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Complete blood count (CBC)

A complete blood count (CBC) is a blood test that measures the number and quality of red blood cells (RBCs or erythrocytes), white blood cells (WBCs or leukocytes) and platelets (thrombocytes).

A CBC includes the following information:

  • RBC count – the total number of RBCs
  • WBC count – the total number of WBCs
  • hemoglobin (Hb or Hgb) level – the total amount of hemoglobin in the blood
  • hematocrit (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  • The tourniquet is removed and the needle is withdrawn.
    • Mild discomfort may be felt when the needle is withdrawn.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:

  • bleeding
  • bruising
  • infection

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.

Common terminology to describe CBC results
TermWhat it means

anemia

a reduction in the number of RBCs or hemoglobin content of RBCs

leukopenia

a reduction in the number of WBCs

leukocytosis

an increase in the number of WBCs

neutropenia

a decrease in the number of neutrophils

thrombocytopenia

a decrease in the number of platelets

thrombocytosis

an increase in the number of platelets

Abnormal CBC Values
ComponentAn increased value may be due toA decreased value may be due to

WBC

  • viral infections
  • overwhelming bacterial infection
  • bone marrow suppression caused by some drugs, chemotherapy or radiation therapy
  • bone marrow disorders, such as leukemia or myelodysplastic syndrome (MDS)
  • anemia
  • immune system suppression

RBC

  • dehydration (such as from severe diarrhea)
  • kidney disease
  • some lung diseases
  • polycythemia vera (a myeloproliferative disorder)
  • anemia due to:
    • prolonged bleeding or blood loss (hemorrhage)
    • insufficient dietary intake of iron or certain vitamins
    • blood disorders
    • chronic disease
  • Hodgkin lymphoma and other lymphomas
  • blood related cancers, such as:
  • some myeloproliferative disorders

Platelets

  • prolonged bleeding or blood loss (hemorrhage)
  • iron deficiency anemia
  • infection (inflammation)
  • surgical removal of the spleen (splenectomy)
  • polycythemia vera (a myeloproliferative disorder)
  • some types of leukemia
  • bone marrow disease
  • diseases of the immune system
  • bacterial infection
  • viral infection
  • chemotherapy
  • radiation therapy
  • multiple blood transfusions
  • certain drugs, such as non-steroidal anti-inflammatory drugs (NSAIDs) including aspirin or ibuprofen

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.

red blood cell (RBC)

A 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.

Also called erythrocyte.

white blood cell (WBC)

A type of blood cell that helps the body fight infection and diseases.

White blood cells are made in the bone marrow and are found in the blood and lymphatic tissue. They play an important role in immune response (the immune system’s reaction to the presence of foreign substances in the body).

Also called leukocyte.

platelets

A type of blood cell that helps blood to clot. Platelets are made in the bone marrow.

Also called thrombocyte.

hemoglobin

A protein found in red blood cells that carries oxygen and gives blood its red colour.

hematocrit

The 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.

For example, a hematocrit of 25% means that there are 25 mL of red blood cells in a 100 mL blood sample.

myeloproliferative disorder

Any of a group of diseases that affect the bone marrow and cause large numbers of abnormal red blood cells, white blood cells or platelets to build up in the bone marrow and blood.

bronchogenic carcinoma

A cancerous (malignant) tumour that starts in the bronchi (the large tubes that connect the windpipe, or trachea, to each of the lungs).

Bronchogenic carcinomas include small cell lung cancer and non–small cell lung cancer.

colony-stimulating factor

A substance that stimulates the bone marrow to produce white blood cells, red blood cells and platelets.

Colony-stimulating factors are found naturally in the body or can be made in the lab.

Also called CSF or hematopoietic growth factor.

myelodysplastic syndrome (MDS)

A group of disorders that affect the bone marrow so it does not produce enough healthy mature blood cells. People with MDS have low white blood cell counts, low platelet counts and increased monocytes in some cases. Signs and symptoms include fever, easy bruising and bleeding, infections, paleness and malaise.

MDS is not cancer but can transform into acute myelogenous leukemia (AML). It may occur on its own or can be a side effect of radiation therapy or chemotherapy.

Also called preleukemia or smouldering leukemia.

multiple myeloma

A type of cancer that starts in plasma cells (white blood cells that produce antibodies to help the body fight infection) in the bone marrow.

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