In some health care facilities, a red blood cell (RBC) count may be requested on the first tube of CSF collected and the last tube collected as a means of differentiating a subarachnoid hemorrhage (SAH) from a traumatic tap. If the RBC counts in the first and last tubes are similar, an SAH is suspected. If the RBC count in the last tube is significantly less than the count in the first tube, a traumatic tap is suspected.
However, a problem occurs with this method if a traumatic tap occurs. It may be difficult to determine if blood from the traumatic puncture is masking an SAH. When a large number of RBCs are present in the last tube, but less than the number in the first tube, the physician must use additional clinical information to determine if an SAH is present.