CSFserum

CSFserum refers to the relationship between concentrations of substances in cerebrospinal fluid (CSF) and in serum. In clinical practice it is used to assess the integrity of the blood–CSF barrier and to detect intrathecal (within the CNS) synthesis of immunoglobulins, which can aid in diagnosing inflammatory or infectious CNS diseases and in monitoring conditions such as multiple sclerosis.

A central measure in CSFserum analysis is the CSF albumin quotient, QAlb, defined as CSF albumin divided by serum albumin. Albun in CSF primarily results from transfer across the blood–CSF barrier, so QAlb serves as a proxy for barrier permeability. Reference values vary with age; a higher QAlb indicates barrier dysfunction.

Immunoglobulin-related CSFserum metrics include the CSF IgG quotient, QIgG = CSF IgG / serum IgG, and the IgG index, calculated as IgG index = QIgG / QAlb. An elevated IgG index suggests intrathecal IgG synthesis. In addition, the presence of oligoclonal bands on CSF electrophoresis is a hallmark of intrathecal immunoglobulin production, particularly in conditions such as multiple sclerosis.

Interpretation of CSFserum results must consider potential limitations: blood contamination from a traumatic tap can elevate CSF protein and skew quotients; delays in processing or assay differences can affect measurements; sampling must be paired with accurate serum values. CSFserum analyses are applied in evaluating inflammatory and infectious CNS diseases, assessing blood–CSF barrier integrity, and supporting diagnoses where intrathecal immunoglobulin synthesis is suspected.