Illustration of red blood cells to highlight complete blood count

What Is a Complete Blood Count?

Medically reviewed on June 27, 2022 by Jordan Stachel, M.S., RDN, CPT. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.

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The blood serves an invaluable purpose in the body as it provides oxygen and nutrients to every organ and tissue. It transports carbon dioxide and other waste material out of the body via the lungs, kidneys, and digestive system.

While it might seem as simple as some red liquid, blood is more complicated than you realize. It’s full of numerous components, such as a red blood cell component and a white blood cell component that determine its general function and can dictate overall health.

Any imbalances or problems with these individual components can lead to health issues. Learn more about a complete blood count or CBC test and why you might need one below.

Understanding a Complete Blood Count

A complete blood count (CBC) is a blood test that measures components in the blood that help determine its proper function. CBC results can then be used to evaluate overall health and identify various disorders like polycythemia, kidney disease, aplastic anemia, and much more. It can also check the blood to determine the concentration and effect of current medication [1].

Using that information, a healthcare provider can detect any abnormal changes in blood cell count, which may indicate an underlying health condition.

The results of a complete blood count can also be used to adjust medication dosage [2].

What Does a Complete Blood Count Measure?

A CBC measures several different components of the blood. Changes or abnormalities in any of these components could point to underlying health problems.

Red Blood Cells

Red blood cells are responsible for transporting fresh oxygen from the lungs to all the other parts of the body. Red blood cells also transport carbon dioxide from the body to the lungs for removal when you exhale. These cells are typically round and flat with an indented center. Much like other blood cells, red blood cells are produced in the bone marrow [3].

Along with the size and shape of these cells, a CBC can determine how many red blood cells you have and the number of new cells you are creating. In terms of count, a normal range for women is about 3.92 to 5.13 trillion cells per liter. For men, 4.35 to 5.65 trillion cells per liter is considered a normal range. However, these normal ranges may vary based on the test [2].

White Blood Cells

White blood cells, also known as “leukocytes,” are an important part of the immune system. They only make up about 1 percent of the total blood, but they are the body’s first line of defense against foreign microbes, including bacteria and microbes. The white blood cells are constantly working to protect the body from attack. When a harmful microbe spreads, white blood cells help to contain and neutralize the infection. They also play a role in healing wounds and otherwise preventing illness [4].

White blood cells are also made in the bone marrow, and they are stored in the lymphatic tissue and blood. A normal white blood cell count is about 3,400 to 9,600 cells per microliter in a healthy adult [2].


Hemoglobin is an iron-rich protein that is found in red blood cells. It’s what gives red blood cells their red color. The protein also gives red blood cells their distinctive shape. Shape and color aside, hemoglobin carries oxygen within red blood cells to other parts of the body [5].

Low hemoglobin commonly points to an iron deficiency. A normal hemoglobin range in men is about 13.2 to 16.6 grams per deciliter. In women, a normal range is about 11.6 to 15 grams per deciliter.


Platelets are the smallest of the red blood cells. In their inactive form, platelets look like small plates. They circulate in the blood and create clots when they notice damaged blood vessels. When you get a cut on your hand, platelets are what keep you from bleeding out. The platelets bind to the site and create a clot that allows the wound to heal properly [6].

In men, a normal platelet count is about 135,000 to 317,000 platelets per microliter. In women, a regular platelet count is about 157,000 to 371,000 platelets per microliter [2]. Generally, having more than 450,000 platelets may point to a condition called “thrombocytosis.” On the other hand, having fewer than 150,000 platelets may mean you have thrombocytopenia [6].


A hematocrit test is another part of the complete blood count test. A hematocrit test measures the number of red blood cells in proportion to the total blood. The results of this test are reported as a percentage that denotes the number of blood cells that are red blood cells. A ratio of red blood cells that is too low or too high can point to certain diseases or disorders [7].

A typical range for women is 35.5 to 44.9 percent. For men, a typical range is 38.3 to 48.6 percent [7].

Why You Might Need a Complete Blood Count

You may need a complete blood count for various reasons. These include the following.

Monitoring Medication

If you have been put on any medication, your healthcare provider may periodically order a complete blood count to determine if the medication is causing any adverse reactions or otherwise harming your health [2].

Monitoring a Condition

Anemia, certain infections, and other blood disorders naturally affect blood count. This may require regular CBC tests to monitor a condition and determine if a disorder is improving [2].


If you are experiencing certain ongoing symptoms, your healthcare provider may order a CBC to assist with the diagnosis. Symptoms include:

  • Unexplained bruising or bleeding
  • Fever
  • Persistent nausea and vomiting
  • General fatigue
  • Weakness, dizziness, or fainting
  • Inflammation
  • Joint pain
  • Abnormal heart rate or blood pressure [1]

A CBC can help to diagnose the cause of these symptoms. A CBC can also confirm an infection diagnosis [2].

Overall Health

A complete blood count may also help evaluate overall health. The test can help you gain insights into health conditions such as leukemia, anemia, and other disorders [2]. Results of the test are typically available a few days later. If your provider notices any abnormalities, they may recommend other tests [1].

What Are the Symptoms of a Low White Blood Cell Count?

What Does High White Blood Cell Count Mean?

What Is Included in a Comprehensive Metabolic Panel?


1. Complete Blood Count. Cleveland Clinic. URL. Accessed June 27, 2022.

2. Complete blood count (CBC). Mayo Clinic. URL. Accessed June 27, 2022.

3. What Are Red Blood Cells? Rochester Medical Center. URL. Accessed June 27, 2022.

4. What Are White Blood Cells? Rochester Medical Center. URL. Accessed June 27, 2022.

5. What's to know about hemoglobin levels? Medical News Today. URL. Accessed June 27, 2022.

6. What Are Platelets and Why Are They Important? Johns Hopkins Medicine. URL. Accessed June 27, 2022.

7. Hematocrit test. Mayo Clinic. URL. Accessed June 27, 2022.

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