What Is an RBC Blood Test?

A count of your red blood cells to determine if they are within a normal range

The red blood cell (RBC) count is a test that measures the number of oxygen-carrying blood cells in your blood. An abnormal RBC test result is often the first sign of an illness.

A high red blood cell count may indicate that you have a condition that's preventing you from getting enough oxygen. A high count for men is anything over 6.1 million cells per microliter (mcL); for women, it's anything above 5.4 million cells/mcL; and for children, it's a count higher than 5.5 million cells/mcL.

A low RBC may be caused by an infection or a medical condition related to anemia. Low RBC counts are those below 4.7 for men, 4.2 for women, and 4.1 for children.

Red blood cell count

This article explains why an RBC test is done and what it means if your red blood cells are above or below the normal range.

What Is a Normal RBC Range?

Your red blood cells (RBC) transport oxygen to cells of the body. 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 reference points.

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

Anything above these RBC levels may be concerning.

High or Low RBC Counts

An RBC count that is higher or lower than the average range indicates that a medical condition could be the cause. Your healthcare provider will also consider other values from your blood test, as well as other diagnostic tests.

High RBC 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 Causes

A low RBC count shows there is a decrease in oxygen-carrying cells in the blood. This is called anemia. A low RBC count can indicate a serious problem, but not always. 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

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
  • 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.

Treating Abnormal RBC Levels

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

You may be able to increase a low RBC count by:

  • Maintaining a healthy, balanced diet.
  • Taking a daily vitamin and iron supplement, if needed.
  • Exercising regularly to improve heart and lung function.
  • Quitting smoking.
  • Avoiding aspirin, which reduces clotting and can cause slow blood loss.
  • Taking 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 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).

  • Is low blood count always serious?

    No, a low RBC count isn't always serious. For example, your RBC count may be low if you are pregnant or menstruating, or if you aren't getting enough iron in your diet. If you do have a low RBC count, however, you may need further testing to rule out more serious causes.

  • What cancers cause high red blood cell count?

    A few cancers are associated with a high RBC count. These include:

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.
  1. Leukemia & Lymphoma Society. Understanding blood counts.

  2. Akunov A, Sydykov A, Toktash T, Doolotova A, Sarybaev A. Hemoglobin changes after long-term intermittent work at high altitudeFront Physiol. 2018;9:1552. doi:10.3389/fphys.2018.01552

  3. MedlinePlus. Blood count tests.

  4. National Heart, Blood, and Lung Institute. Iron-deficiency anemia.

  5. Cleveland Clinic. Function of red blood cells.

Additional Reading

By Amber J. Tresca
Amber J. Tresca is a freelance writer and speaker who covers digestive conditions, including IBD. She was diagnosed with ulcerative colitis at age 16.