Patient Test Information

Red Blood Cell Count (RBC)

  • Why Get Tested?

    To evaluate the number of red blood cells (RBCs); to screen for, help diagnose, or monitor conditions affecting red blood cells

    When To Get Tested?

    As part of a complete blood count (CBC), during a health checkup, or when a healthcare practitioner suspects that you have a condition such as anemia (decreased number of RBCs) or polycythemia (increased number of RBCs)

    Sample Required?

    A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)

    Test Preparation Needed?


  • What is being tested?

    Red blood cells (RBCs), also called erythrocytes, are cells that circulate in the blood and carry oxygen throughout the body. The RBC count totals the number of red blood cells that are present in your sample of blood. It is one test among several that is included in a complete blood count (CBC) and is often used in the general evaluation of a person's health.

    Blood is made up of a few different types of cells suspended in fluid called plasma. In addition to RBCs, there are white blood cells (WBCs) and platelets. These cells are produced in the bone marrow and are released into the bloodstream as they mature. RBCs typically make up about 40% of the blood volume. RBCs contain hemoglobin, a protein that binds to oxygen and enables RBCs to carry oxygen from the lungs to the tissues and organs of the body. RBCs also help transport a small portion of carbon dioxide, a waste product of cell metabolism, from those tissues and organs back to the lungs, where it is expelled.

    The typical lifespan of an RBC is 120 days. Thus the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect RBC production and some conditions may result in significant bleeding. Other disorders may affect the lifespan of RBCs in circulation, especially if the RBCs are deformed due to an inherited or acquired defect or abnormality. These conditions may lead to a rise or drop in the RBC count. Changes in the RBC count usually mirror changes in other RBC tests, including the hematocrit and hemoglobin level.

    • If RBCs are lost or destroyed faster than they can be replaced, if bone marrow production is disrupted, or if the RBCs produced do not function normally, or do not contain enough hemoglobin, then you may develop anemia, which affects the amount of oxygen reaching tissues.
    • If too many RBCs are produced and released, then you can develop polycythemia. This can cause thicker blood, decreased blood flow and related problems, such as headache, dizziness, problems with vision, and even excessive clotting or heart attack.
  • How is the test used?

    A red blood cell (RBC) count is typically ordered as part of a complete blood count (CBC) and may be used as part of a health checkup to screen for a variety of conditions. This test may also be used to help diagnose and/or monitor a number of diseases that affect the production or lifespan of red blood cells.

    When is it ordered?

    An RBC count is ordered as a part of the complete blood count (CBC), often as part of a routine physical or as part of a pre-surgical workup. A CBC may be ordered when you have signs and symptoms suggesting a disease that might affect red blood cell production. Some common signs and symptoms associated with anemia that generally lead to a healthcare practitioner ordering a CBC are:

    • Weakness or fatigue
    • Lack of energy
    • Paleness

    Some signs and symptoms that may appear with a high RBC count include:

    • Disturbed vision
    • Headache, dizziness
    • Flushing
    • Enlarged spleen

    A CBC may also be performed on a regular basis to monitor people who have been diagnosed with conditions such as:

    • Bone marrow disorders 
    • Kidney disease 
    • Bleeding problems 
    • Chronic anemia 
    • Polycythemia
    • Cancer, as chemotherapy or radiation therapy often decreases bone marrow production of all the blood cells

    What does the test result mean?

    Since an RBC count is performed as part of a complete blood count (CBC), results from other components are taken into consideration. A rise or drop in the RBC count must be interpreted in conjunction with other tests, such as hemoglobin, hematocrit, reticulocyte count, and/or red blood cell indices.

    The following table summarizes what results may mean.

    Test Reference Range Examples of Causes of Low Result Examples of Causes of High Result
    Red Blood Cell Count (RBC)

    Conventional Units

    Men: 4.5-5.9 x 106/microliter

    Women: 4.1-5.1 x 106 microliter

    SI Units

    Men: 4.5-5.9 x 1012/L

    Women: 4.1-5.1 x 1012/L

    Known as anemia

    • Acute or chronic bleeding
    • RBC destruction (e.g., hemolytic anemia, etc.)
    • Nutritional deficiency (e.g., iron deficiency, vitamin B12 or folate deficiency)
    • Bone marrow disorders or damage
    • Chronic inflammatory disease
    • Chronic kidney disease

    Known as polycythemia 

    • Dehydration
    • Lung (pulmonary) disease
    • Kidney or other tumor that produces excess erythropoietin
    • Smoking 
    • Living at high altitude 
    • Genetic causes (altered oxygen sensing, abnormality in hemoglobin oxygen release)
    • Polycythemia vera—a rare disease

    from Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed.
    McPherson R, Pincus M, eds. Philadelphia, PA: Elsevier Saunders; 2011.

    Note: Conventional Units are typically used for reporting results in U.S. labs;
    SI Units are used to report lab results outside of the U.S.

