Understanding Your White Blood Cell Differential

The white blood cell differential — often shortened to “the differential” — is a blood test that measures the proportions and absolute numbers of the different types of white blood cells in your blood. It is usually performed as part of a complete blood count (CBC), either automatically with every CBC or as a follow-up when an abnormality is identified.

The differential gives a much more detailed picture of immune function than the total white blood cell count alone. The same total number can mean very different things depending on which type of white blood cell is increased or decreased. This article explains what each cell type does, what the test measures, and what abnormal results may mean.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.

What is a white blood cell differential?

A white blood cell differential measures the prevalence of each of the five main types of white blood cells. White blood cells are the cells of the immune system, and each type has a different role in defending the body against infection, controlling inflammation, and responding to allergens, parasites, and abnormal cells.

The differential reports each cell type in two ways:

As a percentage of all the white blood cells in your blood (for example, “Neutrophils: 65%”). This shows the relative proportions of each cell type.
As an absolute count, expressed as cells per microlitre or per litre of blood (for example, “Absolute neutrophil count: 4,200 cells/µL”). The absolute count is calculated by multiplying the percentage by the total white blood cell count.

Both numbers are useful, but absolute counts are generally more clinically meaningful. A patient can have a “normal” percentage of one cell type but still have an abnormal absolute count if the total white blood cell count is unusually high or low. Most clinical decisions are based on absolute counts.

Why is a differential done?

A differential is performed for many of the same reasons as a CBC:

To investigate symptoms. The pattern of abnormalities in the differential helps narrow down the cause of fever, fatigue, recurrent infections, allergic symptoms, or unexplained inflammation.
To diagnose specific conditions. Certain patterns are highly suggestive of specific diagnoses — for example, a high lymphocyte count in a young adult may suggest viral mononucleosis, while a high eosinophil count may suggest an allergic reaction or parasitic infection.
To help diagnose blood cancers. Conditions such as acute myeloid leukemia, chronic lymphocytic leukemia, and chronic myeloid leukemia often produce characteristic differential patterns.
To monitor treatment. Patients receiving chemotherapy or other treatments that affect the bone marrow have their differentials checked frequently to monitor for low neutrophil counts and assess immune recovery.
To follow chronic conditions. Patients with autoimmune diseases, HIV, or other immune conditions have differentials checked regularly to track immune system function over time.

How is the test performed?

The differential uses the same blood sample as the CBC — there is no separate blood draw required. Most modern laboratories perform an automated differential, in which laboratory equipment counts and classifies white blood cells based on their physical and chemical properties.

If the automated differential produces unusual results or if specific abnormalities are suspected, a manual differential is performed. In a manual differential, a drop of blood is spread on a glass slide, stained, and examined under the microscope by a trained laboratory technician or a pathologist. The technician counts a fixed number of cells (usually 100 or 200) and identifies each one. A manual differential is more time-consuming but can detect abnormal or immature cells that automated counters may miss. The slide examined for the manual differential is also called a peripheral blood smear, and the smear may include additional observations beyond the counts themselves.

The five main types of white blood cells

The differential reports five main types of white blood cells. Each is described in detail below, with typical reference ranges and the conditions most commonly associated with high or low results. The percentages shown are typical reference ranges for adults; absolute counts are also widely used and are usually shown alongside the percentage on your report.

Neutrophils

Neutrophils are the most abundant type of white blood cell, typically accounting for 55%–70% of all white blood cells in adults. They are the body’s first responders to most bacterial and fungal infections, traveling quickly to the site of infection and engulfing and destroying the invading organisms.

The absolute neutrophil count (ANC) is one of the most clinically important numbers in the entire CBC, particularly for patients receiving chemotherapy. A typical reference range for the ANC in adults is 1,500–8,000 cells per microlitre.

The differential may also report band neutrophils, which are slightly immature neutrophils released from the bone marrow before fully developing. A typical reference range is 0%–3%. An increased number of bands (called a “left shift”) suggests the bone marrow is rapidly producing new neutrophils in response to a serious infection or inflammation.

