A calculation is used to correct CSF WBC counts which are falsely increased due to a traumatic tap:
- WBCs added = WBC(blood) x RBC(CSF) / RBC(blood)
The blood WBC count is multiplied by the ratio of the cerebrospinal fluid RBC count to blood RBC count.The result is the number of artificially introduced WBCs. The true CSF white cell count is then calculated by subtracting the artificially introduced WBCs from the actual CSF WBC count.
If the patient's peripheral WBC and RBC counts are within normal limits, some laboratories use the following formula:
- Subtract one white cell from the CSF WBC count for each 750 RBC counted in the spinal fluid.