CSF to Serum Glucose Ratio Calculator
CSF to Serum Glucose Ratio Calculator
Calculate the CSF:serum glucose ratio, a more reliable pointer to bacterial meningitis than the absolute CSF glucose alone.
CSF : Serum Glucose Ratio
CSF ÷ serumCSF glucose 30 mg/dL, serum glucose 110 mg/dL
Formula
- CSF glucose
- mg/dL, from the lumbar puncture sample
- serum glucose
- mg/dL, ideally drawn at or near the time of the lumbar puncture
- normal ratio
- approximately 0.6, reflecting the equilibrium CSF glucose normally reaches with plasma
Worked example
CSF glucose 30 mg/dL, serum glucose 110 mg/dL
30 ÷ 110 = 0.27
Below 0.4 → markedly reduced, strongly suggests bacterial, tuberculous or fungal meningitis
Interpreting the ratio
| Ratio | Interpretation | Typical cause |
|---|---|---|
| < 0.4 | Markedly reduced | Bacterial, tuberculous or fungal meningitis |
| 0.4 – 0.6 | Reduced | Partially treated bacterial meningitis, tuberculous meningitis, carcinomatous meningitis, subarachnoid haemorrhage |
| ≥ 0.6 | Normal | Viral meningitis; no CNS infection |
Why the ratio, and not the CSF glucose alone
CSF glucose reaches the cerebrospinal fluid by facilitated transport across the blood-brain and blood-CSF barriers and equilibrates with plasma glucose over roughly one to three hours. Because of that lag, the absolute CSF glucose value depends heavily on whatever the plasma glucose happened to be over the preceding hours — a CSF glucose of 45 mg/dL is entirely normal at a serum glucose of 75 mg/dL, and distinctly low at a serum glucose of 300 mg/dL. Expressing CSF glucose as a ratio to a simultaneously drawn serum sample corrects for this and makes the number comparable across patients regardless of their concurrent blood glucose.
A ratio below 0.4 strongly suggests bacterial, tuberculous or fungal meningitis: the causative organisms and the accompanying leukocyte response both consume glucose within the CSF, and bacterial infection additionally impairs the glucose transport mechanism itself. Viral meningitis characteristically spares the ratio, since neither of those processes operates to the same degree — a normal ratio is reassuring against bacterial infection but does not exclude a partially treated or early bacterial process.
Because the ratio depends on equilibration, the serum sample should be drawn at or near the time of the lumbar puncture — ideally within the same hour. A serum sample drawn well before or after, particularly if the patient’s glucose is actively changing, produces a ratio that reflects the mismatch in timing rather than the true CSF:serum relationship.
Frequently asked questions
What is a normal CSF to serum glucose ratio?
Approximately 0.6 or above. This reflects the equilibrium CSF glucose normally reaches with plasma glucose.
Why is the ratio more useful than CSF glucose alone?
Because CSF glucose equilibrates with plasma glucose, the absolute value depends heavily on the concurrent serum level — the same CSF glucose can be normal or low depending on the serum value. The ratio corrects for this.
What does a low CSF:serum glucose ratio suggest?
Below 0.4 strongly suggests bacterial, tuberculous or fungal meningitis, where organisms and leukocytes consume CSF glucose and bacterial infection also impairs its transport. Viral meningitis characteristically spares the ratio.
When should the serum sample be drawn relative to the lumbar puncture?
At or near the same time, ideally within the same hour, because CSF glucose equilibrates with plasma over one to three hours and a mistimed serum sample distorts the ratio.
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References
- Spanos A et al. Differential diagnosis of acute meningitis. An analysis of the predictive value of initial observations. JAMA. 1989;262(19):2700–7.
- Brouwer MC et al. Community-acquired bacterial meningitis. Nat Rev Dis Primers. 2016;2:16074.