The majority of CSF is produced by the choroid plexus, which is present throughout the ventricular system and the subarachnoid space that surround the brain and spinal cord. The choroid plexus is formed by the choroidal epithelium, a bilayered structure, and its accompanying blood vessels and interstitial connective tissue. The brain and its surrounding fluid are protected from substances in the blood by the blood–CSF barrier. Only substances that can cross biological membranes because of their lipophilic nature may diffuse unrestricted across the blood-brain barrier. Proteins are not included in this subset of substances.

CSF is clear and colorless because it is comprised of 99% water. The majority of protein in CSF is derived from the serum. When the integrity of the blood–CSF barrier is intact, the CSF to serum albumin ratio is 1:200, implying that normal entry rate of protein from the serum to the CSF space is 200 times less than its rate of exit. Any compromise to the integrity of this barrier results in a higher rate of entry and thus a higher protein level in the CSF. The normal CSF protein ranges from 10 to 40 mg/dL in a child, to as high as 120 mg/dL in a neonate.