pleocytosis

Pleocytosis is the term for an elevated number of white blood cells in the cerebrospinal fluid (CSF). In adults, normal CSF typically contains up to about 0–5 cells per microliter; pleocytosis is defined by a higher count and may reflect inflammation or infection of the central nervous system. The CSF WBCs can be differentially classified as neutrophils, lymphocytes, or eosinophils, and the predominant cell type often helps suggest the underlying cause.

Patterns of cellular predominance assist in narrowing diagnosis. Neutrophilic pleocytosis is most typical of acute bacterial meningitis, though early viral infections can also show neutrophils. Lymphocytic pleocytosis is common in viral meningitis and encephalitis, as well as in tuberculosis and fungal infections. Eosinophilic pleocytosis, a less common pattern, can occur with parasitic infections, certain drug reactions, or hypersensitivity responses, and occasionally in fungal infections.

Diagnosis and evaluation require CSF analysis obtained by lumbar puncture. Key tests include cell count with differential, protein concentration, glucose level, Gram stain and culture, and pathogen-specific assays such as PCR for herpesviruses, bacterial panels, cryptococcal antigen, TB testing, and cytology for malignant cells. Neuroimaging may be performed prior to LP if there is concern for mass effect or raised intracranial pressure. CSF findings must be interpreted in the clinical context, as pleocytosis is a marker of CNS inflammation rather than a diagnosis by itself.