The number of white blood cells in CSF is very low, usually necessitating a manual WBC count.
The WBC differential helps to distinguish many of these causes.
Since white cells may enter the CSF in response to local infection, inflammation, or bleeding, the RBC count is used to correct the WBC count so that it reflects conditions other than hemorrhage or a traumatic tap.
The ratio of RBCs in CSF to blood is multiplied by the blood WBC count.
This value is subtracted from the CSF WBC count to eliminate WBCs derived from hemorrhage or traumatic tap.