Understanding the Red Blood Cell (RBC) Count

The red blood cell (RBC) count is a test used to measure the number of oxygen-carrying blood cells in a volume of blood. It is one of the main tests doctors use to determine how much oxygen is being transported to cells of the body.

An abnormal RBC count is often the first sign of an illness. At other times, the test can point the doctor in the direction of a diagnosis if there are symptoms like unexplained fatigue or shortness of breath.

Keep reading to learn more about your red blood cell count and what it means if it's too low or too high.

Red blood cell count

Complete Blood Count

Usually, the doctor needs to look at more than an RBC count to diagnose a medical condition. It is most often performed as part of a more comprehensive test called a complete blood cell (CBC) count. This test measures several different components of a blood sample, including:

  • Red blood cells (RBC), which transport oxygen to cells of the body
  • Hemoglobin (Hb), the protein in RBCs that carries oxygen and carbon dioxide molecules
  • White blood cells (WBC), which are a part of the immune system
  • Platelets (PLT), the cells responsible for blood clotting
  • Hematocrit (Hct), the ratio of RBCs to the total volume of blood

Based on the composition of the blood, doctors can have a better idea of what to look for when trying to diagnose a medical condition.

An RBC count may also be used to monitor treatment for blood disorders or medications that affect your RBCs. This is especially true for cancer and chemotherapy. Both of them can negatively affect blood counts.

Normal Ranges

An RBC count is the number of red blood cells per a particular volume of blood. It may be reported in millions of cells per microliter (mcL) of blood or in trillions of cells per liter (L) of blood.

The "normal" or "reference" range can sometimes vary depending on whose blood is being tested. If you live in a high-altitude city like Denver, your blood count will be far higher than people who live in low-altitude areas like the Gulf Coast.

This is because when you are at a higher altitude, your body creates more red blood cells so that more oxygen can be carried to your tissues. For this reason, the ranges cannot be considered hard-and-fast values but just a reference point.

The RBC count reference range varies by sex and age:

  • Women: 4.2 to 5.4 million/mcL
  • Men: 4.7 to 6.1 million/mcL
  • Children: 4.1 to 5.5 million/mcL

High or Low RBC Counts

When the RBC count is higher or lower than the average range, it alerts the doctor that a medical condition could be the cause. The other values from the CBC will be considered, as well as other diagnostic tests.

High RBC Count Causes

A high RBC count tells us that there has been an increase in oxygen-carrying cells in the blood. In some cases, this may reveal that the body is trying to make up for some condition that is preventing the body from receiving enough oxygen. In others, the cause may be related to diseases or drugs that increase the production of RBCs.

Some of these causes include:

Low RBC Count Causes

A low RBC count shows there is a decrease in oxygen-carrying cells in the blood. This is called anemia. Various infections, nutritional deficiencies, and medical conditions may cause anemia. Some of these include:

  • Kidney failure
  • Thyroid problems
  • Bleeding, either internal or external
  • Leukemia, a type of cancer of white blood cells
  • Drug side effects, including chemotherapy
  • Multiple myeloma, a type of cancer affecting blood plasma cells
  • Deficiency of erythropoietin, a kidney hormone that promotes RBC growth
  • Deficiencies in iron, folate, vitamin B12, or vitamin B6
  • Hemolysis, the abnormal breakdown of red blood cells
  • Pregnancy


Treatment of an abnormal RBC count is typically focused on addressing the underlying condition. These treatments will vary widely depending on the cause.

But if the cause is a nutritional deficiency, medication use, or a chronic condition, there may be things you can do to not only improve your blood count but your overall health too.

High RBC Count Treatment

If you have a high RBC count:

  • Exercise to improve your heart and lung function.
  • Eat less red meat and iron-rich foods.
  • Avoid iron supplements.
  • Keep yourself well hydrated.
  • Avoid diuretics, including coffee and caffeinated drinks, which can dehydrate you.
  • Stop smoking, especially if you have COPD or pulmonary fibrosis.
  • Avoid the use of steroids, erythropoietin, and other performance-enhancing drugs.

Low RBC Count Treatment

If you have a low RBC count (including anemia):

  • Maintain a healthy, balanced diet.
  • Take a daily vitamin and iron supplement, if needed.
  • Exercise regularly to improve heart and lung function.
  • Stop smoking.
  • Avoid aspirin, which reduces clotting and can cause slow blood loss.
  • Take your thyroid medications as prescribed if you have thyroid problems.


A red blood cell count is used to measure the number of oxygen-carrying red blood cells in your body. When you have a high blood count or a low blood count, it's a sign that you may be sick.

Your doctor will also monitor your RBC count when you have a blood disorder or you're taking medicines that affect it. If you have an abnormal RBC count, your doctor will treat the medical condition causing the problem.

Frequently Asked Questions

  • What causes a low red blood cell count?

    A low red blood cell (RBC) count can be caused by certain infections, nutritional deficiencies, and medical conditions. Examples include internal bleeding or external bleeding, kidney failure, thyroid problems, leukemia, chemotherapy, multiple myeloma, hemolysis, pregnancy, and a deficiency in iron, folate, or vitamins B12 and vitamin B6.

  • What is a normal RBC range?

    Normal RBC ranges differ according to age and sex:

    • Women: 4.2 to 5.4 million/mcL
    • Men: 4.7 to 6.1 million/mcL
    • Children: 4.1 to 5.5 million/mcL
  • What do red blood cells do?

    Red blood cells carry oxygen from the lungs to body tissues. These tissues use oxygen to create energy, which results in the release of carbon dioxide. Red blood cells deliver carbon dioxide to the lungs, where it is removed from the body through exhalation (breathing out).

  • What causes a low white blood cell count?

    A low white blood cell (WBC) count, also known as leukopenia, has a few possible causes:

    • Side effect from a drug
    • Viral infection
    • Side effect of chemotherapy or radiation therapy
    • Immune system disorder
    • Blood or bone marrow disorder
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5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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