What is a Complete Blood Count?

What is a Complete Blood Count?

Overview of a complete blood count

A complete blood count (CBC) is an easy and standard test that screens for some health disorders.

Your doctor can determine if your blood cell count increases or decreases with a CBC. The typical value of a complete blood count varies depending on your gender and age. Therefore, your lab report will indicate the average value range for your gender and age.

A CBC helps diagnose a wide range of conditions, from infection to anemia and even cancer.

Types of blood cells

By measuring changes in your blood cell levels, your doctor can conveniently evaluate your overall health and detect any underlying disorders. There are three types of blood cells, and all can be measured via specific blood tests.

Red blood cells

Red blood cells transport oxygen throughout the body. They also help in flushing out carbon dioxide. A complete blood count (CBC) measures two components of the red blood cell: hemoglobin and hematocrit.

Low levels of hematocrit and hemoglobin are usually indicators of anemia. Anemia is a condition that occurs when the iron is deficient in the blood.

White blood cells

White blood cells are the body’s soldiers. They help your body fight infection. A complete blood count measures the number of white blood cells in your body. It also determines the type of white blood cells in your body. Any white blood cell count abnormality could indicate inflammation, infection, or cancer.


Platelets help with blood clotting and control of bleeding. If your wound or cut stops bleeding, it’s because you have healthy platelets that are doing their job pretty well. Any alterations in platelet levels can increase your risk of bleeding and indicate a severe underlying medical condition.

When is the right time to do a complete blood count?

A CBC may be done as part of a routine check-up. It may also be done if you have unexplained symptoms like bruising or bleeding. The results of a CBC can help your doctor to:

  • Evaluate your overall health
  • Diagnose a health condition
  • Monitor your health problem
  • Monitor your treatment

Preparing for a CBC

Be sure to wear a short-sleeved shirt or a shirt with sleeves that can easily be rolled up.

You can have your meals before a CBC. There are no restrictions per se. However, your healthcare provider may require that you do a fast before the test. This is common in situations where the blood sample will be used for further testing. Your healthcare provider will guide you on how to go about this.

What happens during a complete blood count?

During a blood count, some blood will be drawn from a vein from the back of your hand or inside of your elbow. The test does not take more than a few minutes. The lab technician will:

  • Clean your skin with an antiseptic wipe
  • Place a tourniquet or an elastic band around your upper arm. This will cause the vein to swell with blood.
  • A needle will then be inserted in your vein and a blood sample collected in one or more vials.
  • Takes off the elastic band
  • It covers the area from which blood was drawn with a bandage. This will help to stop any bleeding.
  • The blood sample will be labeled and taken to a laboratory for analysis.

You may feel some slight discomfort during a blood test. For example, when a needle punctures your skin, you might feel a pinching sensation or a prick. Sometimes, one may feel lightheaded or faint at the sight of blood. Afterward, there may be some minor bruising, which usually clears up in a couple of days.

Most results can be gotten within a few hours after testing.

Complete blood count average values table

Blood componentNormal levels
red blood cellIn men: 4.32-5.72 million cells/mcL
In women: 3.90-5.03 million cells/mcL
hemoglobinIn men: 135-175 grams/L
In women: 120-155 grams/L
hematocritIn men: 38.8-50.0 percent
In women: 34.9-44.5 percent
white blood cell count3,500 to 10,500 cells/mcL
platelet count150,000 to 450,000/mcL

It is important to note that a complete blood count is not a definitive diagnostic test. Excessively high or low blood counts could indicate a wide range of conditions. Specialized tests are required to diagnose a specific situation. Health conditions that could cause an abnormal count include:

  • Bleeding disorders
  • Deficiencies of vitamins, minerals, or iron
  • Heart disease
  • Bone marrow problems
  • Autoimmune disorders
  • Cancer
  • Reaction to medication

If your CBC levels are abnormal, your healthcare provider may order a second blood test for confirmation. Other tests may also be done for further evaluation of your condition and to confirm a diagnosis.

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