How do you calculate the cerebrospinal fluid (CSF) to plasma glucose ratio?

How to Calculate the CSF to Plasma Glucose Ratio

The CSF to plasma glucose ratio is calculated by dividing the CSF glucose concentration by the plasma (blood) glucose concentration, both measured in the same units (mg/dL or mmol/L), with plasma glucose obtained immediately before or at the time of lumbar puncture. 1

Critical Timing Requirements

• Plasma glucose must be measured just before the lumbar puncture to ensure accurate interpretation of CSF results 1
• The Association of British Neurologists emphasizes that without simultaneous plasma glucose measurement, interpretation of CSF glucose is very difficult and potentially misleading 1
• Studies demonstrate that failure to measure plasma glucose at the same time as CSF glucose is a common and significant clinical error 1

The Calculation Formula

CSF to Plasma Glucose Ratio = CSF Glucose (mg/dL or mmol/L) ÷ Plasma Glucose (mg/dL or mmol/L)

For example:

• If CSF glucose = 40 mg/dL and plasma glucose = 100 mg/dL
• Ratio = 40 ÷ 100 = 0.40

Normal and Pathological Values

Normal Reference Range

• Normal CSF to plasma glucose ratio is >0.66 (or approximately two-thirds of plasma glucose) 2
• Normal CSF glucose ranges from 2.6-4.5 mmol/L (approximately 47-81 mg/dL) 2

Pathological Thresholds

• Ratio <0.36 has 93% sensitivity and specificity for bacterial meningitis 2
• Ratio <0.31 provides optimal differentiation between bacterial meningitis and non-inflammatory conditions 3
• Ratio <0.5 is typical for tuberculous meningitis 4
• Viral meningitis typically maintains a ratio >0.36 2, 4

Common Clinical Pitfalls to Avoid

Hyperglycemia Confounding

• The absolute CSF glucose value becomes unreliable in patients with hyperglycemia or abnormal glucose metabolism 2
• In diabetic patients, the ratio calculation is essential because absolute CSF glucose may appear falsely normal despite pathology 3
• Using the ratio increases sensitivity for detecting pathologic hypoglycorrhachia with minimal loss of specificity 3

Timing Errors

• If concentrated glucose solution is given intravenously shortly before lumbar puncture, the ratio becomes unreliable 3
• This can produce falsely elevated ratios even in the presence of infection 3

Special Populations

• In post-hemorrhagic hydrocephalus, hypoglycorrhachia (low CSF glucose) is a normal finding and does not indicate infection 5
• The median CSF to plasma glucose ratio in these patients can be as low as 0.235 without infection present 5

Clinical Application Algorithm

1. Obtain plasma glucose immediately before or during lumbar puncture 1

2. Calculate the ratio using the same units for both values 3

3. Interpret based on clinical context:

   • Ratio >0.66: Normal, viral infection possible 2
   • Ratio 0.36-0.66: Borderline, consider repeat testing or alternative diagnoses 2
   • Ratio <0.36: Bacterial meningitis highly likely 2
   • Ratio <0.31: Bacterial meningitis very likely 3
   • Ratio <0.5 with lymphocytic predominance: Consider tuberculous meningitis 4

4. Always correlate with other CSF parameters (protein, cell count, differential) as the ratio alone is insufficient for diagnosis 1