RBC (Red Blood Cells) is the number of red blood cells per unit volume of blood. It is a basic parameter of the complete blood count: red blood cells carry oxygen thanks to hemoglobin, so deviations in RBC directly affect the oxygen supply to tissues.
RBC is assessed together with hemoglobin, hematocrit and the red-cell indices MCV and MCH — these distinguish the types of anemia.
Reference ranges
The reference range depends on sex, age and laboratory (the unit is million cells per µL, equivalent to ×10¹²/L):
| Men | 4.2 – 5.7 million/µL |
| Women | 3.8 – 5.1 million/µL |
| SI unit | = 4.2–5.7 ×10¹²/L |
What an elevated level means
A rise in RBC (erythrocytosis, polycythemia) means thickening of the blood. Causes: dehydration (false rise), smoking, chronic hypoxia in lung and heart disease, living at altitude, sleep apnea, and also polycythemia vera — a bone-marrow disease. An excess of red blood cells raises the risk of thrombosis.
What a low level means
A fall in RBC points to anemia or blood loss. Common causes are iron deficiency, a lack of vitamin B12 and folate, chronic diseases, bone-marrow damage, kidney diseases with reduced erythropoietin. It is accompanied by weakness, shortness of breath, pallor.
How the test is done
Blood is drawn from a vein (less often from a finger), and the count is done by an automatic hematology analyzer. No special preparation is needed, but it is advisable to test fasting and in a calm state. RBC is always interpreted together with hemoglobin, hematocrit and the indices RDW, MCV, MCH.
What affects the value
- Dehydration thickens the blood and inflates RBC; excess fluid lowers it.
- Altitude above sea level and smoking raise the value.
- Pregnancy physiologically lowers RBC because of the rise in plasma volume.
- Body position and a long tourniquet during the draw can slightly change the result.
Calculate using this marker
Calculators where RBC (Red blood cell count) is used directly:
Frequently asked questions
How does RBC differ from hemoglobin?
RBC is the number of red blood cells, while hemoglobin is the mass of the oxygen-carrying protein in them. The two usually change together, but not always: for example, in thalassemia there are many red cells but little hemoglobin. So anemia is assessed primarily by hemoglobin.
Is a high RBC dangerous?
A persistently high RBC makes the blood more viscous and increases the risk of thrombosis, heart attacks and strokes. It is important to find the cause: dehydration is corrected by drinking, while polycythemia vera and chronic hypoxia require treatment by a doctor.