What Is a Complete Blood Count (CBC)?

A complete blood count, commonly referred to as a CBC, is a routine blood test that measures the quantity and proportion of different types of blood cells in the body. This test provides essential insights into a patient’s overall health and can help diagnose a variety of conditions, including infections, diseases, and blood disorders.

The CBC evaluates several key components of blood, including:

  • Red blood cells (RBCs): Transport oxygen throughout the body
  • White blood cells (WBCs): Help fight infection
  • Platelets: Assist in blood clotting
  • Hemoglobin: A protein in red blood cells that carries oxygen and aids in clotting
  • Hematocrit: The ratio of red blood cells to the fluid component of blood (plasma)

By analyzing these elements, a CBC can detect abnormalities that may indicate conditions such as anemia, leukemia, sickle cell disease, or ongoing infection. Because of its diagnostic value, the CBC is often one of the first tests ordered during a routine physical exam.

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How a CBC Is Performed

Obtaining a complete blood count is a simple and minimally invasive procedure. It involves drawing a small sample of blood from a vein—usually in the arm—through a quick process known as venipuncture. The test is safe, fast, and causes minimal discomfort.

CBCs may be ordered to:

  • Confirm overall health
  • Investigate signs of infection
  • Diagnose allergies
  • Identify blood clotting issues
  • Detect blood cell abnormalities such as anemia
  • Uncover underlying diseases

The Role of the Blood Smear Test

In some cases, a blood smear test is performed alongside the CBC for more detailed analysis. During this test, a drop of the patient’s blood is spread on a microscope slide, stained with a special dye, and examined under a microscope.

A blood smear allows doctors to evaluate the size, shape, and number of blood cells more precisely. This visual inspection can reveal abnormalities—such as irregularly shaped red blood cells—that may suggest specific conditions like sickle cell anemia. When combined with the CBC, the blood smear offers a clearer, more comprehensive picture of a patient’s health.