CSF Glucose in Bacterial Meningitis
In a 14-year-old boy with fever, vomiting, lethargy, neck stiffness, and cloudy CSF with elevated WBC count, the expected glucose result would be decreased (option B).
CSF Findings in Bacterial Meningitis
Bacterial meningitis typically presents with characteristic CSF abnormalities:
- Glucose: Decreased in approximately 50-60% of patients with bacterial meningitis 1
- CSF:Serum glucose ratio: A ratio <0.4 has 80% sensitivity and 98% specificity for bacterial meningitis in children over 12 months 1
- Cell count: The patient's CSF shows 100 cells with cloudy appearance, consistent with bacterial meningitis which typically shows 100-5,000 cells/mm³ (though can range from 100-110,000 cells/mm³) 1
Diagnostic Value of CSF Glucose
The UK Joint Specialist Societies guideline states that in bacterial meningitis:
- CSF glucose is typically very low 1
- Normal CSF glucose is about two-thirds of plasma glucose 1
- A CSF:plasma glucose ratio cutoff of 0.36 has 93% sensitivity and specificity for diagnosing bacterial meningitis 1
- A CSF glucose above 2.6 mmol/L is unlikely to be associated with bacterial meningitis 1
Comparison with Other Meningitis Types
Different types of meningitis show characteristic CSF glucose patterns:
| Type | CSF Glucose | CSF:Plasma Glucose Ratio |
|---|---|---|
| Normal | 2.6-4.5 mmol/L | >0.66 |
| Bacterial | Very low | Very low |
| Viral | Normal/slightly low | Normal/slightly low |
| Tuberculous | Very low | Very low |
| Fungal | Low | Low |
| Lyme | Normal to slightly low | Normal (>0.66) |
Clinical Significance
Low CSF glucose in bacterial meningitis has prognostic implications:
- A CSF:plasma glucose ratio of 0.1-0.35 is associated with higher mortality compared to ratios of 0.38-2.0 3
- Recent research confirms that CSF:blood glucose ratio is a precise predictor of bacterial meningitis (optimal cutoff=0.36, sensitivity=92.9%, specificity=92.9%) 4
- Low CSF glucose levels have been associated with sensorineural hearing loss in bacterial meningitis caused by S. pneumoniae and H. influenzae 5
Diagnostic Algorithm
- Obtain CSF and blood glucose simultaneously
- Calculate CSF:blood glucose ratio
- If ratio <0.36, strongly suspect bacterial meningitis 4
- Consider point-of-care glucometer testing which can provide rapid results (within 5 minutes) with good sensitivity (94.1%) and specificity (91%) using a cutoff ratio of 0.46 6
Common Pitfalls
- Relying solely on absolute CSF glucose values without calculating the CSF:serum ratio
- Not obtaining a simultaneous blood glucose sample
- Failing to recognize that partially treated bacterial meningitis may have less pronounced glucose abnormalities
- Overlooking that tuberculous meningitis can also present with very low CSF glucose
In conclusion, based on the clinical presentation (fever, vomiting, lethargy, neck stiffness) and CSF findings (cloudy appearance, elevated WBC count), this 14-year-old boy most likely has bacterial meningitis, and the expected CSF glucose result would be decreased.