CSF/Serum Glucose Ratio in Bacterial Meningitis
A CSF-to-serum glucose ratio <0.4 is highly specific for bacterial meningitis, with a ratio <0.36 providing 93% sensitivity and specificity, and a ratio <0.23 predicting bacterial etiology with 99% certainty when combined with other CSF parameters. 1, 2, 3
Diagnostic Thresholds and Performance
Primary Diagnostic Cutoffs
- CSF/serum glucose ratio <0.36 is the optimal single indicator for bacterial meningitis with 93% sensitivity and 93% specificity 2, 3
- Ratio <0.4 is 80% sensitive and 98% specific for bacterial meningitis in children ≥2 months of age 1
- **Ratio <0.23** predicts bacterial meningitis with 99% certainty when combined with other CSF parameters (protein >120 mg/dL, WBC >12,000/mm³, or neutrophils >11,000/mm³) 1
Absolute CSF Glucose Values
- CSF glucose <34 mg/dL (<1.9 mmol/L) predicts bacterial meningitis with 99% certainty 1
- CSF glucose <40 mg/dL occurs in 50-60% of bacterial meningitis cases 1
- CSF glucose <2.6 mmol/L has 93% sensitivity and specificity for bacterial meningitis 2
Clinical Algorithm for Interpretation
When Ratio is <0.23
- Bacterial meningitis is highly likely (99% certainty) 1
- Initiate empiric antimicrobial therapy immediately 1
- Do not delay treatment waiting for culture results 1
When Ratio is 0.23-0.36
- Bacterial meningitis remains likely 3
- Evaluate additional CSF parameters: neutrophil predominance (≥50%), protein >50 mg/dL, elevated opening pressure 4
- Consider serum inflammatory markers (CRP, procalcitonin) if Gram stain is negative 5
When Ratio is 0.36-0.5
- If ratio <0.5: Consider tuberculous meningitis, which typically presents with CSF/serum ratio <0.5 (very low) with lymphocytic predominance 1, 6, 7
- Bacterial meningitis becomes less likely but cannot be excluded 3
When Ratio is >0.5
- Viral meningitis most likely, as CSF/serum glucose ratio typically remains >0.36 and often normal 1, 2
- Bacterial meningitis is unlikely if ratio is normal 2
Critical Advantages Over Absolute CSF Glucose
The CSF/serum glucose ratio is superior to absolute CSF glucose values because it accounts for hyperglycemia and abnormal glucose metabolism, which can make absolute CSF glucose unreliable. 2, 6, 7
- The ratio remains accurate even after pre-hospital antibiotic administration, unlike CSF lactate measurements which lose diagnostic value after antibiotics 1
- In postoperative neurosurgical patients, CSF lactate (>4.0 mmol/L) was found superior to the glucose ratio, but this does not apply to community-acquired meningitis 5
Special Population Considerations
Neonates
- Term neonates require different thresholds: a ratio of 0.6 is considered abnormal (not 0.4 as in older children) 1
- Neonates have higher normal CSF/serum glucose ratios than older children and adults 1
Tuberculous Meningitis
- CSF glucose <2.2 mmol/L has 68% sensitivity and 96% specificity for TB meningitis 1, 6, 7
- **CSF/serum ratio <0.5** strongly suggests TB meningitis, especially with lymphocytic predominance and markedly elevated protein (>1 g/L) 6, 7
Practical Implementation
Point-of-Care Testing
- Bedside glucometer measurement of CSF/serum glucose ratio shows close correlation with laboratory methods (sensitivity 91.3%, specificity 89.3%) 8
- Glucometer testing significantly reduces turnaround time compared to laboratory methods without sacrificing accuracy 9, 8
- This rapid assessment can facilitate crucial decision-making on antibiotic initiation in the emergency department 9, 8
Prognostic Value
- Lower CSF/serum glucose ratios are associated with higher mortality in patients with confirmed bacterial meningitis 8
- This ratio can serve as both a diagnostic and prognostic marker 8
Common Pitfalls to Avoid
- Do not rely on absolute CSF glucose alone when serum glucose is abnormal; always calculate the ratio 2, 6, 7
- Do not use CSF lactate measurements after antibiotics have been administered, as they lose diagnostic value; the glucose ratio remains reliable 1
- Do not apply adult thresholds to neonates; use ratio <0.6 as the cutoff for this population 1
- CSF protein <0.6 g/L essentially excludes bacterial meningitis, making this a useful negative predictor when combined with normal glucose ratio 2