White Blood Cell (WBC) Differential
As part of a complete blood count (CBC), when you have a routine health examination; when results of a CBC fall outside the reference range; when you have any number of signs and symptoms that may be related to a condition affecting white blood cells, such as infection, inflammation, or cancer; when you are receiving treatment that is known to affect WBCs, such as chemotherapy
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (infants)
White blood cells (WBCs), also called leukocytes, are cells that circulate in the blood and the lymphatic system that help protect the body against infections. They are an important part of the body's immune system and also have a role in inflammation, allergic responses, and protection against cancer. A WBC differential totals the number of each of the different types of WBCs in a person's sample of blood.
There are five types of white blood cells, each with different functions. The differential reveals if the cells are present in normal proportion to one another, if the number of one cell type is increased or decreased, or if abnormal and/or immature cells are present. This information is helpful in diagnosing specific types of illnesses that affect the immune system and the bone marrow.
A differential may be performed in conjunction with a complete blood count (CBC), a test often used as a general health check, or it may be performed in follow-up to abnormal results on a CBC. Most often, a differential is performed on an automated blood analyzer but also may be performed manually by a trained laboratorian who examines a blood smear under a microscope. The values are typically reported as absolute numbers of cells but may be expressed as the relative percentages of the total number of WBCs.
White blood cells develop from precursor cells produced in the bone marrow. The five different types of WBCs include:
- Granulocytes—these white blood cells have granules in their cytoplasm. The granules contain chemicals and other substances that are released as part of an immune response. The three types of granulocytes include:
- Neutrophils (neu) normally make up the largest number of circulating WBCs. They move into an area of damaged or infected tissue, where they engulf and destroy bacteria or sometimes fungi.
- Eosinophils (eos) respond to infections caused by parasites, play a role in allergic reactions (hypersensitivities), and control the extent of immune responses and inflammation.
- Basophils (baso) usually make up the fewest number of circulating WBCs and are thought to be involved in allergic reactions.
- Lymphocytes (lymphs) exist in both the blood and the lymphatic system. They are divided into three types, but the differential does not distinguish among them. All lymphocytes differentiate from common lymphoid progenitor cells in the bone marrow. The differential counts and reports all lymphocytes together. Separate specialized testing (like immunophenotyping) must be done to differentiate the three types:
- B lymphocytes (B cells) are antibody-producing cells that are essential for acquired, antigen-specific immune responses. Plasma cells are fully differentiated B-cells that produce antibodies, immune proteins that target and destroy bacteria, viruses and other "non-self" foreign antigens.
- T lymphocytes (T cells) finish maturing in the thymus and consist of a few different types. Some T cells help the body distinguish between "self" and "non-self" antigens. Others initiate and control the extent of an immune response, boosting it as needed and then slowing it as the condition resolves. Other types of T cells directly attack and neutralize virus-infected or cancerous cells.
- Natural killer cells (NK cells) directly attack and kill abnormal cells such as cancer cells or those infected with a virus.
- Monocytes (mono), similar to neutrophils, move to an area of infection and engulf and destroy bacteria. They are associated more often with chronic rather than acute infections. They are also involved in tissue repair and other functions involving the immune system.
When there is an infection or an inflammatory process somewhere in the body, the bone marrow produces more WBCs, releasing them into the blood. Depending on the cause of infection or inflammation, one particular type of WBC may be increased as opposed to other types. As the condition resolves, the production of that type of WBC subsides and the number drops to normal levels again.
In addition to infections and inflammation, there are a variety of conditions that can affect the production of WBCs by the bone marrow or their survival in the blood, resulting in either increased or decreased numbers. The differential, along with the other components of the CBC, alerts the healthcare provider to possible health issues. Results are often interpreted in conjunction with additional tests such as a blood smear review, which can reveal the presence of abnormal and/or immature populations of WBCs.
