Normal Cerebrospinal Fluid Laboratory Parameters
Normal adult CSF contains ≤5 leukocytes/µL, essentially 0 erythrocytes, protein <0.4-0.6 g/L, glucose 2.6-4.5 mmol/L, CSF:plasma glucose ratio >0.66, opening pressure 12-20 cm CSF, and clear appearance. 1
Standard Adult CSF Parameters
Cell Counts
- Leukocytes: ≤5 cells/µL is the normal upper limit 2, 1
- Erythrocytes: Essentially 0 cells/µL in non-traumatic lumbar punctures 1
- Predominant cell type: None to rare lymphocytes 2
Biochemical Parameters
- Protein: <0.4 g/L (some laboratories use <0.6 g/L as upper limit) 2, 1
- Glucose: 2.6-4.5 mmol/L 2, 1
- CSF:plasma glucose ratio: >0.66 (normally approximately two-thirds of plasma glucose) 2, 1
- Lactate: <2 mmol/L (<35 mg/dL) 2, 1
Physical Characteristics
- Opening pressure: 12-20 cm CSF (approximately 120-200 mm H₂O) 2, 1
- Appearance: Clear and transparent 2, 1
Correction for Traumatic Lumbar Puncture
When blood contaminates CSF during a traumatic tap, corrections are necessary to interpret results accurately:
White Blood Cell Correction
- Standard approximation: Subtract 1 leukocyte for every 700 erythrocytes present 2, 1
- Alternative formula: True CSF leukocytes = Actual CSF leukocytes - [(Blood leukocytes × CSF erythrocytes) / Blood erythrocytes] 1
Protein Correction
- Standard correction: Subtract 0.1 g/dL for every 100 erythrocytes 1
Critical Clinical Caveats
When Normal Values Don't Exclude Disease
- Bacterial meningitis can present with normal or minimally elevated CSF, particularly in early disease, immunocompromised patients, or after antibiotic administration 2, 1
- Approximately 10% of bacterial meningitis cases have <100 cells/mm³ 1
- Normal opening pressure, ≤5 white blood cells/µL, and normal protein essentially exclude meningitis in immunocompetent hosts, but not in immunocompromised patients 2
Viral and Autoimmune Encephalitis
- Normal CSF does not exclude viral encephalitis, as up to 10% of proven cases have completely normal CSF, especially early in illness 2, 3
- Approximately 90% of NMDA receptor encephalitis cases have normal MRI initially, and CSF may show only mild lymphocytosis or be completely normal 3
Utility of CSF Lactate
- CSF lactate <2 mmol/L practically excludes bacterial meningitis with 93% sensitivity and 96% specificity when measured before antibiotic administration 2, 1
- If antibiotics have been given, sensitivity drops to less than 50% 2
Pediatric Considerations
- Normal CSF values vary significantly with age in the first months of life 1
- Neonates have higher leukocyte counts and protein levels that decrease progressively 1
- Opening pressure may be lower in neonates and infants compared to adults 1
Special Circumstances
Neurosyphilis Testing
- CSF VDRL is highly specific (90.3-99.3%) but has limited sensitivity (49-87%) 2, 4
- A non-reactive CSF VDRL does NOT exclude neurosyphilis due to limited sensitivity 4
- False positives are rare but can occur with CNS malignancy or carcinomatosis meningea 4