Spinal fluid or cerebrospinal fluid (CSF) bathes the central nervous system (brain and spinal cord). It is normally water clear with scant cellular content. Normal cells found in the CSF include small numbers of white blood cells (WBC) and occasional macrophages and ependymal lining cells.
Since spinal fluid is obtained by a spinal tap (insertion of a hypodermic needle through the skin into the spinal canal), there will always be a few red blood cells (RBC) introduced into the fluid due to the trauma of the needle stick. So the presence of small numbers of RBC in spinal fluid is normal and of no significance.
The purpose of spinal fluid analysis is to detect inflammatory conditions such as spinal meningitis or encephalitis of the brain, hemorrhage in the brain or spinal cord, autoimmune conditions such as the ascending paralysis of the Guillain-Barre Syndrome, and tumors (especially cancers).
CSF cytology is performed by spinning down the spinal fluid specimen, recovering the cells and spreading them onto a slide. The slide is stained and examined microscopically for cells.
In inflammatory conditions there are increased numbers of WBC and RBC. In brain or spinal cord hemorrhage large numbers of RBC will be present, with far fewer WBC. In cancers that involve the brain or spinal cord (either primary and arising from these structures, or metastatic from cancer arising elsewhere) there will be cancer cells that have been shed into the CSF. These abnormal cells can be picked up on the microscopic cytology examination.
In summary, small numbers of red blood cells are to be expected in CSF analysis, and are considered normal and of no medical significance.