Red Blood Cell Count (RBC)

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.

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

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.

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

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.

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