How to cbc

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Last updated: April 4, 2026

Quick Answer: A Complete Blood Count (CBC) is a common blood test used to evaluate your overall health and detect a wide range of conditions, including anemia, infection, and leukemia. It measures different components of your blood, such as red blood cells, white blood cells, and platelets. Your doctor will typically order a CBC as part of a routine physical exam or if you have symptoms of illness.

Key Facts

A CBC measures red blood cells, white blood cells, and platelets.
It helps diagnose conditions like anemia, infection, and certain cancers.
The test requires a small sample of blood, usually drawn from a vein in your arm.
Results are typically available within a few hours to a couple of days.
No special preparation is usually needed for a CBC, though your doctor might advise fasting in some cases.

What is a Complete Blood Count (CBC)?

A Complete Blood Count (CBC) is one of the most common laboratory tests that healthcare providers order. It provides a broad picture of your general health and can help detect a wide array of diseases and conditions, such as anemia, infection, bleeding disorders, immune system disorders, and cancer. The test analyzes the cellular components of your blood, giving valuable information about their number, size, and shape.

What Does a CBC Measure?

The CBC test analyzes several key components of your blood:

Red Blood Cells (RBCs): These cells are responsible for carrying oxygen from your lungs to your body's tissues and returning carbon dioxide to your lungs to be exhaled. The CBC measures the number of red blood cells, as well as their size (mean corpuscular volume, MCV) and how much hemoglobin they contain (mean corpuscular hemoglobin, MCH). Low red blood cell counts can indicate anemia, while high counts might suggest dehydration or other conditions.
White Blood Cells (WBCs): Also known as leukocytes, white blood cells are crucial for fighting infection and disease. The CBC measures the total number of white blood cells and can also differentiate between the different types of white blood cells (differential count), such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils. An elevated WBC count often indicates an infection or inflammation, while a low count can suggest an immune deficiency or bone marrow problem.
Platelets: These small cell fragments help your blood clot, preventing excessive bleeding. The CBC measures the number of platelets in your blood. Low platelet counts can lead to increased bleeding and bruising, while very high counts can increase the risk of blood clots.
Hemoglobin (Hgb): This is the protein in red blood cells that carries oxygen. The CBC measures the total amount of hemoglobin in your blood. Low hemoglobin levels are a hallmark of anemia.
Hematocrit (Hct): This measures the proportion of your blood volume that is made up of red blood cells. Like hemoglobin, a low hematocrit can indicate anemia.

Why Is a CBC Performed?

A CBC is a versatile test that can be used for several purposes:

Routine Health Screening: It's often included in a general physical examination to check for underlying health problems you might not be aware of.
Diagnosing Symptoms: If you're experiencing symptoms like fatigue, weakness, fever, bruising, bleeding, or signs of infection, a CBC can help pinpoint the cause.
Monitoring Chronic Conditions: For individuals with known conditions like anemia, kidney disease, or cancer, a CBC helps monitor the effectiveness of treatment and track disease progression.
Monitoring Medical Treatments: Certain medications, such as chemotherapy drugs or immunosuppressants, can affect blood cell counts. A CBC is used to monitor these effects and adjust dosages if necessary.

How is a CBC Performed?

Getting a CBC is a straightforward process. A healthcare professional, typically a nurse or phlebotomist, will draw a small sample of blood, usually from a vein in the crook of your elbow. They will clean the area, apply a tourniquet to make the vein more prominent, and then insert a needle to collect the blood into a collection tube. The procedure is generally quick and causes only minor discomfort, similar to a brief pinch.

What to Expect After the Test

Once the blood sample is collected, it's sent to a laboratory for analysis. The results are usually available within a few hours to a couple of days, depending on the laboratory and whether a differential count is included. Your doctor will review the results with you and discuss any implications or further steps needed.

Preparing for a CBC

In most cases, no special preparation is required for a CBC. You can typically eat, drink, and take your regular medications as usual. However, your doctor may ask you to fast (not eat or drink anything except water) for several hours before the test if it's being done in conjunction with other blood tests that require fasting, such as a lipid panel or glucose test. It's always best to follow your doctor's specific instructions regarding preparation.