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.

Red blood cells circulate in the blood and carry oxygen throughout the body. They are produced in the bone marrow and then released into the bloodstream as they mature. RBCs have a typical lifespan of about 120 days and are continuously renewed and replaced as they age and degrade or are lost through bleeding. A relatively stable number of RBCs is maintained in the circulation by increasing or decreasing the rate of production by the bone marrow.

Some conditions affect RBC production and may cause an increase or decrease in the number of mature RBCs released into the blood circulation. Other conditions may affect the lifespan of RBCs in circulation, especially if the RBCs are deformed due to an inherited or acquired defect or abnormality. 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, then a person will become anemic, which affects the amount of oxygen reaching tissues.

If too many RBCs are produced and released, then a person can develop polycythemia. This can cause decreased blood flow and related problems.

While an RBC count can be used to detect a problem with red blood cell production and/or lifespan, 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
• Reticulocyte count
• Iron studies
• Vitamin B12 and folate levels
• In more severe conditions, a bone marrow examination

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. When someone has signs and symptoms suggesting a disease that might affect red blood cell production, a CBC that includes the RBC count may help make a diagnosis. Some common signs and symptoms associated with anemia that generally lead to a health 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

This test may also be performed on a regular basis to monitor people who have been diagnosed with conditions such as blood disorders, kidney disease, bleeding problems, chronic anemia, and polycythemia. Chemotherapy or radiation therapy often decreases bone marrow production of all the blood cells. Thus, a CBC is typically ordered at regular intervals when monitoring people who are undergoing treatment for cancer.

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 parameters, such as hemoglobin, hematocrit, reticulocyte count, and/or red blood cell indices.

Blood or red cell loss that occurs suddenly or over time and diseases and conditions that decrease red blood cell production in the bone marrow will result in a low RBC count.

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

• Trauma
• Red blood cell destruction, for example 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, myelodysplasia, or lymphoma or other cancers that spread to the 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 stimulates 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 excess RBCs inappropriately.

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.