In a few serious diseases, some immature forms of the cells are released from the bone marrow into the circulation and may be detected by the WBC differential. This may occur with bacterial infection, leukemia, bone marrow involvement by solid tumor, myelodysplastic syndrome, or myeloproliferative neoplasms, for example. Some immature cells that may be detected include metamyelocytes, myelocytes, promyelocytes, and/or blasts.
If results indicate a problem, a wide variety of other tests may be performed in order to help determine the cause. A healthcare provider will typically consider an individual's signs and symptoms, medical history, and results of a physical examination to decide what other tests may be necessary. For example, as needed, a bone marrow biopsy will be performed to evaluate the bone marrow status.
How is the sample collected for testing?
A blood sample is drawn from a vein in the arm or from a fingerstick (for children and adults) or heelstick (for infants).
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The white blood cell differential is often used as part of a complete blood count (CBC) as a general health check. It may be used to help diagnose the cause of a high or low white blood cell (WBC) count, as determined with a CBC. It may also be used to help diagnose and/or monitor other diseases and conditions that affect one or more different types of WBCs.
The differential totals the number of each type and determines if the cells are present in normal proportion to one another, if one cell type is increased or decreased, or if immature cells are present. This information is useful in helping to diagnose the specific cause of an illness, such as:
- Infections caused by bacteria, viruses, fungi or parasites
- Allergies, asthma
- Immune disorders (e.g., autoimmune disorders, immune deficiency)
- Leukemia (e.g., chronic myeloid leukemia, chronic lymphocytic leukemia)
- Myelodysplastic syndrome
- Myeloproliferative neoplasms (e.g., myelofibrosis)
Some diseases trigger a response by the immune system that causes an increase in certain types of WBCs. A differential may give clues to the specific cause of that immune response. For example, it may help determine whether an infection is caused by bacteria or by viruses.
Other conditions affect the production of certain WBCs by the bone marrow or their survival in the circulation, resulting in either an increase or decrease in their number. A differential informs the healthcare provider as to which type of WBC is low or high.
An abnormal differential result may be followed by other tests such as a blood smear, bone marrow biopsy, chromosome analysis, or immunophenotyping (e.g., flow cytometry). These tests can reveal the presence of abnormal and/or immature populations of WBCs.
When is it ordered?
The differential is often performed as part of the complete blood count (CBC), which may be ordered at the time of a routine health exam.
- Fever, chills
- Body aches, pain
- A variety of other signs and symptoms, depending on the site of suspected infection or inflammation
If a differential is not done at the same time as a CBC, it may be ordered when results from the CBC are not within the reference ranges.
What does the test result mean?
The results indicate the number and/or the percentage of each type of white blood cell that is present in a person's sample of blood.
Results of a differential are usually reported as absolute values of the five types of WBCs and/or may be reported as a percent of the total number of WBCs. Absolute values are calculated by multiplying the total number of WBCs by the percentage of each type of white cell. This information can aid in diagnosing illness and monitoring therapy. Neutrophils typically make up the highest number of WBC, followed by lymphocytes, then monocytes, eosinophils and basophils.
Care must be taken when interpreting the results of a differential. A healthcare provider will consider an individual's signs and symptoms and medical history as well as the degree to which the cells are increased or decreased. A number of factors can cause a transient rise or drop in the number of any one type of cell. A persistent increase or decrease will usually prompt further testing to determine the cause.
The following table gives some examples as to what the results of a differential may indicate:
Possible Causes of High and Low WBC Differential Results
Type of WBC Abbreviations Examples of causes of a high count Examples of causes of a low count Neutrophils (Absolute neutrophil count, percent neutrophils)
Neu, Polys, PMNs, ANC, % Neu
Known as neutrophilia
- Acute bacterial infections and also some infections caused by viruses and fungi
- Inflammation (e.g., inflammatory bowel disease, rheumatoid arthritis)
- Tissue death (necrosis) caused by trauma, major surgery, heart attack, burns
- Physiological (stress, rigorous exercise)
- Pregnancy—last trimester or during labor
- Chronic leukemia (e.g., myelogenous leukemia)
Known as neutropenia Lymphocytes (Absolute lymphocyte count, percent lymphocytes
Lymphs, lym, ly, ALC, % lymphs
Known as lymphocytosis Known as lymphopenia or lymphocytopenia Monocytes (Absolute monocyte count, percent monocytes)
Monos, AMC, % monos
Known as monocytosis
- Chronic infections (e.g., tuberculosis, fungal infection)
- Infection within the heart (bacterial endocarditis)
- Collagen vascular diseases (e.g., lupus, scleroderma, rheumatoid arthritis, vasculitis)
- Inflammatory bowel disease
- Monocytic leukemia
- Chronic myelomonocytic leukemia
- Juvenile myelomonocytic leukemia
Known as monocytopenia
Usually, one low count is not medically significant.
