Normal CSF Glucose and Protein Values
In normal cerebrospinal fluid, the glucose level ranges from 2.6-4.5 mmol/L (approximately 47-81 mg/dL), protein is <0.4 g/L (<40 mg/dL), and the CSF-to-plasma glucose ratio is >0.66 (approximately two-thirds of plasma glucose). 1
Normal Reference Values
- CSF Glucose (absolute): 2.6-4.5 mmol/L 1
- CSF Protein: <0.4 g/L 1
- CSF/Plasma Glucose Ratio: >0.66 1
- CSF White Cell Count: <5 cells/μL 1
Critical Clinical Context: Why the Ratio Matters More Than Absolute Values
The CSF-to-plasma glucose ratio is far more diagnostically useful than the absolute CSF glucose value alone, particularly when serum glucose is abnormal. 1, 2, 3
Key Reasoning:
- Normal CSF glucose is approximately two-thirds (66%) of plasma glucose 1
- In patients with hyperglycemia or hypoglycemia, the absolute CSF glucose can be misleading 2, 4
- The ratio remains stable across different blood glucose levels in patients with normal glucose metabolism (range 0.35-0.95) 5
- Always obtain simultaneous plasma glucose when performing lumbar puncture to calculate this ratio accurately 4
Pathological Thresholds for Common Conditions
Bacterial Meningitis:
- CSF/plasma glucose ratio <0.36 has 93% sensitivity and specificity for bacterial meningitis 1, 6
- Absolute CSF glucose typically <2.6 mmol/L 1
- CSF protein typically >2.2 g/L (>220 mg/dL) 1
Tuberculous Meningitis:
- CSF/plasma glucose ratio typically <0.5 2, 3
- Absolute CSF glucose <2.2 mmol/L has 68% sensitivity and 96% specificity 2, 3
- CSF protein markedly elevated, often >1 g/L 2, 3
Viral Meningitis:
- CSF/plasma glucose ratio remains normal or slightly low but stays >0.36 2
- CSF protein mildly elevated 1, 4
- Absolute CSF glucose typically normal or slightly low 1
Common Clinical Pitfalls
A CSF glucose >2.6 mmol/L makes bacterial meningitis unlikely 1, but this threshold becomes unreliable in patients with hyperglycemia or abnormal glucose metabolism 2, 5.
A CSF protein <0.6 g/L essentially excludes bacterial meningitis 1, 4, making this a useful negative predictor.
In patients with abnormal or unclear glucose metabolism, always use the CSF/average blood glucose ratio rather than absolute values to determine if CSF glucose is truly abnormal 5.
The CSF/blood glucose ratio of 0.31 provides excellent differentiation between infected and uninfected patients, with ratios below this level seen in bacterial meningitis even when absolute CSF glucose is not severely depressed 7.