Complete Blood Count (CBC)

Complete Blood Count (CBC), is a blood test used to evaluate your overall health and detect a wide range of disorders, including anemia, infection, and leukemia. A complete blood count test measures several components and features of your blood, including:

Red blood cells, which carry oxygen
White blood cells, which fight infection
Hemoglobin, the oxygen-carrying protein in red blood cells
Hematocrit is the proportion of red blood cells to the fluid component, or plasma, in your blood
Platelets, which help with blood clotting

What is it used for?

A complete blood count is a common blood test that is often part of a routine checkup. Complete blood counts can help detect a variety of disorders including infections, anemia, diseases of the immune system, and blood cancers.

What does the test measure?

A CBC involves multiple measurements that include the number of blood cells and some of their physical features. A standard CBC includes several elements related to red blood cells, white blood cells, and platelets that are described in the following sections.

Red blood cell measurements

Red blood cells (RBCs) are also called erythrocytes. They carry oxygen from your lungs to the tissues and organs in your body. A CBC test includes several basic measurements of RBCs:

• RBC count is the total number of red blood cells in your blood sample.
• Hemoglobin measures the amount of this oxygen-carrying protein that is found inside RBCs.
• Hematocrit measures the proportion of your total blood volume that consists of red blood cells.

A CBC also provides details about the physical features of red blood cells. These are known as RBC indices. There are several kinds of RBC indices:

• Mean corpuscular volume (MCV) is a measurement of the average size of red blood cells.
• Mean corpuscular hemoglobin (MCH) is the average amount of hemoglobin inside each red blood cell.
• Mean corpuscular hemoglobin concentration (MCHC) is a calculated measurement of how concentrated hemoglobin is within red blood cells.
• Red cell distribution width (RDW) is a measurement of the variation in the size of your red blood cells.

The CBC may include the reticulocyte count, which is the total number of newly released young red blood cells in your blood sample. It may also be measured as a percentage.

White blood cell measurements

White blood cells (WBCs) are also called leukocytes. They are an important part of the body’s immune system.

A standard CBC measures the WBC count, which is the total number of white blood cells in a sample of blood.

A common variation of the CBC is the complete blood count with differential. The white blood cell differential is a breakdown of the amount of each of the five different types of WBCs:

• Neutrophils: Neutrophils make up the greatest percentage of WBCs and are produced by the bone marrow to fight a diverse array of inflammatory and infectious diseases.
• WBC
• Monocytes: Monocytes work in conjunction with neutrophils to combat infections and other illnesses while removing damaged or dead cells.
• Eosinophils: Eosinophils are WBCs that are activated in response to allergies and some types of infections.
• Basophils: Basophils are involved in the early identification of infections as well as wound repair and allergic reactions.

Initial blood testing may include a CBC with differential, or this test may be done after an initial standard CBC was abnormal. Because each white blood cell type has a different function, the CBC with differential can be used to identify abnormal levels of specific WBCs, which may offer clues about an underlying health concern.

Platelet measurements

Platelets, also called thrombocytes, are cell fragments that circulate in the blood and play an essential role in blood clotting. When there is an injury and bleeding begins, platelets help stop bleeding by sticking to the injury site and clumping together to form a temporary plug.

A standard component of the CBC is the platelet count, which is the number of platelets in your blood sample. In some cases, your doctor may have the laboratory also measure the mean platelet volume (MPV), which determines the average size of platelets.

How you prepare

If your blood sample is being tested only for a complete blood count, you can eat and drink normally before the test. If your blood sample will be used for additional tests, you may need to fast for a certain amount of time before the test. Your doctor will give you specific instructions.

Results

The following are normal complete blood count results for adults:

What the results may indicate

Results in the following areas above or below the normal ranges on a complete blood count may indicate a problem.

• Red blood cell count, hemoglobin, and hematocrit. The results of your red blood cell count, hemoglobin, and hematocrit are related because they each measure aspects of your red blood cells. If the measures in these three areas are lower than normal, you have anemia. Anemia causes fatigue and weakness. Anemia has many causes, including low levels of certain vitamins or iron, blood loss, or an underlying condition. A red blood cell count that’s higher than normal (erythrocytosis), or high hemoglobin or hematocrit levels, could point to an underlying medical condition, such as polycythemia vera or heart disease.
• White blood cell count. A low white blood cell count (leukopenia) may be caused by a medical condition, such as an autoimmune disorder that destroys white blood cells, bone marrow problems, or cancer. Certain medications also can cause white blood cell counts to drop. If your white blood cell count is higher than normal, you may have an infection or inflammation. Or, it could indicate that you have an immune system disorder or a bone marrow disease. A high white blood cell count can also be a reaction to the medication.
• Platelet count. A platelet count that’s lower than normal (thrombocytopenia) or higher than normal (thrombocytosis) is often a sign of an underlying medical condition, or it may be a side effect of the medication. If your platelet count is outside the normal range, you’ll likely need additional tests to diagnose the cause.

Is there anything else I need to know about a complete blood count?

A complete blood count is only one tool your healthcare provider uses to learn about your health. Your provider will consider your medical history, symptoms, and other factors to make a diagnosis.