Repeated low counts can indicate:
- Bone marrow damage or failure
- Hairy-cell leukemia
Eosinophils (Absolute eosinophil count, percent eosinophils)
Eos, AEC, % eos
Known as eosinophilia Known as eosinopenia
This is often difficult to determine because numbers are normally low in the blood. One or an occasional low number is usually not medically significant.
Basophils (Absolute basophil count, percent basophils)
Baso, ABC, % baso
Known as basophilia
- Rare allergic reactions (e.g., hives, food allergy)
- Inflammation (rheumatoid arthritis, ulcerative colitis)
- Some leukemias (e.g., chronic myeloid leukemia)
Known as basopenia
As with eosinophils, numbers are normally low in the blood; usually not medically significant.
In certain cases, immature and/or abnormal forms of the cells may be present in the blood and may be detected with a differential. Immature forms include metamyelocytes, myelocytes, promyelocytes, and/or blasts. Further work-up (e.g., bone marrow biopsy) may be necessary.
Is there anything else I should know?
Can a white blood cell (WBC) differential be performed on samples other than blood?
Yes. WBC differentials, along with WBC counts, may be performed on many different types of body fluids. A common reason that this is done is to more directly assess one area of the body that may be infected or inflamed. For example, if meningitis is suspected, then a WBC count plus differential may be performed on a sample of cerebrospinal fluid (CSF). Many other examples are listed in the article on Body Fluid Analysis.
If I have an abnormal result on my WBC differential, what other tests might my healthcare provider order?
Other general tests to check your health may include a comprehensive metabolic panel (CMP). Depending on your signs, symptoms, medical history, physical exam and suspected condition, your healthcare provider may choose to order a variety of other tests. A few general examples include:
• Autoimmune diseases: ANA
• Allergies: Allergy tests
My report mentions a "left shift." What does this mean?
A "left shift" is a phrase used to note that there are a high number of young, immature white blood cells present. Most commonly, this means that there is an infection or inflammation present and the bone marrow is producing more WBCs and releasing them into the blood before they are fully mature. This is a natural immune response to infection and inflammation.
My complete blood count (CBC) report includes a result for immature granulocytes (IG). What are they?
Some automated hematology analyzers report the total number of immature granulocytes (IG) present in a person's blood sample. Immature granulocytes are white blood cells that have not fully developed before being released from the bone marrow into the blood. They may include metamyelocytes, myelocytes, and promyelocytes. These cells are normally only present in the bone marrow because they are precursors of neutrophils, the predominant type of white cells in blood. The presence of immature granulocytes in the blood may occur in various diseases, such as infection or a blood cancer, and thus will often prompt further investigation, which may include additional laboratory testing.
On This Site
Tests: White Blood Cell Count, Complete Blood Count, Blood Smear, Bone Marrow Aspiration and Biopsy, Immunophenotyping, Chromosome Analysis
Conditions: Bone Marrow Disorders, Leukemia, Myeloproliferative Neoplasms, Lymphoma, Autoimmune Diseases, Allergies
Elsewhere On The Web
National Institute of Allergy and Infectious Diseases: Immune System
American Society of Hematology: Blood Disorders
Kidshealth.org: Immune System
MedlinePlus Medical Encyclopedia: Immune Response
National Cancer Institute: Leukemia