The hematocrit is the percent of whole blood that is comprised of red blood cells. The hematocrit is a compound measure of RBC number and size. The HCT is almost always ordered as part of the complete blood count ( CBC ). The CBC measures the number of red blood cells (RBCs), white blood cells (WBCs), the total amount of hemoglobin in the blood, and the fraction of the blood composed of cells (hematocrit).