Cerebrospinal Fluid (CSF) Analysis

Cerebrospinal fluid (CSF) analysis is the laboratory examination of fluid obtained by lumbar puncture from the subarachnoid space. By measuring opening pressure, appearance, cell count and differential, glucose, and protein, and by performing microbiological and other studies, it provides a direct window onto inflammatory, infectious, haemorrhagic, and neoplastic processes of the central nervous system.

Definition

CSF analysis is the laboratory examination of cerebrospinal fluid sampled by lumbar puncture, characterising its appearance, opening pressure, cell count and differential, glucose, total protein, and microbiological or cytological findings to assess disease of the central nervous system.

Scope

This topic covers the core laboratory components of CSF examination: physical appearance and pressure, the cell count and differential, glucose and protein concentrations, and the microbiological and cytological studies that follow. It explains how these parameters are interpreted against the blood-brain barrier and the normal composition of CSF. It is reference-educational and does not describe how to perform lumbar puncture or how to manage central-nervous-system disease.

Core questions

• What constitutes the normal physical and biochemical composition of CSF, and how is it maintained?
• How do the cell count, glucose, and protein shift across infectious, inflammatory, and haemorrhagic processes?
• How can a traumatic lumbar puncture be distinguished from true CSF abnormality?

Key concepts

• Opening pressure and appearance
• Cell count and differential (pleocytosis)
• CSF-to-serum glucose ratio
• CSF total protein and barrier integrity
• Xanthochromia
• Traumatic tap versus true haemorrhage

Mechanisms

CSF is produced largely by the choroid plexus and circulates through the ventricular and subarachnoid spaces; its composition is held close to constant by the blood-brain barrier, which normally excludes cells and most plasma proteins. Disease shifts these parameters in characteristic patterns. Inflammation and infection produce pleocytosis - a neutrophil-predominant rise in bacterial meningitis, a lymphocytic rise in viral and tuberculous processes. Glucose, normally a fraction of the blood level, falls when organisms and inflammatory cells consume it, lowering the CSF-to-serum ratio. Protein rises when the barrier is disrupted or local synthesis occurs. Xanthochromia - a yellow discoloration from breakdown of red cells - indicates blood that has been present long enough to be degraded, helping separate subarachnoid haemorrhage from a traumatic tap, which is also suggested by a falling red-cell count across sequential tubes (brouwer-2010; strasinger-2014; fishman-1992).

Clinical relevance

CSF analysis is central to the laboratory recognition of meningitis, subarachnoid haemorrhage, and other central-nervous-system disorders, and the pattern of cell count, glucose, and protein helps point toward bacterial, viral, or other causes. The entry describes how these findings are generated and interpreted as evidence; it is not a basis for individual diagnosis or treatment.

Evidence & guidelines

Major reviews summarise the diagnostic value of CSF cell count, glucose, protein, and microbiology in acute bacterial meningitis and the patterns that distinguish it from viral and tuberculous disease (brouwer-2010). Reference texts detail the normal composition of CSF, the interpretation of xanthochromia, and the correction for a traumatic tap (fishman-1992; strasinger-2014).

History

Systematic CSF analysis followed the introduction of lumbar puncture as a clinical procedure at the close of the nineteenth century, which made the subarachnoid space accessible for sampling. Twentieth-century work established the normal composition of CSF and the diagnostic patterns of cell count, glucose, and protein in meningitis and haemorrhage, consolidated in reference texts and reviews (fishman-1992; brouwer-2010).

Seminal works

• brouwer-2010
• fishman-1992

Frequently asked questions

Which CSF findings suggest bacterial rather than viral meningitis?

Bacterial meningitis classically produces a high white-cell count with neutrophil predominance, a low CSF glucose (and low CSF-to-serum glucose ratio), and an elevated protein, whereas viral processes typically show a lymphocytic pleocytosis with relatively preserved glucose.

What is xanthochromia?

Xanthochromia is a yellowish discoloration of the CSF caused by the breakdown of red blood cells; because it takes time to develop, it helps distinguish a subarachnoid haemorrhage from blood introduced by a traumatic lumbar puncture.