Cerebrospinal fluid analysis from physiological principles and lumbar puncture to analytical interpretation in clinical neurology

Cerebrospinal fluid (CSF) is a clear, sterile ultrafiltrate of plasma produced primarily by the choroid plexuses at a rate of approximately 500 mL per day, with a circulating volume of 125–150 mL in adults. Disruption of the blood–brain barrier (BBB) and blood-CSF barrier (BCB) underpins a wide spectrum of neurological diseases. This review provides a comprehensive and practically oriented analysis of CSF physiology, the technical conduct of lumbar puncture (LP), and the structured clinical interpretation of CSF biomarkers across three diagnostic phases: emergency, basic, and extensive. A narrative review of the literature was conducted, integrating current guidelines, reference intervals, and diagnostic accuracy data for the main CSF analytes and biomarkers used in clinical neurology practice. Emergency analysis — encompassing global cell counts, CSF glucose (and CSF/serum ratio), lactate, and macroscopic appearance — targets acute inflammation and hemorrhage, including subarachnoid hemorrhage. Basic analysis evaluates BBB/BCB integrity and intrathecal inflammation through total protein quantification (age-partitioned), albumin quotients, immunoglobulin levels, and oligoclonal bands, with established roles in diagnosing multiple sclerosis, Guillain-Barré syndrome, and infectious meningitis. Extensive analysis incorporates specialized assays for neurodegenerative markers; alterations in CSF amyloid-beta 42 have been shown to precede clinical Alzheimer’s onset by up to 18 years in longitudinal cohort studies, followed sequentially by changes in p-Tau-181 (11 years) and t-Tau (10 years). Rigorous LP technique, systematic pre-analytical care, and a structured, phase-based approach to CSF interpretation are essential for the accurate diagnosis of infectious, inflammatory, and neurodegenerative CNS pathologies. The integration of classical and emerging biomarkers positions CSF analysis as an increasingly critical tool in clinical neurology.