As blood continuously circulates through the body, it performs many vital functions—including carrying oxygen to and waste products from body tissues, helping to fight infection, and maintaining blood vessel integrity. Whole blood is made up of 2 major components: plasma (which is essentially water that contains hundreds of dissolved substances) and blood cells (which include red and white blood cells and platelets). A complete blood count (CBC) assesses the number and size of these different types of cells.

The following tests are commonly included in a CBC, or in some cases may be performed alone:

Red blood cell (RBC) count estimates the number of RBCs in the blood. The primary function of RBCs is to carry hemoglobin or oxygen from the lungs to the body’s tissues and to carry carbon dioxide (a waste product) from the tissues back to the lungs for elimination. A low RBC count indicates anemia, low red blood cell count, while a high count carries a risk that the RBCs will clump together and block capillaries.

Hemoglobin—the iron-containing pigment that gives RBCs their red color—is a protein that enables the RBCs to transport oxygen and carbon dioxide. The total hemoglobin test estimates the amount of hemoglobin in the blood, and gives an idea of the blood’s ability to carry oxygen. (Though not part of a CBC, a laboratory technique called electrophoresis may be used to detect the presence of abnormal hemoglobins, which often do not work as well as normal ones.)

Hematocrit—which assesses the amount of space (volume) RBCs take up in the blood—is determined by measuring the volume of RBCs after a blood sample is spun in a centrifuge. (Values are given as a percentage of the blood that is red cells.)

Red blood cell indices incorporate the results of the RBC count, hemoglobin, and hematocrit tests to provide further information about the size, hemoglobin concentration, and hemoglobin content of an average RBC. These values help to categorize different types of anemia.

Reticulocyte count measures the number of newly formed RBCs, or reticulocytes, in the blood and gives an idea of RBC production by the bone marrow. (This value is expressed as a percentage of the total RBC count.) This test helps to distinguish between different types of anemias, or to monitor treatment for anemia.

White blood cell (WBC) count estimates the number of WBCs in the blood. These cells—which fight infection and promote wound healing—are drawn to sites of infection and inflammation, where they engulf and digest invaders such as bacteria and other microorganisms or foreign bodies. This test may be done alone to detect the presence of an infection (values may rise dramatically) or to monitor response to cancer treatment.

White blood cell differential measures the proportion of the various types of WBCs in the blood as well as their structure. (There are five different kinds of WBCs that respond in varying degrees to the presence of infection or inflammation.) This test helps to determine the severity of an infection, suggests the type of organism responsible, and also provides important information about the immune system.

Platelet count is an estimate of the number of platelets in the blood. Platelets, or thrombocytes, are blood cells that clump together, or aggregate, at sites of blood vessel injury and work together with various clotting factors to promote the formation of blood clots. This process, called coagulation, is essential to stop bleeding and repair tissue injuries.

Blood smear is a test that involves spreading blood on a slide which is then stained with a special dye and examined under a microscope. This test can provide additional diagnostic information to a CBC by identifying changes in cell color, size, and shape and the type of cells in circulation, as well as the presence of cell inclusions. For example, sickle cell anemia can be detected by the presence of characteristic unusually shaped red cells, and leukemia by the presence of very immature white cells (blast cells) in the blood.

Purpose of the CBC

  • Used as a general screening test to provide information about the state of a person’s health, for example, before a scheduled surgery
  • To detect or evaluate blood cell disorders, such as anemia (an abnormal decrease in RBCs), polycythemia (an excess of RBCs), leukopenia (an abnormal decrease in WBCs), leukemia (a type of cancer that affects the WBCs), and thrombocytopenia (a decrease in blood platelets)
  • To monitor drug therapy and drug toxicity
  • To detect blood loss, abnormal production or destruction of blood cells, acute and chronic infection, allergies, and blood clotting disorders

Who Performs CBC

  • A nurse or a technician will draw the blood sample

Special Concerns about CBC

  • None

Before the CBC

  • Certain medications may alter CBC test values. Inform your doctor of any medications you regularly take. You may be asked to discontinue certain of these agents before the test.

What You Experience during CBC

  • Nurse or technician ties a rubber strap (tourniquet) around the upper arm to mildly restrict the blood flow, keeping blood in the vein.
  • A needle with an attached tube is inserted into a vein, usually in the bend of the elbow or the top of the hand.
  • Once the blood sample is obtained, the needle is withdrawn, a bandage is placed over the puncture site and firm pressure is applied until the bleeding stops.
  • and the blood sample is sent to a laboratory for analysis.
  • CBC takes less than five minutes.

Risks and Complications of CBC

  • CBC is considered a safe procedure. Risks associated with having blood drawn include fainting or feeling lightheaded, lump or bruise (hematoma) at the site caused by blood accumulating under the skin, and pain (especially if multiple punctures are needed to locate a vein).

After the CBC

  • Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.
  • Resume your normal activities and any medications withheld before the test.
  • Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.

CBC Results

  • Your blood sample is sent to a laboratory for analysis. The doctor will review the results—and consider them along with your symptoms, your physical exam, and the results of other tests—for evidence of a blood disorder or another problem.
  • A CBC is often used as an initial test to indicate potential health problems. Abnormal results may necessitate additional tests, such as a bone marrow aspiration and biopsy, to establish a definitive diagnosis.
  • If an abnormality is found and the doctor can make a definitive diagnosis, appropriate treatment will begin.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 04 Jan 2012

Last Modified: 23 Oct 2014