The red blood cell count (RBC) measures the total number of red blood cells in a litre of blood. Red blood cells (erythrocytes) are the most abundant cells in the blood and are responsible for transporting oxygen from the lungs to every tissue in the body, and carrying carbon dioxide back to the lungs for exhalation.

A low RBC count alongside low haemoglobin and haematocrit confirms anaemia. The pattern of RBC count with other indices (MCV, MCH, MCHC) helps classify the type of anaemia. A high RBC count (polycythaemia) can be primary (polycythaemia vera) or secondary to chronic hypoxia, dehydration, or exogenous erythropoietin administration. RBC is always interpreted as part of the complete blood count.