC count – the total number of RBCs
- WBC count – the total number of WBCs
- hemoglobinhemoglobinA protein found in red blood cells that carries oxygen and gives blood its red colour. (Hb or Hgb) level – the total amount of hemoglobin in the blood
- hematocrithematocritThe proportion (by volume) of red blood cells in a blood sample. Hematocrit is a measure of both the number and size of red blood cells in the blood sample. (Hct) – the fraction of the blood made up of RBCs
- mean corpuscular volume (MCV) – the average RBC size
- mean corpuscular hemoglobin (MCH) – the average amount of hemoglobin per RBC
- mean corpuscular hemoglobin concentration (MCHC) – the average concentration of hemoglobin per RBC
- platelet count – the number of platelets
- WBC differential count – the different types of WBCs expressed as a percentage
Why a CBC is done
A CBC may be done to:
- provide information about a person’s general health
- assess how well the blood-forming organs (for example, bone marrow and spleen) are functioning
- check for anemia (a reduction in the number of healthy red blood cells)
- check for infection
- provide a baseline to compare with future CBCs during and after treatment
- monitor the effects of therapy, especially therapies that can cause bone marrow suppression (a condition in which the bone marrow does not produce normal numbers of red blood cells, white blood cells and platelets)
How a CBC is done
A CBC test is usually done in a private laboratory or hospital laboratory. No special preparation is usually needed.
- Blood is usually taken from a vein in the arm.
- A tourniquet or elastic band is wrapped around the upper arm to apply pressure to the area and make the vein swell.
- The person may be asked to open and close the fist to make the veins stand out more.
- The skin is cleaned and disinfected.
- A needle is inserted into the vein and a small amount of blood is removed.
- The person will feel a prick or stinging sensation.
- The sample is collected in a tube and labelled with the person’s name and other identifying information.
- The tourniquet is removed and the needle is withdrawn.
- Mild discomfort may be felt when the needle is withdrawn.
- Pressure is applied to the area where the needle was inserted until bleeding stops.
- A band aid may be applied.
- The sample is sent to a laboratory to be analyzed by special machines, examined under a microscope or both.
Potential side effects
Potential side effects of having a CBC include:
What the results mean
Normal ranges may vary from person to person and laboratory to laboratory. Many factors can affect CBC results. An abnormal result will not necessarily identify the problem, so further investigation may be needed.
Note: Not all factors or conditions that can increase or decrease blood counts are listed above. Only the main cancer-related ones have been included.
What happens if a change or abnormality is found
The doctor will decide if more tests, procedures, follow-up care or treatment are needed. Sometimes therapies or doses may need to be adjusted to avoid complications if blood cell counts are low.