What does a cerebrospinal fluid cytology with a total nucleated cell count of 200 cells/µL, 20% polymorphonuclear leukocytes, and 80% lymphocytes indicate, and how should it be managed?

CSF Lymphocytic Pleocytosis: Likely Viral or Tuberculous Meningitis

This CSF profile with 200 cells/µL, 80% lymphocytes, and 20% polymorphonuclear cells most likely indicates viral meningitis, though tuberculous or fungal meningitis must be excluded based on additional CSF parameters (glucose, protein) and clinical context. 1

Interpretation of the Cell Count

The total nucleated cell count of 200 cells/µL is significantly elevated (normal <5 cells/µL) and falls within the typical range for several conditions 1:

• Viral meningitis: typically 5-1000 cells/µL with lymphocyte predominance
• Tuberculous meningitis: typically 5-500 cells/µL with lymphocyte predominance
• Fungal meningitis: typically 5-500 cells/µL with lymphocyte predominance
• Partially treated bacterial meningitis: may show lymphocytic predominance after antibiotics 1

The 80% lymphocyte predominance is the critical finding here. While bacterial meningitis classically shows neutrophil predominance (>100 cells/µL typically), the lymphocytic pattern strongly argues against untreated bacterial disease 1.

Critical Next Steps for Diagnosis

You must immediately obtain these additional CSF parameters to differentiate between causes:

1. CSF glucose and simultaneous plasma glucose

   • CSF glucose >2.6 mmol/L makes bacterial meningitis unlikely 1
   • CSF:plasma glucose ratio <0.36 suggests bacterial meningitis (93% sensitivity/specificity) 1
   • Very low glucose points to tuberculous or bacterial meningitis
   • Normal/slightly low glucose suggests viral meningitis 1

2. CSF protein

   • Markedly raised (>0.6 g/L) suggests tuberculous meningitis 1
   • Mildly raised suggests viral meningitis 1

3. CSF lactate (if antibiotics not yet given)

   • 35 mg/dL suggests bacterial over viral meningitis (93% sensitivity, 96% specificity) 1

   • High negative predictive value helps rule out bacterial disease 1

4. Gram stain and culture - gold standard for bacterial meningitis (70-85% diagnostic if no prior antibiotics) 1

5. CSF PCR - essential, especially if antibiotics already started

   • 87-100% sensitivity for viral pathogens 1
   • Multiplex PCR can detect multiple pathogens simultaneously 1

Management Algorithm

Immediate empiric treatment decisions depend on clinical presentation:

If patient appears septic or has altered mental status:

• Start empiric antibiotics immediately (ceftriaxone + vancomycin ± ampicillin if >50 years or immunocompromised for Listeria coverage) while awaiting culture results 1
• The 20% neutrophils could represent early bacterial disease, partially treated bacterial meningitis, or early enteroviral infection 1, 2

If patient is clinically stable with typical viral meningitis presentation:

• Withhold antibiotics if CSF glucose normal, protein mildly elevated, and lactate <35 mg/dL 1
• Await PCR results for viral pathogens
• Consider HSV encephalitis if any altered mental status or focal findings - start acyclovir empirically 2

If tuberculous meningitis suspected (subacute presentation, markedly elevated protein, very low glucose):

• Start anti-tuberculous therapy immediately - do not wait for culture confirmation
• Add corticosteroids (dexamethasone) to reduce mortality and morbidity

Critical Pitfalls to Avoid

Do not assume this is "just viral" without complete workup. The 20% neutrophils warrant careful consideration of:

• Partially treated bacterial meningitis - CSF may remain culture-positive up to 48 hours after antibiotics, but sterilization can occur within 2-4 hours for meningococcus/pneumococcus 1
• Listeria monocytogenes - characteristically causes lymphocytic predominance even in untreated bacterial meningitis 1
• Early enteroviral meningitis - may show neutrophil predominance initially 2
• Tuberculous meningitis - neutrophils may predominate early in disease course 1

Blood contamination consideration: The presence of 20% PMNs could theoretically reflect blood contamination if significant RBCs are present, as even minimal blood contamination introduces PMNs (1:1000 correction factor commonly used) 1, 3. Check the RBC count - if >100 RBCs/µL, this could account for some PMNs 3.

Clinical Context is Paramount

No CSF parameters give absolute indication of cause - interpretation requires integration with 1:

• Duration and tempo of symptoms (acute vs. subacute)
• Fever pattern and severity
• Presence of rash (petechial suggests meningococcal)
• Immunocompromised status
• Recent antibiotic exposure
• Epidemiologic exposures
• Neuroimaging findings

The lymphocytic predominance with this cell count provides strong direction toward viral or chronic meningitis, but definitive management hinges on the complete CSF profile and clinical picture.