At a Glance

Why Get Tested?

To determine your general health status; to screen for, diagnose, or monitor any one of a variety of diseases and conditions that affect blood cells, such as anemia, infection, inflammation, bleeding disorder or cancer

When To Get Tested?

As part of a routine medical exam; when you have signs and symptoms that may be related to a condition that affects blood cells; at regular intervals to monitor treatment or disease status or when you are receiving treatment known to affect blood cells

Sample Required?

A blood sample drawn from a vein in your arm or a fingerstick or heelstick (newborns)

Test Preparation Needed?

None

What is being tested?

The complete blood count (CBC) is a group of tests that evaluate the cells that circulate in blood, including red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs). The CBC can evaluate your overall health and detect a variety of diseases and conditions, such as infections, anemia and leukemia.

Blood cells are produced and mature primarily in the bone marrow and, under normal circumstances, are released into the bloodstream as needed. The three types of cells evaluated by the CBC include:

Red Blood Cells

Red blood cells, also called erythrocytes, are produced in the bone marrow and released into the bloodstream when they mature. They contain hemoglobin, a protein that transports oxygen throughout the body. The typical lifespan of an RBC is 120 days. Thus, the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect the production of new RBCs and/or their lifespan, in addition to those conditions that may result in significant bleeding.

RBCs normally are uniform in size and shape, but their appearance can be affected by a variety of conditions, such as vitamin B12 and folate deficiencies and iron deficiency. An example of a common condition affecting RBCs is anemia, which results from low red blood cell counts and low hemoglobin. Various diseases can lead to anemia, so additional tests are often needed to determine the cause. For more details, see the articles on Red Blood Cell Count, Hemoglobin, and Hematocrit.

White Blood Cells

White blood cells, also called leukocytes, are cells that exist in the blood, the lymphatic system, and tissues and are an important part of the body’s natural defense (immune) system. They help protect against infections and also have a role in inflammation, and allergic reactions. There are five different types of WBCs and each has a different function. They include neutrophils, lymphocytes, basophils, eosinophils, and monocytes.

WBCs are present in the blood at relatively stable numbers. However, these numbers may temporarily shift higher or lower depending on what is going on in the body. For instance, an infection can stimulate your bone marrow to produce a higher number of neutrophils to fight off a bacterial infection. With allergies, there may be an increased number of eosinophils. An increased number of lymphocytes may be produced with a viral infection. In certain diseases, such as leukemia, abnormal (immature or mature) white cells may rapidly multiply. For additional details, see the articles White Blood Cell Count and WBC Differential.

Platelets

Platelets, also called thrombocytes, are actually tiny cell fragments that circulate in blood and are essential for normal blood clotting. When there is an injury and bleeding begins, platelets help stop bleeding by adhering to the injury site and clumping together to form a temporary plug. They also release chemical signals that attract and promote clumping of additional platelets and eventually become part of a stable blood clot at the site of the injury that remains in place until the injury heals.

If you have a disease or condition that causes low platelets (thrombocytopenia) or dysfunction of platelets, you may be at an increased risk of excessive bleeding and bruising. An excess of platelets (thrombocytosis) can cause excessive clotting. For more information, see the article Platelet Count.

What is included in a CBC?

A CBC is typically performed using an automated instrument that measures various parameters, including cell counts and the physical features of some of the cells. A standard CBC includes:

Red blood cell (RBC) tests:

• Red blood cell (RBC) count is a count of the actual number of red blood cells in your blood sample.

• Hemoglobin measures the total amount of the oxygen-carrying protein in the blood, which generally reflects the number of red blood cells in the blood.

• Hematocrit measures the percentage of your total blood volume that consists of red blood cells.

• Red blood cell indices provide information on the physical features of the RBCs:

• Mean corpuscular volume (MCV) is a measurement of the average size of your red blood cells.

• Mean corpuscular hemoglobin (MCH) is a calculated measurement of the average amount of hemoglobin inside your red blood cells.

• Mean corpuscular hemoglobin concentration (MCHC) is a calculated measurement of the average concentration of hemoglobin inside your 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 also include reticulocyte count, which is a measurement of the absolute count or percentage of newly released young red blood cells in your blood sample.

White blood cell (WBC) tests:

• White blood cell (WBC) count is a count of the total number of white blood cells in your blood sample.

• White blood cell differential may be included as part of the CBC or may be done in follow up if the WBC count is high or low. The WBC differential identifies and counts the number of the five types of white blood cells present (neutrophils, lymphocytes, monocytes, eosinophils, and basophils). The individual count can be reported as an absolute count and/or as a percentage of total.

Platelet tests:

• The platelet count is the number of platelets in your blood sample.

• Mean platelet volume (MPV) may be reported with a CBC. It is a measurement of the average size of platelets.

• Platelet distribution width (PDW) may also be reported with a CBC. It reflects how uniform platelets are in size.

CBC results that are outside the established reference intervals may indicate the presence of one or more diseases or conditions. Typically, other tests are performed to help determine the cause of abnormal results. Often, a blood smear will be examined using a microscope. A trained laboratory professional will evaluate the appearance and physical features of the blood cells, such as size, shape and color, noting any abnormalities that may be present. This information gives the healthcare practitioner additional clues as to the cause of abnormal CBC results.