Complete Blood Count (CBC)

The CBC (complete blood count) is a simple blood test that measures the number of red blood cells, white blood cells, and platelets in the blood.

The CBC is a very common test. It is often used as a screening test that gives a picture of a person’s overall health status. The CBC can be used as a first step to screen for a wide range of medical conditions including inflammatory diseases. A CBC is usually ordered when a person has signs and symptoms of any of the following rheumatologic diseases:

  • Rheumatoid Arthritis
  • Ankylosing Spondylitis
  • Psoriatic Arthritis
  • Systemic Sclerosis
  • Giant Cell Arteritis
  • Granulomatosis with Polyangiitis
  • Microscopic Polyangiitis
  • Eosinophilic Granulomatosis with Polyangiitis
  • Behçet’s Disease
  • Takayasu Arteritis
  • Reactive Arthritis
  • Enteropathic Arthritis

If any component of a person’s CBC is found to be outside of its reference range (i.e. higher or lower than normal), other tests may be ordered for further investigation.

Science Behind this Test

Red Blood Cells

This part of the CBC measures the number of red blood cells (RBCs) in the blood as well as hemoglobin. Hemoglobin is important because it carries oxygen to the tissues. If there isn’t enough hemoglobin then the tissues don’t get enough oxygen. Some of the reasons why hemoglobin might be low include:

  • Bleeding
  • Iron deficiency (hemoglobin contains iron)
  • Chronic diseases such as Rheumatoid Arthritis can make it difficult for the bone marrow to make new RBCs
  • In some autoimmune diseases such as Systemic Lupus Erythematosus, the immune system attacks and destroys RBCs
  • Abnormal (cancerous) cells in the bone marrow can interfere with the body’s ability to make hemoglobin

White Blood Cells

This part of the CBC measures the number of white blood cells (WBCs) in the blood. WBCs are important for fighting infections. There are five major types of WBCs. Neutrophils are a type of WBC that are very important in fighting bacteria. A very low number of neutrophils can increase a person’s risk of infection.

Platelets

This part of the CBC measures the number of platelets in the blood. Platelets are small, stick cells. Their job is to stop bleeding. If there aren’t enough platelets, it can be difficult to stop bleeding.

Some of the reasons why platelets may be low include:

  • In some autoimmune diseases such as Systemic Lupus Erythematosus, the immune system attacks and destroys platelets
  • Platelets can get trapped in the spleen
  • Consuming too much alcohol can cause a reduction in platelets
  • Some medications can affect the number of platelets
  • Abnormal (cancerous) cells in the bone marrow can interfere with the body’s ability to make platelets

CBC results

The CBC test results are given as the number of cells per litre of blood. For example, a white blood cell count of 1.5 x 109 per litre. The test result will also give the “reference range” meaning the number of cells that are typically found in a healthy adult. For example, the reference range for white blood cells is 4.5 – 11.0 x 109 per litre.

The CBC results are important to consider because:

  • It can give a picture of a person’s overall health status
  • It can be used to screen for a wide range of medical conditions and inflammatory diseases
  • It can be used to monitor a person’s condition after a diagnosis is made
  • It can be used to assess a person’s response to treatment