적혈구
Red blood cells deliver oxygen from the lungs to the tissues and on their return carry carbon dioxide back to the lungs where it is exhaled. These functions are mediated by the cells' hemoglobin. The analyzer counts red blood cells, reporting the result in units of 106 cells per microlitre of blood (× 106/μL) or 1012 cells per litre (× 1012/L), and measures their average size, which is called the mean cell volume and expressed in femtolitres or cubic micrometres. By multiplying the mean cell volume by the red blood cell count, the hematocrit (HCT) or packed cell volume (PCV), a measurement of the percentage of blood that is made up of red blood cells, can be derived; and when the hematocrit is performed directly, the mean cell volume may be calculated from the hematocrit and red blood cell count. Hemoglobin, measured after the red blood cells are lysed, is usually reported in units of grams per litre (g/L) or grams per decilitre (g/dL). Assuming that the red blood cells are normal, there is a constant relationship between hemoglobin and hematocrit: the hematocrit percentage is approximately three times greater than the hemoglobin value in g/dL, plus or minus three. This relationship, called the "rule of three", can be used to confirm that CBC results are correct.