Complete Blood Count (CBC)

About the Test

Purpose of the test

The purpose of a CBC is to give your health care provider details about the state of your health. It is an important medical tool because it uses one sample to analyze the complete spectrum of cells found in the blood as well as some of the characteristics of those cells.

Because it provides information about every type of cell in the blood, the CBC can provide information related to a wide variety of medical problems.

The primary uses for the CBC are diagnosis, monitoring, and screening:

• Diagnosis is determining the cause of a patient’s symptoms. The CBC can identify many different abnormalities in the blood that can be linked to distinct medical problems. For this reason, the CBC is frequently used as a diagnostic test. In many cases, it can confirm or rule out certain conditions and may be used alongside other tests to arrive at a definitive diagnosis.
• Monitoring is the process of following a patient’s condition over time. A CBC can be used to monitor patients who have previously been diagnosed with blood cell disorders. It can help see how a person’s condition has responded to treatment and may be used to watch for side effects of some medical treatments.
• Screening is testing to find health problems before there are any symptoms. In some cases, a doctor may prescribe a CBC as a screening test during routine check-ups.

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 RBCs, WBCs, and PLT that are described in the following sections.

Red blood cell measurements

RBC, also called erythrocytes, 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 RBCs 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 RBCs.

A CBC also provides details about the physical features of RBC. These are known as RBC indices, of which there are several kinds:

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

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

White blood cell measurements

WBCs, also called leukocytes, are an important part of the body’s immune system.

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

A common variation of the CBC is the CBC with differential. The WBC 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 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 abnormal initial standard CBC. Because each WBC 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 (PLT), 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, PLT 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 PLT count, which is the number of PLT in your blood sample.

In some cases, your doctor may have the laboratory also measure the mean PLT volume (MPV), which determines the average size of PLT.

When should I get a complete blood count?

The CBC is a very common test. You may have a CBC performed when you have a routine health examination, are being diagnosed or assessed for a disease or condition, or when your provider is monitoring the effectiveness of a treatment.

Because blood counts can be affected by a diverse range of health conditions, the CBC may be recommended by your doctor to help identify the cause of many different kinds of symptoms. Your health care provider is in the best position to address whether a CBC is appropriate in your specific situation.

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