A blood sample is spun in a machine called a centrifuge, which makes it separate into red blood cells, white blood cells, and plasma. The red cells are the heaviest, so they sink to the bottom. The percentage of red cells is then determined and compared with the normal range. The amount of hemoglobin in the blood is tested by adding certain chemicals to the blood and then measuring the intensity of the red color that signifies the presence of hemoglobin. A hemoglobin test is often done at the same time as a hematocrit.
Cerebrospinal fluid is a clear liquid that surrounds the brain and the spinal cord.
The pressure in the spinal column can be measured at the same time. The fluid is examined to see if it is clear or cloudy and to see if it contains any blood. It is then tested for viruses, bacteria, and other signs of infection. Under the usual circumstances, there is no risk from a lumbar puncture. To test the rate of settling, a chemical is added to a blood sample to keep it from clotting.