Increased numbers of red blood cells (RBCs) can make it difficult to count nucleated cells. Making excessive dilutions to dilute out the RBCs sufficiently can result in also diluting out the nucleated cells to the point where none are observed. A better strategy is to perform the total cell count and then use a commercially available Spinal Diluting Fluid to minimally dilute the CSF. Spinal Diluting Fluid is a mixture of acetic acid and crystal violet. The acetic acid will lyse the RBCs and the crystal violet will slightly stain the nucleated cells so they can be more accurately counted.
Historically, calculations were used to correct the nucleated cell count for the presence of red blood cells. The theory was that a specific number of the nucleated cells present were peripheral WBCs that had been introduced into the CSF along with the red cells, thus inflating the nucleated cell count and perhaps indicating a false meningitis. Studies have shown that using a standard correction factor actually tends to over-correct the nucleated cell count and current recommendations are to generally assume there are 1 to 2 WBCs present for every 1000 RBCs.
