Hematocrit is a measure of the portion of blood occupied by cells (red blood cells, white blood cells and platelets). A hematocrit of 0.40 L/L (litre of cells per litre of blood) indicates that 40% of the blood volume is composed of cells, with plasma making up the other 60%.
Since red blood cells normally make up about 98% of blood cells, a measurement of hematocrit, which is included in a complete blood count, provides essentially the same information as a red blood cell count (see that section) or hemoglobin levels when testing for anemia or polyglobulia (abnormally high red blood cell count). Hematocrit is also a function of the size of the red blood cells (see section on MCV). If red blood cell size decreases, hematocrit decreases and vice versa.