A complete blood count (CBC) is a common test that can help identify many different conditions. It is useful in monitoring a person’s recovery from injuries, surgery, or another health condition.
This article discusses what a CBC measures and what the results mean.
A CBC is a common procedure that measures the different components in the blood.
Doctors will measure different blood cell levels and compare them with the expected levels for a person of the same age and sex. Any differences can indicate a condition or other problem.
It may be part of a routine health checkup, or a doctor may order the test when a person shows symptoms of an underlying health condition. It can also help monitor treatment or an existing health problem.
CBC tests provide a measure of overall health and can detect several conditions, such as anemia.
Depending on the reason for the test, it may be necessary to avoid eating or drinking before the test. This is because certain foods and beverages may influence the results.
Doctors will usually perform the test using one of two methods. They may insert a needle into a vein in the arm to withdraw blood, or they may use a tiny needle to prick the skin and take a drop of blood onto a piece of glass.
Both tests will include a small pricking sensation.
CBCs measure the three primary components of blood: red blood cells, white blood cells, and platelets.
Red blood cells
For red blood cells, a CBC measures:
- how many red blood cells are present, or red blood cell count
- the proportion of red blood cells to other cells, or hematocrit
- the physical features of the red blood cells, such as their volume
- the number of young red blood cells in circulation
Red blood cells contain hemoglobin, which carries oxygen around the body. They are typically flat and rounded with a dip toward the center.
The body produces red blood cells in the bone marrow before releasing them into the bloodstream.
Red blood cells have a lifespan of around 120 days, and the bone marrow must keep producing enough red blood cells to replace those that die or are lost through bleeding.
Some conditions can affect the production of red blood cells. For example, having a low red blood cell count can indicate anemia.
White blood cells
A CBC will measure the white blood cell count and the count of each white blood cell type.
White blood cells are a part of the immune system and fight off infections. The body produces these cells in the bone marrow. It stores 80–90% of them in the bone marrow until the immune system needs them.
Small numbers of white blood cells normally circulate in the blood. Having a high white blood cell count can indicate an infection or inflammatory condition.
There are several types of white blood cells. A rise in each type can mean something different. For example, an increase in neutrophils can indicate a bacterial infection, while increased eosinophils may be a sign of an allergy.
The CBC will also measure the:
- platelet count
- average platelet volume
- average platelet width
Platelets, or thrombocytes, are small blood cells that form blood clots to prevent excessive bleeding.
Platelets have a short lifespan of approximately 10 days. The body typically produces platelets when it needs to repair damage.
Some conditions can cause high or low platelet counts. For example, an autoimmune condition or an infection can cause a low platelet count.
Having a high platelet count increases the risk of blood clotting. Sometimes, the body temporarily increases platelet levels, such as after surgery or an injury. Some conditions, such as cancer, also cause long-term increases.
According to the Leukemia and Lymphoma Society, normal ranges for the main components of blood are as follows:
|Red cells per microliter (µL) of blood||White cells per µL of blood||Platelets per µL of blood||Hemocrit percentage of blood composed of red cells||Hemoglobin (grams per deciliter)|
It is worth noting that these are typical ranges. However, ranges may vary by the laboratory that analyzes them.
The results of a CBC can mean many different things.
Red blood cell count
A high red blood count is known as polycythemia.
Some causes of a high red blood cell count include:
- carbon monoxide exposure
- chronic lung disease
- alcohol use disorder
- heart conditions
- kidney disease
- liver disease
- polycythemia vera, which is a rare disease
Some causes of a low red blood cell count include:
- inflammatory bowel disease
- use of certain drugs
- bone marrow disorders
- chronic kidney disease
- iron, vitamin B12, or folate deficiency
Reticulocytes are young red blood cells. The amount of hemoglobin in these red blood cells can help determine the body’s iron status.
This can help identify an iron deficiency.
White blood cell count
According to one 2020 article, a high white blood cell count could occur due to:
- medication use
- an immune condition
- allergic reactions
A low white blood cell count, or leukopenia, can occur due to:
- bone marrow disorders
- autoimmune conditions
- severe infection, such as sepsis
- lymphoma, which is a type of cancer
- dietary deficiencies
- conditions that affect the immune system, such as HIV or AIDS
Immature granulocytes (IG) are immature white blood cells. These cells are typically only present in the bone marrow.
If there is a presence of IG, it may indicate an infection or a type of blood cancer. As a result, a person will require further investigation and tests.
The National Heart, Lung, and Blood Institute note that a high platelet count can occur due to:
- recovery from injury or surgery
- a vitamin B12 deficiency
- inflammatory conditions
Some causes of a low platelet count include:
- certain medications
- chronic bleeding
- autoimmune conditions
Immature platelet fraction
Immature platelet fraction (IPF) refers to the number of the newly released platelets, or reticulated platelets. The bone marrow produces these platelets, and they do not enter the bloodstream until they have matured.
When a person’s platelet numbers are low, it causes the bone marrow to produce platelets at a faster rate. As a result, the bone marrow is unable to keep up, and it releases the reticulated platelets into the bloodstream.
Low IPF results can suggest a decrease in platelet production, while high IPF results can indicate a loss of platelets in the blood.
Blood tests that include platelet counts can help determine if a person requires a platelet transfusion. They can also help monitor how the bone marrow recovers after a bone marrow transplant.
CBCs can provide information on a wide range of conditions and a person’s overall health.
A doctor may order one during a routine checkup or if there are signs and symptoms of an underlying problem.
People undergoing treatment for chronic conditions may regularly visit a doctor for a CBC.
Anyone experiencing persistent symptoms should contact a doctor for a checkup.
A CBC is a safe and common test for many conditions. Doctors may provide one as part of a routine checkup.
Many conditions affect the number and characteristics of red and white blood cells and platelets. A CBC can determine if these changes are different from what they should be and whether or not they require attention.