Red Blood Cell Count
RBC· also: Erythrocytes, Red Cell Count
Clinical Overview
The red blood cell (RBC) count measures the number of erythrocytes per microliter of blood. RBCs carry oxygen bound to hemoglobin and remove carbon dioxide. The RBC count is used together with hemoglobin and hematocrit to classify anemia and guide its investigation.
Why This Test Matters
RBC count distinguishes between different types of anemia: a low RBC with a high MCV suggests macrocytic anemia (B12/folate deficiency), while a low RBC with a low MCV suggests microcytic anemia (iron deficiency or thalassemia). A high RBC count (erythrocytosis) points to polycythemia vera or secondary erythrocytosis from chronic hypoxia.
Reference RangesWHO/IFCC standards
| Age Group | Sex | Reference Range | Unit | Notes |
|---|---|---|---|---|
| Adults (18–64) | Male | 4.5 – 5.9 | ×10⁶/µL | — |
| Adults (18–64) | Female | 4 – 5.2 | ×10⁶/µL | — |
| Children / Pediatric | All | 3.8 – 5.5 | ×10⁶/µL | Varies widely by age |
Also reported in: ×10¹²/L.
What Causes Abnormal Results?
High RBC Causes
- Polycythemia vera (primary bone marrow disorder)
- Secondary polycythemia from chronic hypoxia (COPD, sleep apnea)
- High-altitude adaptation
- Dehydration (relative erythrocytosis)
- Erythropoietin-producing tumors (renal cell carcinoma)
- Androgen or erythropoietin misuse in athletes
Low RBC Causes
- Iron deficiency anemia
- Aplastic anemia (bone marrow failure)
- Hemolytic anemia
- Vitamin B12 or folate deficiency
- Chronic kidney disease
- Leukemia and lymphoma affecting bone marrow
- Chemotherapy or radiation therapy
- Blood loss (acute hemorrhage or chronic bleeding)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting is required. The RBC count is routinely measured as part of a CBC. Avoid vigorous exercise immediately before the test, as it can cause transient changes in blood counts.
Factors That Can Affect Results
- Altitude of residence (higher altitude = naturally higher RBC)
- Dehydration (falsely raises RBC count)
- Pregnancy (plasma volume expansion dilutes RBC count)
- Recent blood transfusion
- Sample hemolysis (falsely lowers measured RBC)
- Cold agglutinins (red cells clump, affecting automated count)
Related Topics
Frequently Asked Questions
What does a high RBC count mean?
A high RBC count (erythrocytosis) means the body is producing more red blood cells than normal. This is most commonly a physiological response to chronic low oxygen levels (seen in COPD, sleep apnea, or living at high altitude). Less commonly, it is caused by polycythemia vera, a bone marrow disorder. Dehydration can also cause an apparent rise.
Is RBC count the same as hemoglobin?
No. RBC count measures how many red blood cells are present, while hemoglobin measures the amount of hemoglobin protein in the blood. They usually correlate, but can diverge — for example, in thalassemia, the RBC count may be normal or even high while hemoglobin is low, because the cells are small and poorly filled with hemoglobin.
How is RBC count used to classify anemia?
RBC count is used alongside MCV and hemoglobin. Microcytic anemia (low MCV) with low RBC suggests iron deficiency or thalassemia. Macrocytic anemia (high MCV) with low RBC suggests B12 or folate deficiency. Normocytic anemia (normal MCV) with low RBC can indicate anemia of chronic disease, hemolysis, or bone marrow failure.