What is the likely cause of meningitis in a patient with sudden severe headache, photophobia, fever, and cerebrospinal fluid (CSF) analysis showing pleocytosis (elevated white blood cell count), elevated protein, and low glucose, is it bacterial, viral, or fungal?

Bacterial Meningitis is the Most Likely Diagnosis

This CSF profile is highly consistent with bacterial meningitis and requires immediate empiric antibiotic therapy. The combination of high WBC count (850 cells/μL), low CSF:serum glucose ratio (45/95 = 0.47), and elevated protein (120 mg/dL) strongly indicates bacterial rather than viral or fungal etiology 1.

CSF Analysis Interpretation

Why This is Bacterial Meningitis

• CSF WBC count of 850 cells/μL exceeds the threshold of 321 cells/μL, which has 80.6% sensitivity and 81.4% specificity for bacterial meningitis 2
• The CSF:serum glucose ratio of 0.47 is above the bacterial meningitis cutoff of 0.36 (93% sensitivity and specificity), but the absolute CSF glucose of 45 mg/dL is still concerning 1
• Protein of 120 mg/dL is elevated, consistent with bacterial meningitis, though not as dramatically elevated as typical (bacterial usually >0.6 g/L or 60 mg/dL) 1
• Lymphocyte predominance (85%) does NOT rule out bacterial meningitis - Listeria monocytogenes and partially treated bacterial meningitis commonly present with lymphocytic predominance 1, 3

Why This is NOT Viral Meningitis

• Viral meningitis rarely produces total CSF WBC counts >2000 cells/μL, even with early neutrophil predominance 1, 4
• The CSF glucose of 45 mg/dL is lower than expected for viral meningitis, where glucose is typically normal or only slightly low 4
• Viral meningitis typically has CSF WBC counts of 5-1000 cells/μL, and this patient's 850 cells/μL is at the upper limit but combined with other parameters suggests bacterial etiology 4

Why This is NOT Fungal Meningitis

• Fungal meningitis typically presents with more gradual onset over days to weeks, not sudden severe headache 1
• Fungal meningitis usually shows CSF glucose much lower than 45 mg/dL (typically very low) 1, 4
• This patient lacks typical risk factors for fungal meningitis (no mention of severe immunosuppression, HIV, or chronic illness) 1

Critical Clinical Pearls

Listeria Must Be Considered

• In a 35-year-old female with lymphocytic predominance, Listeria monocytogenes is a critical consideration 1
• Listeria characteristically causes lymphocytic rather than neutrophilic CSF pleocytosis 1, 3
• Empiric coverage MUST include ampicillin (ceftriaxone alone does NOT cover Listeria) 1, 5

Common Diagnostic Pitfalls

• Do not be falsely reassured by lymphocyte predominance - this occurs in Listeria and partially treated bacterial meningitis 1, 3
• Do not wait for culture results - bacterial meningitis has high mortality, and antibiotics must be started within one hour of presentation 1, 5
• The CSF:serum glucose ratio of 0.47, while above 0.36, should be interpreted in context - the absolute CSF glucose of 45 mg/dL with elevated protein and high WBC count still indicates bacterial disease 1

Immediate Management Algorithm

1. Start empiric antibiotics immediately (within 1 hour): vancomycin + ceftriaxone + ampicillin for this age group 1, 5, 6
2. Blood cultures before antibiotics, but do not delay treatment 5
3. Consider dexamethasone if not already given (ideally before or with first antibiotic dose) 1
4. Send CSF for Gram stain, culture, and PCR testing (though Gram stain sensitivity is only 50-99%) 1, 3
5. Monitor closely for complications including increased intracranial pressure and seizures 1

Most Likely Pathogens in This Age Group

• Streptococcus pneumoniae (most common in adults <50 years) 1, 7
• Neisseria meningitidis (second most common) 1, 2
• Listeria monocytogenes (especially given lymphocytic predominance) 1

The CSF lactate level, if available, would provide additional diagnostic certainty - levels >35 mg/dL have 93% sensitivity and 96% specificity for bacterial meningitis 1, 4.