The tubes used for CSF aspiration are generally pre-numbered 1 through 3 or 4, with tube 1 being the first tube filled.
- If possible, tube 1 should be reserved for non-routine studies.
- Tube 2 can be used for immunology and chemistry testing.
- Tube 3 can be used for microbiology testing.
- Hematology analysis is typically performed on the last tube collected (3 or 4) to assure that any peripheral blood that may have contaminated the sample during the lumbar puncture has cleared.
When only three tubes are obtained:
- Tube 2 is often reserved for microbiology
- Tube 3 is shared, with the hematology testing performed first, followed by any chemistry or immunology testing.
The first tube obtained should not be used for microbiology or hematology testing because of possible contamination from the lumbar puncture. However, 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.
One problem that could occur with this method is if a traumatic tap is overlying a subarachnoid hemorrhage and the presence of blood in the last tube is due to an SAH. The two-tube count can raise the suspicion of an SAH, but a question still remains whether this is the cause or is the residual blood due only to the traumatic tap, but the volume of CSF collected is insufficient to allow the CSF to clear. 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.
