When a doctor see's a patience and they have anemic like tendencies they send them to the lab for a hematocrit test. After the whole blood is drawn by the phlebotomist they put the blood in the centrifuge to separate the RBCs from the rest of the substances. The doctor's are looking for a low RBC count. The red blood cells are produced in the bone marrow. Erythropoietin is a hormone and is made in the kidney's to control the production of the RBCs.
RBC's divided by the total blood and that's how you can calculate the amount of red blood cells. The results below are 0.9cm/ 3.5cm to get 26%
The picture below is showing anemic and normal results of a hematocrit test.