Historically, hematocrit was measured by adding Heparin to slim test tube containing a fixed amount of blood and running it in a centrifuge. The red blood cells, being more dense than the rest of the blood components, would collect at the bottom of the tube. There would be a clear demarcation between the red cells and the rest of the blood that could be read on a scale. However, now the process is automated and is calculated by multiplying the red blood cell count by the mean cell volume. However, this method is a bit less accurate.
High hematocrit can indicate dengue shock syndrome, polycythemia vera, blood doping, chronic obstructive pulmonary disease, anabolic steroid abuse, or just dehydration. Low hematocrit can indicate hemmorhage or anemia.
Hematocrit differs from red blood cell count in that it measures the total volume of red blood cells, which in low hematocrit and normal blood count, could indicate a cell weakening disease such as sickle cell anemia. It differs from a hemoglobin count in that hemoglobin in dead or damaged red blood cells will show as normal but the hematocrit will show up as low.