CSF Analysis Reference Ranges and Disease-Specific Findings
Normal Adult CSF Parameters
In healthy adults, cerebrospinal fluid should contain ≤5 white blood cells/µL, essentially 0 red blood cells in non-traumatic taps, CSF/plasma glucose ratio of 0.6-0.7, and protein <0.6 g/L. 1, 2
Cell Counts
- White blood cells: ≤5 cells/µL 3, 1, 4, 2
- Red blood cells: Essentially 0 cells/µL in non-traumatic lumbar puncture 1, 2
- Opening pressure: 12-20 cm CSF 3, 4
Biochemical Parameters
- CSF glucose: 2.6-4.5 mmol/L (approximately 2/3 of plasma glucose) 3
- CSF/plasma glucose ratio: 0.6-0.7 (normally >0.66) 3, 1, 2
- Protein: <0.4-0.6 g/L 3, 1, 2
- Lactate: Normal range varies by age; 5th percentile 0.88-1.41 mmol/L, 95th percentile 2.00-2.71 mmol/L 5
Bacterial Meningitis
Bacterial meningitis typically presents with turbid CSF, WBC >100 cells/µL (often >2,000), neutrophil predominance, very low glucose (CSF/plasma ratio <0.36), and markedly elevated protein, though 10% of cases have <100 cells/mm³. 3, 4
CSF Characteristics
- Appearance: Turbid, cloudy, or purulent 3, 4
- Opening pressure: Raised (>20 cm CSF) 3, 4
- WBC count: Typically >100 cells/µL, often >2,000 cells/µL 3, 4
- Predominant cell type: Neutrophils 3, 4
- Protein: Markedly raised; unlikely if <0.6 g/L 3
- Glucose: Very low; CSF/plasma ratio typically <0.36 (93% sensitivity/specificity) 3
- Lactate: >35 mg/dL has 93% sensitivity and 96% specificity 3, 1
Critical Diagnostic Caveats
- 10% of bacterial meningitis cases present with <100 cells/mm³ 3, 1, 2
- Lymphocyte predominance can occur with Listeria or partially treated bacterial meningitis 3
- Normal CSF parameters do not exclude meningitis in immunocompromised patients 2
Viral Meningitis
Viral meningitis characteristically shows clear CSF with 5-1,000 WBCs/µL, lymphocyte predominance (though neutrophils may predominate early in enteroviral disease), normal or slightly low glucose, and mildly elevated protein. 3, 4
CSF Characteristics
- Appearance: Clear 3, 4
- Opening pressure: Normal or mildly raised 3, 4
- WBC count: 5-1,000 cells/µL 3, 4
- Predominant cell type: Lymphocytes (neutrophils may predominate early, especially in enteroviral disease) 3, 4
- Protein: Mildly raised 3, 4
- Glucose: Normal or slightly low 3, 4
Special Considerations
- 5-10% of HSV encephalitis cases have completely normal initial CSF with no pleocytosis and negative HSV PCR 1, 2
- Repeat lumbar puncture at 24-48 hours is recommended if initial CSF is normal but clinical suspicion remains high 1, 2
- Neutrophil predominance unlikely if total WBC >2,000 cells/mm³ 3
Tuberculous Meningitis
Tuberculous meningitis presents with raised opening pressure, clear or cloudy CSF, 5-500 WBCs/µL with lymphocyte predominance, markedly raised protein, and very low glucose. 3, 4
CSF Characteristics
- Appearance: Clear or cloudy 3
- Opening pressure: Raised 3
- WBC count: 5-500 cells/µL 3
- Predominant cell type: Lymphocytes 3
- Protein: Markedly raised 3
- Glucose: Very low 3
- CSF/plasma glucose ratio: Very low 3
Fungal Meningitis
Fungal meningitis demonstrates raised opening pressure, clear or cloudy appearance, 5-500 WBCs/µL with lymphocyte predominance, raised protein, and low glucose. 3
CSF Characteristics
- Appearance: Clear or cloudy 3
- Opening pressure: Raised 3
- WBC count: 5-500 cells/µL 3
- Predominant cell type: Lymphocytes 3
- Protein: Raised 3
- Glucose: Low 3
- CSF/plasma glucose ratio: Low 3
Correction for Traumatic Tap
When blood contamination occurs during lumbar puncture, subtract 1 WBC for every 700 RBCs present in CSF and reduce protein by 0.1 g/dL for every 100 RBCs. 1, 2
WBC Correction Methods
- Simple method: Subtract 1 WBC for every 700 RBCs 1, 2
- Alternative formula: True CSF WBC = Actual CSF WBC - [(Peripheral blood WBC × CSF RBC) / Peripheral blood RBC] 1, 2
- If corrected WBC count exceeds 10 times the predicted contamination, this strongly suggests true meningitis 1
Protein Correction
Critical Diagnostic Thresholds
Normal opening pressure, <5 WBC/µL, and normal CSF protein essentially exclude meningitis in immunologically normal hosts. 1, 4
Key Decision Points
- CSF glucose >2.6 mmol/L is unlikely to be bacterial meningitis 3
- CSF protein <0.6 g/L suggests against bacterial meningitis 3
- CSF/plasma glucose ratio <0.36 has 93% sensitivity/specificity for bacterial meningitis 3
- CSF lactate >35 mg/dL has 93% sensitivity and 96% specificity for bacterial meningitis (if antibiotics not yet given) 3, 1
Gram Stain and Culture Performance
- Gram stain sensitivity: 50-99% (dependent on organism and prior antibiotics), specificity 97-100% 4
- Culture diagnostic yield: 70-85% in untreated cases 4
- CSF PCR sensitivity: 87-100%, specificity 98-100% (especially valuable if antibiotics given prior to LP) 4
Special Population Considerations
Immunocompromised Patients
- Maintain high suspicion for infection regardless of cell count until cultures are final 1, 2
- Bacterial meningitis may present with minimal or absent pleocytosis 2
- Additional testing required: HSV PCR, CMV testing, fungal studies, cryptococcal antigen 1, 2
Procedural Safety
- Check platelet count before lumbar puncture in hematologic malignancy patients 2
- Platelet transfusion recommended when count <20,000/µL 2
- Known thrombocytopenia is a contraindication to immediate lumbar puncture 2
Essential CSF Collection and Processing
Collect at least 22 mL of CSF from adults, with the first 2-2.5 mL allocated for cell count with differential, microscopy, culture, and sensitivities. 4
Volume Requirements
- Total collection: At least 22 mL 4
- Cell count/culture: First 2-2.5 mL (minimum 5 mL optimal) 4
- Viral studies: 2 mL 4
- Tuberculous meningitis: 6 mL 4
- Protein/glucose: 1-2 mL 4