Complete Blood Count (CBC)

About The Test

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 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.
• Lymphocytes: Lymphocytes such as B-cells and T-cells are found primarily in the lymph system and fight bacteria and other pathogens in the blood.
• 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 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 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.