CSF Glucose Ratio in Bacterial Meningitis
A CSF-to-blood glucose ratio <0.4 is highly specific for bacterial meningitis, and a ratio <0.23 predicts bacterial etiology with 99% certainty when combined with other CSF parameters. 1
Diagnostic Thresholds and Performance
Primary Diagnostic Cutoffs
- CSF/blood glucose ratio <0.4 is 80% sensitive and 98% specific for bacterial meningitis in children ≥2 months of age 1
- CSF/blood glucose ratio <0.23 is an individual predictor of bacterial (versus viral) meningitis with 99% certainty 1
- CSF/blood glucose ratio <0.36 provides optimal discrimination with 92.9% sensitivity and 92.9% specificity (area under curve 0.97) 2
- Absolute CSF glucose <34 mg/dL predicts bacterial meningitis with 99% certainty 1
Why the Ratio Matters More Than Absolute Values
The CSF/blood glucose ratio is superior to absolute CSF glucose values because serum hyperglycemia or hypoglycemia can render absolute CSF glucose misleading 3. The ratio remains diagnostically accurate even after antibiotics have been administered prior to lumbar puncture 2.
Differential Diagnosis Using CSF Glucose Ratio
Bacterial Meningitis
- CSF/blood glucose ratio typically <0.36 2
- Absolute CSF glucose typically <40 mg/dL in 50-60% of cases 1
- CSF glucose <35 mg/dL with CSF/blood ratio <0.23 indicates bacterial etiology 1
Viral Meningitis
- CSF/blood glucose ratio typically >0.36 and often normal 4, 5
- CSF glucose is normal or only slightly decreased 4
- Ratio remains above 0.36 even when mildly decreased 4, 5
Tuberculous Meningitis
- CSF/blood glucose ratio typically <0.5 (very low) 3
- Absolute CSF glucose <2.2 mmol/L has 68% sensitivity and 96% specificity for TB meningitis 3
- CSF/serum ratio <0.5 is highly suggestive of TB meningitis when lymphocytic pleocytosis is present 3
Clinical Algorithm for Interpretation
Step 1: Calculate the ratio immediately
- Obtain simultaneous blood and CSF glucose measurements 2
- Calculate CSF/blood glucose ratio at bedside using point-of-care glucometer (turnaround time 5 minutes vs. 112 minutes for laboratory) 6
Step 2: Apply diagnostic thresholds
- Ratio <0.23: Bacterial meningitis highly likely (99% certainty when combined with other parameters) 1
- Ratio 0.23-0.36: Bacterial meningitis probable; consider empirical antibiotics 2
- Ratio 0.36-0.5: Consider tuberculous meningitis if lymphocytic predominance present 3
- Ratio >0.5: Viral meningitis more likely; bacterial meningitis less probable 4, 5
Step 3: Integrate with other CSF parameters
- The combination of CSF glucose <34 mg/dL, CSF/blood ratio <0.23, protein >120 mg/dL, WBC >12,000/mm³, or neutrophils >11,000/mm³ predicts bacterial meningitis with 99% certainty 1
Critical Pitfalls to Avoid
Neonatal Considerations
- Term neonates have higher normal CSF/blood glucose ratios 1
- A ratio of 0.6 is considered abnormal in neonates (not 0.4 as in older children) 1
Timing and Antibiotic Effects
- The CSF/blood glucose ratio remains accurate even after pre-hospital antibiotic administration 2
- However, CSF lactate measurements lose diagnostic value after antibiotics are given 4
Serum Glucose Confounders
- Never rely on absolute CSF glucose alone when serum glucose is abnormal (diabetes, stress hyperglycemia, hypoglycemia) 3
- Always calculate the ratio to account for serum glucose abnormalities 3
Early Viral Meningitis
- Neutrophils may predominate early in viral meningitis (especially enteroviral), but the CSF/blood glucose ratio typically remains >0.36 4, 5
- Total CSF WBC rarely exceeds 2000/mm³ in viral meningitis even with neutrophil predominance 4
Practical Implementation
Point-of-care testing is highly accurate for CSF glucose measurement using standard glucometers, with no significant difference from laboratory methods (p=0.46) 7. Bedside glucometer testing provides results in 5 minutes with sensitivity 94.1% and specificity 91% for detecting bacterial meningitis using a cutoff of 0.46 6.
Do not delay antibiotics while waiting for imaging or laboratory glucose results if bacterial meningitis is suspected clinically—obtain blood cultures and start empirical therapy immediately 1.