Step by Step
Bact
Bacterial meningitis — cloudy, high protein, low glucose
Bacterial meningitis produces cloudy CSF, a white blood cell count of 1,000-10,000 dominated by neutrophils (PMNs), protein above 100 mg/dL, and glucose below 45 (a CSF-to-serum glucose ratio under 0.6). Opening pressure is also typically elevated.
Vir
Viral meningitis — clear, normal glucose
Viral meningitis produces clear CSF, a much lower white blood cell count (10-500) dominated by lymphocytes rather than neutrophils, only mildly elevated protein, and — critically — normal glucose.
TB/F
TB and fungal meningitis — the hybrid pattern
TB and fungal meningitis produce a pattern that looks like a hybrid of the other two: lymphocytic predominance like viral meningitis, but with high protein and low glucose like bacterial meningitis. Onset is also classically slower than bacterial meningitis. Opening pressure is elevated specifically in bacterial and cryptococcal meningitis.
A patient with a subacute course of headache and confusion over several weeks has a CSF analysis showing lymphocytic predominance but low glucose — this hybrid pattern (lymphocytes like viral, but low glucose like bacterial) points toward TB or fungal meningitis rather than a typical viral process.
Applied Walkthrough
1
A patient presents with a gradual, multi-week onset of headache and mild confusion. CSF analysis shows lymphocytic predominance, elevated protein, and low glucose.
2
Ask: does the lymphocytic predominance mean this is viral meningitis? Not necessarily — while lymphocytic predominance is typical of viral meningitis, the low glucose here is the key detail that doesn't fit that pattern at all, since viral meningitis classically has normal glucose.
3
This combination — lymphocytes (like viral) plus low glucose (like bacterial) plus a slow, subacute onset — points specifically toward TB or fungal meningitis, a hybrid pattern that's easy to misread as viral if you only look at the cell type without checking glucose.
4
Recognizing this exception is exactly why CSF glucose matters as much as cell type when narrowing down a diagnosis before culture results are available.
Exam Application
Exams test the specific CSF values for each meningitis type (bacterial: cloudy, PMN-predominant, high protein, low glucose; viral: clear, lymphocyte-predominant, normal glucose) and specifically the TB/fungal exception — lymphocytic predominance combined with low glucose, which doesn't fit the typical viral pattern.
⚠ Common Trap
The most common trap is assuming lymphocytic predominance always means viral meningitis. TB and fungal meningitis are also lymphocyte-predominant, but have low glucose (like bacterial meningitis) rather than the normal glucose seen in viral meningitis — checking glucose specifically prevents this misclassification.
✓ Quick Self-Check
1. What are the classic CSF findings in bacterial meningitis?
Cloudy fluid, WBC 1,000-10,000 with neutrophil (PMN) predominance, protein above 100 mg/dL, and glucose below 45 (CSF:serum ratio under 0.6).
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2. What are the classic CSF findings in viral meningitis?
Clear fluid, WBC 10-500 with lymphocyte predominance, mildly elevated protein, and normal glucose.
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3. What CSF pattern is seen in TB and fungal meningitis, and how does it differ from typical viral meningitis?
Lymphocytic predominance with high protein and low glucose — the low glucose distinguishes it from viral meningitis, which has normal glucose despite the shared lymphocytic predominance.
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4. In which two types of meningitis is CSF opening pressure classically elevated?
Bacterial meningitis and cryptococcal meningitis.
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5. Why can't cell type alone (lymphocytes vs. neutrophils) reliably distinguish viral from TB/fungal meningitis?
Because both are lymphocyte-predominant; glucose level is the key differentiator — normal in viral, low in TB/fungal.
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