In a healthy person, there should be little or no antibodies in the CSF. Where there is a viral meningitis or encephalitis, antibodies may be produced against the virus by lymphocytes in the CSF. The finding of antibodies in the CSF is said to be significant when ratio between the titre of antibody in the serum and that in the CSF is less than 100. But this does depend on an intact blood-brain barrier. The problem is that in many cases of meningitis and encephalitis, the blood-brain barrier is damaged, so that antibodies in the serum can actually leak across into the CSF. This also happens where the lumbar puncture was traumatic in which case the spinal fluid would be bloodstained. So really, one should really check the integrity of the blood-brain barrier before making a definite diagnosis. One way to check the integrity of the blood brain barrier is to use a surrogate antibody that most individuals would have, such as measles virus, since most people would have been vaccinated. So the patient's serum and CSF for measles antibody. If the blood-brain barrier is intact, there should be little or no measles antibodies in the CSF.