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The main functions of the red blood cell (RBC) (erythrocyte) are to carry oxygen from the lungs to the body tissues and to transfer carbon dioxide from the tissues to the lungs. This process is achieved through hemoglobin (Hb) in the RBCs, which combines easily with oxygen and carbon dioxide and gives arterial blood a bright red appearance. To enable maximal use of Hb, the RBC is shaped like a biconcave disc; this affords more surface area for the Hb to combine with oxygen. RBCs also can change shape when necessary to allow for passage through the smaller capillaries.


The RBC count determines the total number of erythrocytes in a microliter (cubic millimeter) of blood.


Normal reference values in adults


* Men: 4.2-5.4 - 106/mm3 or - 1012/L (average, 4.8)


* Women: 3.6-5.0 - 106/mm3 or - 1012/L (average, 4.3).


Anemia (decreased RBC count)

Causes of anemia include:


* lymphomas


* multiple myeloma, myeloproliferative disorders, leukemia


* acute and chronic hemorrhage


* systemic lupus erythematosus


* Addison disease


* rheumatic fever


* infective endocarditis, chronic infection.



Erythrocytosis (increased RBC count)

Causes of primary erythrocytosis include:


* polycythemia vera


* other myeloproliferative disorders


* renal disease


* extrarenal tumors


* high altitude


* pulmonary disease


* cardiovascular disease


* alveolar hypoventilation


* hemoglobinopathies


* tobacco use, carboxyhemoglobinemia.



Relative erythrocytosis (decrease in plasma volume)

Relative erythrocytosis causes:


* dehydration


* Gaisbock syndrome.1



Interfering factors


* Posture: When a blood sample is obtained from a healthy person in a recumbent position, the RBC count is 5% lower.


* Dehydration: Hemoconcentration in dehydrated adults may obscure significant anemia.


* Age: The normal adult RBC count is reached at age 14 years and is maintained until older age, when there is a gradual drop.


* Venipuncture: Falsely high counts may occur because of prolonged venous stasis during venipuncture.


* Stress: Stress can cause a higher RBC count.


* Altitude: Higher altitude increases RBC count. Decreased oxygen content of the air stimulates RBC count increases.


* Pregnancy: There is a relative decrease in RBC count when the body fluid increases in pregnancy, with the normal number of erythrocytes becoming more diluted.


* Medications: Many medications may cause a change in RBC count.


* Anticoagulants: The EDTA blood sample tube must be at least three-fourths filled or values will be invalid because of cell shrinkage caused by the anticoagulant.


* Blood clots: The blood sample must not be clotted (even slightly) or the values will be invalid.


Source: Fischbach FT, Fischbach MA. A Manual of Laboratory and Diagnostic Tests. 10th ed. Philadelphia, PA: Wolters Kluwer; 2018.




1. Tefferi A. Diagnostic approach to the patient with polycythemia. UpToDate. 2019. [Context Link]