Interpreting CSF Results
Tip for juniors: Always interpret CSF in context of the clinical picture
(fever, neck stiffness, rash, seizures, focal neurology, immunocompromise, recent antibiotics).
Normal CSF does not completely exclude meningitis early in the illness.
Key CSF Parameters and Their Normal Values
| Parameter | Normal Value |
|---|---|
| Opening pressure | 70–180 mmH2O |
| Closing pressure | Similar to opening pressure |
| Appearance | Clear and colourless |
| Total protein | 15–45 mg/dL |
| Glucose | 50–80 mg/dL (≈ 2/3 of simultaneous blood glucose) |
| White blood cells (WBC) | 0–5 cells/µL (mostly lymphocytes) |
| Red blood cells (RBC) | 0 cells/µL |
Common CSF Patterns in Various Conditions
| Condition | Opening Pressure | Appearance | WBC Count | Protein | Glucose |
|---|---|---|---|---|---|
| Normal | 70–180 mmH2O | Clear and colourless | 0–5 cells/µL | 15–45 mg/dL | 50–80 mg/dL (or ~2/3 of blood glucose) |
| Bacterial meningitis | Raised | Cloudy or purulent | High (predominantly neutrophils) | Markedly increased | Low |
| Viral meningitis | Normal or slightly raised | Clear | Increased (predominantly lymphocytes) | Normal or slightly increased | Normal or slightly low |
| Fungal or tuberculous meningitis | Raised | Clear or slightly cloudy | Increased (predominantly lymphocytes) | Increased | Low |
| Subarachnoid haemorrhage | Raised | Xanthochromic or bloody | Variable | Increased | Usually normal |
| Multiple sclerosis | Normal | Clear | Normal or mildly ↑ | Increased (esp. IgG / oligoclonal bands) | Normal |
| Guillain–Barré syndrome | Normal | Clear | Normal | Increased (albuminocytologic dissociation) | Normal |
How to Approach CSF Interpretation
- Check the basics: Is the sample traumatic (lots of RBCs)? Was it taken before or after antibiotics?
- Look at appearance: Clear vs cloudy vs frankly bloody vs xanthochromic.
-
Check WBC count and differential:
- Neutrophil predominant → think bacterial (or early viral).
- Lymphocyte predominant → viral, TB, fungal, partially treated bacterial, malignancy.
- Interpret protein: High in bacterial, TB, fungal, GBS; mild in viral.
- Interpret glucose: Compare to blood glucose. Low CSF glucose strongly suggests bacterial / TB / fungal infection or malignancy.
- Always correlate with the patient: fever, rash, neck stiffness, immunocompromise, focal neurology, seizures, recent trauma or surgery.
Traumatic Tap vs Subarachnoid Haemorrhage (SAH)
- Traumatic tap: RBC count typically falls from tube 1 → tube 4, and the supernatant becomes clear after centrifugation (no xanthochromia initially).
- SAH: RBC count similar in all tubes, and xanthochromia appears after a few hours.
- When in doubt, treat the patient and discuss with a senior / neurosurgery rather than relying on CSF alone.
Red flags:
- Clinical features of meningitis or encephalitis with any abnormal CSF → start empiric antibiotics/antivirals, don’t wait.
- Immunocompromised patients can have “near-normal” CSF – have a low threshold for treatment.
- Very high opening pressure, reduced GCS, focal neurology → discuss with senior urgently; consider imaging if not already done.