Interpreting Cerebrospinal Fluid (CSF) Test Results
CSF test results must be interpreted in the context of clinical presentation, as no single parameter provides an absolute diagnosis. The pattern of abnormalities across multiple parameters is more informative than any individual value.
Normal CSF Parameters
| Parameter | Normal Range |
|---|---|
| Opening Pressure | 12-20 cm CSF |
| Appearance | Clear, colorless |
| White Blood Cell Count | <5 cells/μL |
| Protein | <0.4 g/L |
| Glucose | 2.6-4.5 mmol/L |
| CSF:Plasma Glucose Ratio | >0.66 |
Interpretation of Abnormal Findings
CSF Cell Count
- Elevated WBC (>5 cells/μL): Indicates inflammation within the CNS
- Neutrophil predominance: Suggests bacterial infection (early viral infections may also show neutrophil predominance initially)
- Lymphocyte predominance: Suggests viral, fungal, or tuberculous infection (partially treated bacterial meningitis may also show lymphocytic predominance) 1
- Normal WBC: Does not exclude infection, especially early in disease course or in immunocompromised patients 1
CSF Protein
- Elevated (>0.4 g/L): Indicates blood-brain barrier disruption
- Markedly elevated (>1 g/L): Suggests bacterial or tuberculous meningitis
- Mildly elevated (0.4-1 g/L): Common in viral meningitis, multiple sclerosis
- Albuminocytological dissociation (elevated protein with normal cell count): Characteristic of Guillain-Barré syndrome 2
CSF Glucose
- Low CSF:plasma glucose ratio (<0.36): Strong indicator of bacterial meningitis (93% sensitivity, 93% specificity) 3
- Normal/slightly low: Typical of viral meningitis
- Very low: Characteristic of bacterial or tuberculous meningitis 1
CSF Lactate
- Elevated (>35 mg/dL): Highly suggestive of bacterial meningitis (93% sensitivity, 96% specificity) if measured before antibiotic administration 1
Diagnostic Patterns
Bacterial Meningitis
- Turbid/cloudy appearance
- Elevated opening pressure
- Neutrophil predominant pleocytosis (typically >100 cells/μL)
- Markedly elevated protein
- Low CSF:plasma glucose ratio (<0.36)
- Elevated lactate (>35 mg/dL) 1, 3
Viral Meningitis/Encephalitis
- Clear appearance
- Normal/mildly elevated opening pressure
- Lymphocyte predominant pleocytosis (typically 5-1000 cells/μL)
- Mildly elevated protein
- Normal/slightly low glucose
- Normal lactate 1
Tuberculous Meningitis
- Clear or cloudy appearance
- Elevated opening pressure
- Lymphocyte predominant pleocytosis (typically 5-500 cells/μL)
- Markedly elevated protein
- Very low glucose 1
Special Considerations
Traumatic Lumbar Puncture
- Artificially elevates WBC count and protein
- Correction for WBC: Subtract 1 white cell for every 7000 red blood cells/μL
- Correction for protein: Subtract 0.1 g/dL protein for every 100 red blood cells 1, 2
Partially Treated Bacterial Meningitis
- May show lymphocytic predominance instead of expected neutrophilic pattern
- CSF lactate sensitivity drops to <50% if antibiotics given before LP 1
Timing Considerations
- Early in disease course, cell differential may be misleading:
10% of bacterial meningitis cases initially show lymphocytic predominance
- Viral meningitis may initially show neutrophil predominance 4
- If initial CSF is normal but clinical suspicion remains high, repeat LP in 24-48 hours 1
Specialized Testing
PCR Testing
- Indicated for suspected viral encephalitis (HSV, VZV, enteroviruses)
- Consider in patients with abnormal CSF parameters (elevated WBC, abnormal protein/glucose)
- In adults, PCR testing rarely positive with completely normal CSF parameters 5
- HSV PCR may be negative early in disease course; consider repeat testing if high clinical suspicion 1
Alzheimer's Disease Biomarkers
- CSF biomarkers (Aβ42, tau, p-tau) useful when diagnostic uncertainty remains after structural imaging
- Particularly valuable in early MCI or atypical presentations
- Interpretation requires consideration of pretest probability, age, clinical presentation, and possibility of multiple pathologies 1
Pitfalls to Avoid
- Relying on a single parameter: No single CSF parameter is definitive for diagnosis
- Neglecting to measure plasma glucose: CSF:plasma glucose ratio is more informative than absolute CSF glucose
- Misinterpreting traumatic tap: Failure to correct for blood contamination can lead to misdiagnosis
- Ruling out infection with normal initial CSF: Early infections may have normal parameters
- Overreliance on cell differential early in disease: Cell predominance patterns may be atypical early in disease course
Remember that CSF analysis is just one component of diagnosis. Clinical presentation, neuroimaging findings, and other laboratory tests should be considered together for accurate diagnosis and management.