CSF is comprised of >99% water. Other components of CSF include the major ions, oxygen, sugars, lactate, proteins (e.g., albumin, globulins), amino acids, urea, ammonia, creatinine, lipids, neurotransmitters and their metabolites, hormones, and vitamins. The blood–CSF barrier protects the brain and CSF from potentially harmful or toxic substances in the blood. Only lipophilic substances can diffuse unrestricted across the blood–CSF barrier. In addition, substances such as glucose that are lipophobic are transported across the blood–CSF barrier into the CSF space via facilitated diffusion using a proteolipid carrier. If the integrity of the blood–CSF barrier is compromised, abnormal amounts of protein, cells, and glucose may be present in the CSF.

The composition of CSF and the mechanism of transport across this barrier are important when evaluating CSF fluid samples obtained by lumbar puncture (LP) in an attempt to diagnose and treat central nervous system (CNS) disease. The most common CSF studies obtained include gram stain and culture, cell count and differential, as well as glucose and protein concentrations. Abnormal results of these studies help to determine whether CNS disease is present and also help indicate what type of disease is present. CSF for analysis is obtained via a LP and should be clear and colorless secondary to the large water content of the fluid if no CNS disease is present. The CSF typically contains no red blood cells (RBCs) and 0 to 1 white blood cells (WBCs). Often, the LP can be a technically challenging procedure and not uncommonly may result in minor trauma to the blood–CSF barrier, leading to bleeding within the CSF space. As a result, some of the lab values, such as the protein count and WBC count, may be elevated secondary to the increased RBC count of the CSF, but not necessarily secondary to the presence of CNS disease, such as an infection or an inflammatory process.