Is it possible to measure serotonin with a sample of spinal fluid?
It is entirely possible to sample serotonin levels from cerebral spinal fluid (CFS) taken via a lumbar puncture, also known as a spinal tap, to the spinal column from the central nervous system (CNS).
This is the most accurate method of measuring neurotransmitter levels in the brain* without risking damage to cerebral tissue, although it is safer and almost as accurate to take the sample from platelets extracted a blood sample. The reason it is safer is because you don't have to go through nerve tissue, and because CSF does not contain the defences of blood to deal with toxins and pathogens (potential infections).
note. It is perhaps more accurate to use a tissue sample, although this would take longer to analyse and damage grey matter. In animal studies we use in vivo microdialysis, but this is only worthwhile in controlled (extremely clean) experimental conditions.