CSF cell count and analysis

Principle

• Cerebrospinal fluid is examined for cells, chemistry and microorganisms to diagnose infection, hemorrhage, inflammation and malignancy.

Specimen

• Collected by lumbar puncture into sterile numbered tubes (typically 3–4).
• Common allocation: tube 1 chemistry and serology, tube 2 microbiology, tube 3 cell count, tube 4 additional tests; follow local policy.
• Deliver immediately and process at once (STAT); cells begin to lyse within about 1 hour. Never refrigerate the microbiology tube.

Reagents and equipment

• Improved Neubauer hemocytometer, diluent (saline, or dilute acetic acid to lyse RBCs for WBC count), cytocentrifuge, Wright or Romanowsky stain, Gram stain.

Procedure

• Note appearance: clear, cloudy, bloody or xanthochromic (compare with water against white background).
• Count undiluted fluid if clear, or dilute if cloudy or bloody.
• Cells/µL = (cells counted × dilution) ÷ (number of large squares counted × 0.1 µL).
• Prepare a cytocentrifuge slide for the differential count.

Results

• Typical adult values (local ranges apply): WBC 0–5/µL, mainly lymphocytes and monocytes; no RBCs; neonates have higher counts.
• Glucose about 60–70% of plasma glucose; protein about 15–45 mg/dL (lumbar).
• Bacterial meningitis: high WBC with neutrophils, low glucose, high protein.
• Viral meningitis: lymphocytes, normal glucose, normal or mildly raised protein.
• Tuberculous or fungal meningitis: lymphocytes, low glucose, high protein.
• Traumatic tap: RBC count falls from tube 1 to later tubes, fresh blood, may clot, no xanthochromia.
• Subarachnoid hemorrhage: evenly bloody tubes, xanthochromia developing after a few hours, erythrophages or siderophages.

Quality control and pitfalls

• Run cell count controls and check hemocytometer technique; confirm high counts.
• Delays cause cell lysis and falsely low counts.
• Correct WBC for blood contamination in traumatic taps using the peripheral WBC:RBC ratio.

Clinical use

• Meningitis, encephalitis, subarachnoid hemorrhage, CNS malignancy and demyelinating disease work-up.