Complete blood count (CBC)

A complete blood count (CBC) is a blood test that examines the numbers and features of blood cells. The 3 types of cells it examines are red blood cells (RBCs), white blood cells (WBCs) and platelets.

A CBC measures the following:

• total number of red blood cells (the RBC count)
• total amount of hemoglobin in the blood
• percentage of blood made up of red blood cells (the hematocrit)
• average red blood cell size (the mean corpuscular volume)
• average weight of hemoglobin per red blood cell (the mean corpuscular hemoglobin)
• average amount of hemoglobin per red blood cell (the mean corpuscular hemoglobin concentration)
• total number of white blood cells
• number of each type of white blood cell (the WBC differential), including neutrophils (the absolute neutrophil count, or ANC)
• number of platelets (the platelet count)

A CBC is a common blood test. It is often done as part of a routine checkup, but can be done at any time.

A CBC may be done to:

• learn information about your general health
• check how well the bone marrow and spleen are working
• help diagnose diseases and conditions that affect blood cells, such as anemia, infection, blood disorders or leukemia
• provide a baseline to compare with future CBCs
• check for bone marrow suppression
• monitor a condition (as a part of follow-up)

A CBC is usually done in a community lab or hospital.

You may be given special instructions to follow before having a CBC done. Some medicines may affect CBC results, so you may be asked to stop taking certain medicines before a CBC. Check with the lab to see if you should avoid any medicines and for how long.

Blood is usually taken from a vein in the arm. An elastic band (tourniquet) is wrapped around your upper arm to apply pressure to the area and make the veins easier to see. You may be asked to make a fist so 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. You may feel a prick or stinging sensation.

The blood is collected in a tube and labelled with your name and other identifying information. Sometimes more than one tube of blood is collected. The tourniquet is removed and the needle is withdrawn. You may feel mild discomfort when the needle is withdrawn. Pressure is applied to the area where the needle was inserted until bleeding stops. A small bandage may be put on the area.

The blood collected is examined by a specialist in the lab (a lab technologist) using microscopes and other special equipment.

A CBC does not usually cause any side effects. If side effects happen, they are usually minor and happen at the needle site. Side effects that may happen include:

• discomfort
• bleeding
• bruising
• swelling
• infection

CBC results are given as numbers and often depend on certain factors including sex, age and medical history. They should be compared to a normal reference range or to previous results to have meaning. Normal ranges for CBCs may vary slightly from lab to lab.

A CBC usually provides general information that can give doctors clues to possible health problems. Information from a CBC helps doctors decide whether other tests or procedures are needed to make a diagnosis. The information may also help your doctor develop or revise treatment plans.

A doctor familiar with your medical history and overall health is the best person to explain your CBC results and what they mean for you.

Common terms used to describe CBC results are:

• anemia – not having enough healthy RBCs or hemoglobin
• leukopenia – a low number of WBCs
• neutropenia – a low number of neutrophils
• leukocytosis – an increased number of WBCs
• thrombocytopenia – a low number of platelets
• thrombocytosis – an increased number of platelets

CBC results can be low or high for many reasons. Some examples of abnormal CBC results related to cancer are listed below.

A low RBC count may be due to:

• anemia due to prolonged bleeding or blood loss (hemorrhage), a diet lacking iron or certain vitamins, certain types of chemotherapy, blood disorders or chronic disease
• Hodgkin lymphoma and other lymphomas
• cancers of the blood, such as leukemia and multiple myeloma
• some myeloproliferative disorders

A high RBC count may be due to:

• dehydration, such as from severe diarrhea
• kidney tumours
• lung diseases
• polycythemia vera (a myeloproliferative disorder)

A low WBC count may be due to:

• viral infection
• severe bacterial infection
• bone marrow suppression caused by treatments like chemotherapy or radiation therapy
• bone marrow diseases, such as leukemia or myelodysplastic syndrome (MDS)
• anemia

A high WBC count may be due to:

• infection
• leukemia
• some myeloproliferative disorders
• some types of cancer, such as bronchogenic carcinoma
• certain drugs, such as colony-stimulating factors
• stress, allergies or tissue injury

A low platelet count may be due to:

• some types of cancer, such as leukemia or lymphoma
• autoimmune diseases
• bacterial infection
• viral infection
• chemotherapy or radiation therapy
• having many blood transfusions
• certain drugs, such as nonsteroidal anti-inflammatory drugs (NSAIDs) including acetylsalicylic acid (ASA, Aspirin, salicylate) and ibuprofen (Motrin, Advil, Nuprin)

A high platelet count may be due to:

• prolonged bleeding or blood loss (hemorrhage)
• anemia from low iron levels
• infection (inflammation)
• surgical removal of the spleen (splenectomy)
• polycythemia vera
• some types of leukemia

Your doctor may recommend more tests, procedures, follow-up care or treatment. Sometimes therapies or doses may need to be changed to avoid problems when blood cell counts are low.

Blood samples may need to be taken from very small blood vessels (called capillaries) in a finger or heel of an infant or young child. This is done by pricking the skin with a needle.

Preparing children before a test or procedure can help lower their anxiety, increase their cooperation and develop their coping skills. This includes explaining to children what will happen during the test, such as what they will see, feel and hear.

Parents or caregivers can help reassure and prepare children by giving them brief but accurate information about what will happen, such as:

• A big rubber band that feels like a balloon will be placed around your arm. This will feel tight, like someone is squeezing your arm.
• When your skin is cleaned it will feel cold.
• When the needle is placed in your arm, you will feel a prick or pinch. It may sting or hurt a little, or it may not hurt at all.
• Once the needle is in your arm, you will see the blood come out into a tube. You can look away if you don’t want to watch this.

Children may worry about not having enough blood after some has been taken from their arm. You can reassure them that bodies make new blood all the time and that their body will not run out of blood.

Parents or caregivers can help distract children during blood tests to help make it easier for them.

Toddlers (1 to 2 years) might like to watch bubbles or toys that move or make sounds, such as magic wands, light-up toys or pinwheels. They may want to hold their favourite toy.

Preschoolers (3 to 5 years) may want to hold a favourite toy or watch toys that light up and make sounds.

School-age children (6 to 12 years) may want to:

• look at items, such as magic wands, light-up toys, video games and picture or “search and find” books
• bring their favourite stuffed animal or toy to hold
• blow bubbles or practise deep breathing while blowing bubbles
• imagine their favourite place
• hear a joke or story or tell you a joke or story

Teenagers (13 to 18 years) can try deep breathing or imagining their favourite place. They also might want to hear a joke or story.

Preparing a child for a blood test depends on the age and experience of the child. Find out more about helping your child cope with tests and treatments.