Cerebrospinal fluid (CSF) is the clear fluid that surrounds the brain and spinal cord. It is contained in the meninges, specifically between the piebrae (which covers the central nervous system) and the arachnoid (which lines the inner side of the dura mater), i.e. in the subarachnoid space. It protects the brain and spinal cord from injury by absorbing movement and shock that could damage them, and also serves as a nutrient delivery and waste disposal system for the brain. Cerebrospinal fluid is continuously produced in areas of the brain called the lateral ventricles and more specifically by the choroid plexuses and is absorbed into the bloodstream.

The composition of the cerebrospinal fluid is thus supposed to reflect the physiopathological state of the brain: inflammation, infection, presence of pharmacological molecules, etc. A small amount of cerebrospinal fluid is collected during a medical procedure called a lumbar puncture in the lower back, between the 4th and 5th lumbar vertebrae5 . It is done at this level so as not to impact the spinal cord ending higher up.