A differential blood test enables a doctor to determine how many white blood cells are in the body. There are five types of white blood cell, and the test also shows how many of each type are present.
The results provide information about the condition of a person's immune system and how it responds to diseases and other threats.
In this article, learn more about how doctors use the differential blood test and how they interpret its results.
A doctor will often order this test when trying to confirm a diagnosis.
They may be looking for signs of an acute illness, such as the flu or a urinary tract infection (UTI).
Or, they may be checking for a chronic condition, such as an autoimmune disorder or one that affects the bone marrow.
The bone marrow is responsible for producing white blood cells, so changes in white blood cell counts can indicate how well the bone marrow is functioning.
A doctor may order a differential blood test if a person has symptoms, such as:
While a differential blood test can indicate problems with the white blood cells, it will not be the only test that doctors use to make a diagnosis.
To perform the test, a nurse or medical professional draws a blood sample from a vein in the arm or finger. When testing an infant, a doctor will draw blood from the heel.
There is no need to fast or make any special preparations for a differential blood test.
There are five types of white blood cell in the body:
- Neutrophils: According to the American Association for Clinical Chemistry, neutrophils are the most common type of white blood cell. They are responsible for destroying bacteria in injured or infected tissue.
- Monocytes: Monocytes are similar to neutrophils. They destroy bacteria, but usually those causing chronic infections. Monocytes also play a role in repairing damaged tissues.
- Eosinophils: These are responsible for treating infections caused by parasites. Eosinophils also control the immune system's response to allergic reactions.
- Basophils: Basophils are the least common type of white blood cell. Their function is still unclear, but they may play a role in allergic reactions.
- Lymphocytes: There are three types of lymphocytes. B lymphocytes produce antibodies to attack specific viruses, bacteria, and other foreign invaders. T lymphocytes help to identify cells that require an immune response. Natural killer cells, the third type, destroy cancer cells and viruses.
Each type of white blood cell plays an essential role in the immune system.
When a person receives their differential blood test results, they should also get a reference range of normal values from the laboratory.
Looking at this reference range can help a person to tell if their white blood cell levels are low, normal, or high.
Overall, a higher-than-average white blood cell count may indicate the presence of an infection.
Some labs give a percentage of the cells that are present. While different labs have different ranges, the following is an example of a normal range:
- Basophils: 0.5–1.0 percent
- Eosinophils: 1–4 percent
- Lymphocytes: 20–40 percent
- Monocytes: 2–8 percent
- Neutrophils: 40–60 percent
Other labs may give the number of each type of white blood cell present. Labs usually express these figures in the thousands.
Normal values for neutrophils are typically between 2,500 and 6,000 cells. A person with a very low neutrophil count will have fewer than 1,000 of these cells. Doctors call this neutropenia.
Normal levels also depend on gender, age, and pregnancy. For this reason, it is important to examine laboratory results carefully when determining if levels are high or low.
While the results of a differential blood test give information about all five types of white blood cell, a doctor is usually focusing on just one or two types.
Depending on the type of cell, high or low levels can indicate different issues, such as:
- High: A basophil count can point to certain types of leukemia, including chronic myeloid leukemia. A high count can also indicate that a person has severe allergic reactions. People with inflammatory disorders, such as rheumatoid arthritis or ulcerative colitis, may also have high basophil counts.
- Low: A low basophil count does not typically suggest a medical condition. However, stress, allergic reactions, steroid use, and hyperthyroidism can each cause a basophil count to be low.
- High: A high eosinophil count tends to result from an allergic reaction, such as asthma, eczema or a reaction to a medication. Inflammatory disorders, such as celiac disease or inflammatory bowel disease (IBD), can also cause high eosinophil levels.
- Low: Eosinophils are usually present in such low quantities that low readings do not tend to indicate issues. However, stress or steroid use can also cause an eosinophil count to be low.
- High: A high lymphocyte level can indicate an acute viral infection, such as chicken pox, herpes, or hepatitis. Or, a lymphocyte count may be high because of a bacterial infection, such as tuberculosis or pertussis, or a condition such as lymphocytic leukemia or lymphoma.
- Low: A low lymphocyte level can point to an autoimmune disorder, such as lupus or rheumatoid arthritis. The presence of HIV, tuberculosis, hepatitis, or the flu can also cause a lymphocyte count to be low.
- High: A high monocyte count can result from a chronic infection, such as tuberculosis, or a fungal infection. The presence of a condition such as endocarditis (bacterial inflammation of the heart), IBD, monocytic leukemia, juvenile myelomonocytic leukemia, scleroderma, or rheumatoid arthritis can also cause a count to be high.
- Low: Most doctors do not consider a single low monocyte count to be significant. However, low monocyte results on several tests can indicate hairy cell leukemia or bone marrow damage.
- High: A high neutrophil level can indicate an acute bacterial infection, inflammation, tissue death (such as after a heart attack), stress on the body, or chronic leukemia. A level may also be high because a person is in the last trimester of pregnancy.
- Low: A neutrophil count may be low after an adverse drug reaction or chemotherapy treatments. Illnesses, such as myelodysplastic syndrome, autoimmune disorders, bone marrow cancers, and aplastic anemia can also cause low neutrophil counts.
A differential blood test is one of many lab tests that a doctor can use to confirm a diagnosis of an infection or illness.
Values can vary from lab to lab, and a person should carefully review their results with the doctor.