The white blood cell differential assesses the ability of the body to respond to and eliminate infection. It also detects the severity of allergic and drug reactions plus the response to parasitic and other types of infection. It is essential in evaluating the reaction to viral infections and response to chemotherapy. It can also identify various stages of leukemia.

The white blood cell differential is normally run as part of the complete blood count (CBC), which is ordered for many different reasons.

The results indicate the percentage of each type of white blood cell that is present.

Neutrophils can increase in response to bacterial infection or inflammatory disease. Severe elevations in neutrophils may be caused by various bone marrow disorders, such as chronic myelogenous leukemia. Decreased neutrophil levels may be the result of severe infection or other conditions, such as responses to various medications, particularly chemotherapy.

Eosinophils can increase in response to allergic disorders, inflammation of the skin, and parasitic infections. They can also increase in response to some infections or to various bone marrow disorders. Decreased levels of eosinophils can occur as a result of infection.

Basophils can increase in cases of leukemia, chronic inflammation, the presence of a hypersensitivity reaction to food, or radiation therapy.

Lymphocytes can increase in cases of viral infection, leukemia, cancer of the bone marrow, or radiation therapy. Decreased lymphocyte levels can indicate diseases that affect the immune system, such as lupus, and the later stages of HIV infection.

Monocyte levels can increase in response to infection of all kinds as well as to inflammatory disorders. Monocyte counts are also increased in certain malignant disorders, including leukemia. Decreased monocyte levels can indicate bone marrow injury or failure and some forms of leukemia.

Since percentages might be misleading in some patients, absolute values of the various types of WBCs can also be reported, such as the absolute neutrophil count (ANC), also known as the absolute granulocyte count or AGC. Absolute values are calculated by multiplying the number of WBCs by the percentage of each type of white cell and can aid in diagnosing illness and monitoring therapy.

Eating, physical activity, and stress may alter white blood cell differential values.

Long-term use of steroids or long-term exposure to toxic chemicals (such as lye or insecticides) can increase the risk of an abnormal differential.