CSF Analysis: Bacterial vs Viral vs TB — High-Yield Guide for MRCP Part 1
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CSF Analysis: Bacterial vs Viral vs TB — High-Yield Guide for MRCP Part 1

TL;DR:

 In MRCP Part 1, CSF interpretation is about pattern recognition. Use opening pressure, cell type, protein, and the CSF:serum glucose ratio to rapidly separate bacterial, viral, and TB meningitis. TB is classically lymphocytic with very high protein and low glucose; viral preserves glucose; bacterial shows neutrophils with low glucose.


Why this topic matters for MRCP Part 1

CSF analysis appears frequently because it tests applied physiology, microbiology, and clinical reasoning in a time-pressured format. Questions are usually short and hinge on one or two discriminators (e.g., glucose ratio or protein magnitude). Mastery here yields quick marks.


A 30-second framework for any CSF stem

Read CSF in the same order every time:

  1. Opening pressure

  2. Cell count and differential

  3. Protein

  4. Glucose (always interpret with serum glucose)

  5. Targeted tests (Gram stain/culture, PCR, AFB/ADA)


High-yield comparison table

Feature

Bacterial meningitis

Viral meningitis

TB meningitis

Opening pressure

Raised

Normal or mildly raised

Raised

WBC count

High

Moderate

Moderate

Differential

Neutrophils

Lymphocytes

Lymphocytes

Protein

High

Normal–mild ↑

Very high

Glucose

Low

Normal

Low

CSF:serum glucose ratio

<0.4

Normal

<0.4

Helpful tests

Gram stain/culture

Viral PCR (e.g., enterovirus, HSV)

AFB stain/culture, TB PCR; ↑ ADA

Time course

Acute, toxic

Acute/subacute

Subacute–chronic

Exam pearl: The CSF:serum glucose ratio is more reliable than absolute glucose. A ratio <0.4 strongly favours bacterial or TB meningitis.


The five most tested subtopics

  1. Glucose ratio over absolute glucose.

  2. Cellular evolution in TB (early neutrophils → lymphocytes).

  3. Protein magnitude (TB often markedly raised).

  4. Opening pressure as a discriminator (viral often normal).

  5. Choosing the right test (PCR vs AFB/ADA) based on the pattern.

Ten high-yield points to memorise

  1. Neutrophils + low glucose → think bacterial first.

  2. Lymphocytes + normal glucose → usually viral.

  3. Lymphocytes + very high protein + low glucose → TB.

  4. Always pair CSF glucose with serum glucose.

  5. Prior antibiotics may sterilise cultures but chemistry remains abnormal.

  6. Early TB can be neutrophilic (classic trap).

  7. Raised opening pressure supports bacterial/TB over viral.

  8. ADA supports TB only in context; not standalone.

  9. HSV encephalitis: lymphocytes, normal glucose—PCR confirms.

  10. Use clinical tempo (acute vs subacute) to reinforce the pattern.

MRCP Part 1 candidate revising CSF analysis and meningitis patterns.

Mini-case (single best answer)

A 31-year-old presents with 6 days of headache and fever. CSF shows raised opening pressure; WBC 160/µL (88% lymphocytes); protein 2.6 g/L; CSF glucose 1.9 mmol/L with serum glucose 5.4 mmol/L.

Most likely diagnosis: TB meningitis.Why: Subacute course, lymphocytes, very high protein, and a low CSF:serum glucose ratio (~0.35).


Common exam traps (avoid these)

  • Assuming lymphocytes always mean viral.

  • Ignoring the glucose ratio.

  • Over-weighting early negative cultures.

  • Forgetting early TB can look neutrophilic.

  • Missing raised opening pressure as a clue.

Practical study checklist

  • Reproduce the comparison table from memory.

  • Practise ratio calculations quickly.

  • Drill short stems with mixed CSF patterns.

  • Link CSF findings to time course and systemic features.


FAQs

How do I quickly tell bacterial from viral meningitis on CSF?

Low glucose with neutrophils points to bacterial; normal glucose with lymphocytes suggests viral.

Is CSF protein always raised in TB meningitis?

Typically yes, and often markedly—this helps separate TB from viral meningitis.

Can TB meningitis present with neutrophils initially?

Yes. Early TB may be neutrophilic before becoming lymphocytic.

Which glucose value should I trust?

The CSF:serum glucose ratio is preferred for exam accuracy.


Ready to start?

Consolidate this topic within the MRCP Part 1 syllabus, then test retention under exam conditions:


Sources

 
 
 
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