Causes of a high neutrophil count (neutrophilia):

Bacterial infection, the most common cause
Acute inflammation, including conditions such as appendicitis or pancreatitis
Physical or emotional stress, recent surgery, or trauma
Pregnancy
Smoking
Corticosteroid medications such as prednisone
Bone marrow disorders, including chronic myeloid leukemia

Causes of a low neutrophil count (neutropenia):

Chemotherapy and other medications, the most common cause
Viral infections
Severe bacterial infections that overwhelm the bone marrow’s ability to produce cells (called sepsis)
Autoimmune diseases such as lupus
Vitamin B12 or folate deficiency
Bone marrow disorders, including aplastic anemia and acute myeloid leukemia

Neutropenia significantly increases the risk of serious infection. An ANC below 1,000 cells per microlitre is considered moderate neutropenia; below 500 is severe neutropenia, which often requires protective measures such as isolation or preventive antibiotics. Patients receiving chemotherapy whose ANC drops below a certain threshold may have their treatment delayed or have medications added to stimulate neutrophil production.

Lymphocytes

Lymphocytes are the second most common white blood cells, typically accounting for 20%–40% of the total in adults. They are the central cells of the adaptive immune system — the part of the immune system that learns to recognize specific threats and remembers them. There are three main subtypes: B lymphocytes (which produce antibodies), T lymphocytes (which directly attack infected or abnormal cells), and natural killer (NK) cells (which kill virus-infected cells and some cancer cells). The standard differential reports lymphocytes as a single category; further subtyping requires specialized testing such as flow cytometry.

Causes of a high lymphocyte count (lymphocytosis):

Viral infections, particularly mononucleosis (Epstein-Barr virus), cytomegalovirus, and acute viral hepatitis
Pertussis (whooping cough) and some other bacterial infections
Toxoplasmosis and other parasitic infections
Tuberculosis and chronic infections
Chronic lymphocytic leukemia (CLL) and other lymphoid cancers
Stress reactions and recovery from acute infection

Causes of a low lymphocyte count (lymphopenia):

Acute viral infections, including HIV in advanced stages
Corticosteroid medications
Chemotherapy and radiation therapy
Autoimmune diseases such as lupus
Severe stress, including major surgery or critical illness
Inherited immune deficiency disorders
Aging (a mild decline in lymphocyte count is common in older adults)

Monocytes

Monocytes are larger white blood cells that typically account for 2%–8% of the total. They circulate in the blood for a few days before migrating into tissues, where they mature into macrophages — large immune cells that engulf and digest microorganisms, dead cells, and debris. Monocytes are particularly important in chronic infections and in clearing damaged or abnormal cells from the body.

Causes of a high monocyte count (monocytosis):

Chronic bacterial infections such as tuberculosis
Recovery from acute infection
Inflammatory conditions such as inflammatory bowel disease and rheumatoid arthritis
Some viral infections
Certain blood cancers, including chronic myelomonocytic leukemia (CMML) and some types of acute leukemia

Causes of a low monocyte count (monocytopenia):

Chemotherapy
Severe infections
Aplastic anemia and other bone marrow failure conditions
Hairy cell leukemia, a specific type of blood cancer in which monocytopenia is a characteristic finding

Eosinophils

Eosinophils are white blood cells that typically account for 1%–4% of the total. They play an important role in allergic reactions and in the body’s defense against parasitic infections. They are named for the bright pink colour they take on when stained with a dye called eosin.

Causes of a high eosinophil count (eosinophilia):

Allergic conditions, including asthma, hay fever, eczema, and food allergies
Parasitic infections, particularly with worms
Drug reactions
Autoimmune and inflammatory conditions, including some forms of vasculitis and inflammatory bowel disease
Some cancers, including Hodgkin lymphoma and certain blood cancers
Hypereosinophilic syndrome, a rare condition in which eosinophils are persistently very high without an identified cause

Causes of a low eosinophil count (eosinopenia):

Acute stress responses, including major infection or surgery
Corticosteroid medications
Cushing’s syndrome (overproduction of cortisol by the body)

A low eosinophil count is common and rarely clinically significant on its own.