    Some causes of a low RBC count (anemia) include:

    • Trauma that leads to loss of blood
    • Conditions that cause red blood cells to be destroyed, such as hemolytic anemia caused by autoimmunity or defects in the red cell itself; the defects could be a hemoglobinopathy (e.g., sickle cell anemia), thalassemia, an abnormality in the RBC membrane (e.g., hereditary spherocytosis), or enzyme defect (e.g., G6PD deficiency).
    • Sudden (acute) or chronic bleeding from the digestive tract (e.g., ulcers, polyps, colon cancer) or other sites, such as the bladder or uterus (in women, heavy menstrual bleeding, for example)
    • Nutritional deficiency such as iron deficiency or vitamin B12 or folate deficiency
    • Bone marrow damage (e.g., toxin, radiation or chemotherapy, infection, drugs)
    • Bone marrow disorders such as leukemia, multiple myeloma, myelodysplastic syndrome, or lymphoma or other cancers that spread to the bone marrow
    • Chronic inflammatory disease or condition
    • Kidney failure—severe and chronic kidney diseases lead to decreased production of erythropoietin, a hormone produced by the kidneys that promotes RBC production by the bone marrow.

    Some causes of a high RBC count (polycythemia) include:

    • Dehydration—as the volume of fluid in the blood drops, the count of RBCs per volume of fluid artificially rises.
    • Lung (pulmonary) disease—if someone is unable to breathe in and absorb sufficient oxygen, the body tries to compensate by producing more red blood cells.
    • Congenital heart disease—with this condition, the heart is not able to pump blood efficiently, resulting in a decreased amount of oxygen getting to tissues. The body tries to compensate by producing more red blood cells.
    • Kidney tumor that produces excess erythropoietin
    • Smoking
    • Genetic causes (altered oxygen sensing, abnormality in hemoglobin oxygen release)
    • Polycythemia vera—a rare disease in which the body produces too many RBCs

    My RBC count is slightly out of range. What does this mean?

    Your RBC count is interpreted by your healthcare practitioner within the context of other tests that you have had done as well as other factors, such as your medical history. A single result that is slightly high or low may or may not have medical significance. There are several reasons why a test result may differ on different days and why it may fall outside a designated reference range.

    • Biological variability (different results in the same person at different times): If you have the same test done on several different occasions, there's a good chance that one result will fall outside a reference range even though you are in good health. For biological reasons, your values can vary from day to day.
    • Individual variability (differences in results between different people): References ranges are usually established by collecting results from a large population and determining from the data an expected average result and expected differences from that average (standard deviation). There are individuals who are healthy but whose tests results, which are normal for them, do not always fall within the expected range of the overall population.

    A test value that falls outside of the established reference range supplied by the laboratory may mean nothing significant. Generally, this is the case when the test value is only slightly higher or lower than the reference range and this is why a healthcare practitioner may repeat a test on you and why they may look at results from prior times when you had the same test performed.

    However, a result outside the range may indicate a problem and warrant further investigation. Your healthcare provider will consider your medical history, physical exam, and other relevant factors to determine whether a result that falls outside of the reference range means something significant for you. For more, read the articles on Reference Ranges and What They Mean.

    If my RBC Count is out of range, what other tests might be done?

    An RBC count can be used to detect a problem with red blood cell production and/or lifespan, but it cannot determine the underlying cause. In addition to the full CBC, some other tests may be performed at the same time or as follow up to help establish a diagnosis. Examples include:

    • Blood smear—a laboratory professional examines the blood under the microscope to confirm results of a CBC and/or to look abnormal blood cells
    • Reticulocyte count—determines the number of young (immature) red blood cells
    • Iron studies—iron is important in the production of red blood cells
    • Vitamin B12 and folate levels—these vitamins are also important for red blood cell production
    • In more severe conditions, a bone marrow aspiration and biopsy—usually done by a pathologist to help detect abnormalities in the bone marrow and determine the cause of low or high blood cell counts or abnormal blood cells

    How treatable are abnormal red blood cell counts?

    First, a healthcare practitioner must determine the cause of someone's abnormal RBC count so the appropriate treatment can be prescribed. For some anemias, treatment may include a dietary supplement or a change in diet to include nutritional foods. In some instances, it may only require a change in the person's current medication. For more severe cases, treatment may involve transfusion with blood from a donor. For some, prescribing a drug to stimulate red cell production in the bone marrow may be required, especially for people who have received chemotherapy or radiation treatments.

    Can my RBC count be tested at my doctor’s office?

    Maybe. Some healthcare practitioners' offices are equipped with laboratory instruments and staffed by trained laboratorians who are able to perform this test.

    Do diet and nutrition help keep RBCs at healthy levels?

    Yes, to the extent that if you eat a well-balanced diet, you can prevent anemia due to a lack of iron, vitamin B12, or folate in the foods you eat. Sometimes use of a supplement is recommended if you are at risk of a vitamin deficiency. However, the most common cause of vitamin B12 deficiency is malabsorption, and the most common cause of iron deficiency is bleeding. These conditions and other RBC problems that are caused by diseases other than nutritional deficiencies will not be corrected by diet.

    Are there symptoms I should recognize if my RBCs are dangerously low or high?

    Fatigue and weakness may indicate a low or high RBC count. Fainting, pallor, shortness of breath, dizziness, and/or altered mental status can also indicate a low RBC count. Disturbed vision, headache, and flushing may be present with increased numbers of RBCs.

    Is there anything else I should know?

    A recent blood transfusion can affect results of an RBC count.

    Alteration of the number of RBCs is often temporary and can be easily corrected and/or returned to normal levels by treating and resolving the underlying condition.

    During pregnancy, body fluids tend to accumulate, thus decreasing the RBC count in relation to fluid volume.

    Living at high altitudes causes an increase in RBC count; this is the body's response to the decreased oxygen available at these heights.

    Women tend to have slightly lower RBC counts than men.

  • View Sources

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