A white blood cell count and white cell differential is used to screen for or diagnose a wide range of conditions that change the number of white blood cells (WBCs). These include infection, immune deficiency, bone marrow disease, inflammation, allergy, and those disorders that affect the shape or size of the white blood cells. 

White blood cells are composed of several different types of cells and the test counts both the total number of white cells and the number of the individual cell types — this is called a white cell differential.

A WBC is often performed as part of a full blood count (FBC) when you have a routine health examination. It can also be used to monitor your condition when you are undergoing treatment that affects white blood cells. 

What is being tested?

The white blood cell (WBC) count indicates the number of white blood cells in a sample of blood. This count provides a clue to the presence of illness. White blood cells are made in the bone marrow and protect the body against infection and aid in the immune response. If an infection develops, white blood cells attack and destroy the bacteria or other micro-organisms causing the infection.

Blood is composed of three main types of cells; red blood cells, white blood cells and platelets that are suspended in fluid, called plasma. All these cells are made in the bone marrow and are released into circulation throughout the body.

There are five types of WBCs that each have a different function:

• Three types are referred to as Granulocytes because they contain granules in their cytoplasm that release chemicals as part of the immune response, these include:
  • neutrophils: most common type of circulating WBCs. These cells move to the damaged or infected tissue where they engulf and destroy bacteria
  • eosinophils: respond to infections caused by parasites, play a role in allergic reactions and control the extent of immune responses
  • basophils: least commonly found in circulation and are involved in allergic reactions.

• Lymphocytes: these cells exist in the lymphatic system and blood and are sub divided into three types:
• B lymphocytes (B cells): produce antibodies as part of the immune response
• T lymphocytes (T cells): recognise foreign substances and processes them for removal
• Natural Killer cells (NK cells): directly attack and destroy cancerous cells or cells infected with a virus
• Monocytes: similar to neutrophils, migrate to the site of infection, engulf and destroy bacteria.

When a person has an inflammatory process or infection somewhere in the body, the bone marrow will produce more WBCs and release them into the circulation where they migrate to the site of inflammation or infection. The bone marrow will cease to produce WBCs and levels will drop back to normal when the condition resolves.

Immune disorders and cancer can also affect the production of WBCs by the bone marrow, this will result in either an increase or decrease number of WBCs in the blood. Results are often interpreted along with a WBC differential.

How is it used?

The WBC count is used as part of the full blood count (FBC) to:

• Screen for a variety of diseases and conditions
• To help diagnose an infection, an allergy, or leukaemia and to monitor the progression of these conditions
• To help monitor the body’s response to various treatments and to monitor bone marrow function as treatments such as radiation and chemotherapy are known to affect WBCs.

Conditions or drugs that weaken the immune system, such as HIV infection or chemotherapy, cause a decrease in white blood cells. The WBC count detects dangerously low numbers of these cells.

When is it requested?

A WBC count is normally ordered as part of the full blood count (FBC), which is requested for a wide variety of reasons. A WBC count also may be used to monitor recovery from illness. Counts that continue to rise or fall to abnormal levels may indicate that the condition is getting worse. Counts that return to normal indicate improvement.

What does the result mean?

An elevated number of white blood cells is called leukocytosis. This can result from:

• bacterial infections 
• inflammation or inflammatory conditions like vasculitis or rheumatoid arthritis
• leukaemia, myeloproliferative neoplasms
• allergies
• conditions that result in tissue death such as trauma or stress or surgery.

A WBC count of 11.0 – 17.0 x 109/L cells would be considered mild to moderate leukocytosis.

A decreased WBC count is called leukopenia. It can result from many different situations, such as:

• lymphoma
• chemotherapy, radiation therapy
• autoimmune disorders – where the body attacks and destroys its own WBCs
• diseases of the immune system (e.g. HIV)

A count of 3.0 – 3.5 x 109/L cells would be considered mild leukopenia.

Is there anything else I should know?

Eating, physical activity and stress can cause an increased WBC count.

Pregnancy in the final month and labour may be associated with increased WBC levels.

If you have had your spleen removed, you may have a persistent mild to moderate increased WBC count.

The WBC count tends to be lower in the morning and higher in the late afternoon. WBC counts are age-related.

On average, normal newborns and infants have higher WBC counts than adults. It is not uncommon for the elderly to fail to develop leukocytosis as a response to infection.

There are many drugs that cause both increased and decreased WBC counts.

Common questions

• Are there different types of WBCs?

Yes. There are five main types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Immature forms may also be present in certain conditions. These include metamyelocytes, myelocytes, promyelocytes and blast cells.

• How do you treat abnormal WBC levels?

Treatment depends upon the cause. Infections usually cause increased WBC counts and may be treated with antibiotics. Leukaemias require chemotherapy and other treatments.

More information

RCPA Manual: White cell count

RCPA Manual: White cell count differential