Basophils

Basophils are the least common type of white blood cell, typically accounting for only 0.5%–1% of the total. They release histamine and other chemicals during allergic reactions and play a role in inflammation. Their function in the immune system is less well understood than the other white blood cell types.

Causes of a high basophil count (basophilia):

Chronic myeloid leukemia (CML) and other myeloproliferative neoplasms — basophilia is a particularly characteristic feature of CML
Allergic reactions and chronic inflammation
Hypothyroidism
Some chronic viral infections

Causes of a low basophil count (basopenia):

Acute infections
Hyperthyroidism
Pregnancy and ovulation
Corticosteroid medications

A low basophil count is common in healthy people and is rarely clinically significant.

What about immature or abnormal cells?

In addition to the five main cell types, the differential may report other findings if abnormal or immature cells are present:

Bands. Slightly immature neutrophils, mentioned above. A small number is normal; an increased number suggests the bone marrow is responding rapidly to infection.
Blasts. The earliest, most immature blood-forming cells. Blasts are normally found only in the bone marrow, not in the blood. The presence of blasts in the blood is always abnormal and may indicate acute leukemia, severe infection, or another bone marrow disorder. Blasts in the blood usually prompt urgent investigation, often including a bone marrow biopsy.
Promyelocytes, myelocytes, and metamyelocytes. Intermediate stages between blasts and mature neutrophils. Their presence in the blood, sometimes called a “left shift” or “leukoerythroblastic picture,” may indicate severe infection, bone marrow stress, or a bone marrow disorder.
Atypical lymphocytes. Lymphocytes with unusual size or shape under the microscope. These are most commonly seen in viral infections such as mononucleosis, but can also be seen in some lymphoid cancers.
Reactive changes. A general term for white blood cells that look unusual but appear to be responding to an underlying condition such as infection or inflammation rather than indicating a primary blood disorder.

If immature or abnormal cells are reported, a peripheral blood smear is usually examined and additional testing may be ordered.

What happens after the differential?

If your differential is normal, no further investigation is usually needed. If a result is abnormal, the next steps depend on which cell type is affected, by how much, what other CBC findings are present, and what symptoms or other test results are available. Some possibilities include:

Repeat the test. Mildly abnormal results often resolve on their own. A repeat differential in a few days or weeks may be all that is needed.
Examine a peripheral blood smear. When abnormal cells are detected by automated counters, or when the pattern is unusual, a smear allows direct microscopic evaluation of the cells.
Order specific infection testing. Patterns suggestive of specific infections — viral mononucleosis, HIV, tuberculosis, parasitic infections — may prompt targeted blood testing.
Order flow cytometry. If the lymphocyte count is high or unusual lymphocytes are seen, flow cytometry can identify specific lymphocyte subtypes and help diagnose conditions such as chronic lymphocytic leukemia.
Refer to a hematologist. Persistent or significantly abnormal results, particularly when accompanied by other CBC abnormalities, may prompt referral to a specialist in blood diseases.
Perform a bone marrow biopsy. If multiple components of the CBC are abnormal, blasts are seen in the blood, or other findings suggest a bone marrow disorder, a bone marrow biopsy is often the next step. The article Understanding your bone marrow biopsy report explains this procedure in detail.

An abnormal differential, like an abnormal CBC, is a starting point rather than a diagnosis. Your doctor will interpret the results in the context of your symptoms, medical history, and other test findings.

Questions to ask your doctor

Were any cell types in my differential outside the reference range?
If a result is abnormal, what is the most likely cause given my symptoms and history?
Could any of my medications be affecting my results?
Were any immature or abnormal cells reported?
Do I need a peripheral blood smear or any other follow-up tests?
Should the test be repeated, and if so, when?
Should I be referred to a hematologist?
If I have a low neutrophil count, are there precautions I should take to avoid infection?