Normally in blood, there are 700 RBCs for each WBC. By adjusting the white cell count seen in a bloody CSF for this factor, the number of WBCs in "excess" (and an approximate number of the WBC before the blood was introduced) can be given. This is used for traumatic taps or situations of subarachnoid hemorrhage. If the WBC and/or RBC are abnormal, then a more complex calculation is required.
Specimen:
CSF WBC and RBC are done on the same tube of blood.
corrected WBC =
= (WBC in bloody CSF) - ((WBC in blood) * (RBC in CSF) / (RBC in blood))
where:
• WBC is white blood cells per µL
• RBC is red blood cells per µL
If the CBC of the peripheral blood is normal (no anemia or leukocytosis):
corrected WBC =
= (WBC of bloody CSF) - ((RBC in CSF) / 700)
where:
• 700 is a rough estimate of the RBC/WBC ratio in the peripheral blood with a normal CBC. If the CBC is known then the actual ratio can be calculated.
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