CSF Analysis for CNS Diseases

Standard CSF Analysis Components

• The American College of Critical Care Medicine recommends a comprehensive CSF analysis panel including cell count with differential, glucose and protein concentrations, Gram stain, and bacterial cultures as the baseline workup for all patients with suspected CNS infection 1, 2
• Cell count with differential is mandatory for every CSF sample, as it provides critical diagnostic information about the inflammatory process 1, 2
• Protein concentration should be measured, with normal values typically <220 mg/dL; elevations suggest infection or inflammation 1, 2
• Glucose measurement with simultaneous serum glucose is essential; normal CSF glucose is >35 mg/dL with CSF-to-blood ratio >0.23 1, 2
• Gram stain and bacterial culture must be performed on all samples when infection is suspected 1, 2
• Opening pressure should be measured when clinically indicated, as normal pressure (<250 mm H₂O), fewer than 5 WBCs/μL, and normal protein essentially excludes meningitis in immunocompetent patients 1, 2

Characteristic CSF Findings by Disease

• Bacterial meningitis typically presents with ≥2,000 total WBCs/μL or ≥1,180 neutrophils/μL, CSF glucose <35 mg/dL with CSF-to-blood ratio <0.23, and protein concentration ≥220 mg/dL 1, 2
• Viral meningitis/encephalitis typically shows mild to moderate lymphocytic pleocytosis (5-1,000 cells/μL), normal or mildly elevated protein, and normal glucose 2, 3
• Autoimmune encephalitis commonly demonstrates mild to moderate lymphocytic pleocytosis (20-200 cells), elevated protein, and elevated IgG index/synthesis rate and oligoclonal bands in some cases 4, 5

Specialized Testing Based on Clinical Context

• For suspected bacterial meningitis, Gram stain, culture, and antimicrobial susceptibility testing are the cornerstone of bacterial diagnosis, and empirical antibiotics should be started immediately after blood cultures if lumbar puncture is delayed 1, 2
• For suspected viral encephalitis, PCR testing for HSV-1/2 is mandatory, and VZV PCR and IgG/IgM should be included 2, 3, 5
• For immunocompromised patients, expanded testing is essential and should include cryptococcal antigen testing, fungal stains and cultures, acid-fast bacilli smears and cultures, and PCR for opportunistic pathogens 1, 2
• For suspected autoimmune encephalitis, CSF analysis is the most important test and should be performed regardless of MRI findings, and neuronal antibody panels should be tested in both CSF and serum 4, 5

Critical Timing and Contraindications

• Proceed with LP immediately in any febrile patient with altered consciousness or focal neurologic signs, and in suspected meningitis or encephalitis 1, 3, 4, 5
• Obtain CT/MRI before LP in patients with new focal neurologic findings or signs of increased intracranial pressure 1, 4, 5
• If imaging delays LP and bacterial meningitis is suspected, start empirical antibiotics immediately after blood cultures 1

Cerebrospinal Fluid Interpretation in Suspected CNS Infection or Inflammation

Diagnostic Parameters and Considerations

• The Infectious Diseases Society of America recommends that a CSF lactate level ≥35 mg/dL (or ≥2 mmol/L) has 93% sensitivity and 96% specificity for bacterial vs. viral differentiation when measured before antibiotics 6, 7
• A CSF lactate <2 mmol/L effectively rules out bacterial disease 6, 7
• HSV encephalitis can be hemorrhagic with elevated CSF red cell count in approximately 50% of cases 6, 7
• An acellular CSF is described for VZV, EBV, and CMV, occurring more frequently in immunocompromised patients 6, 7
• CSF lactate measurement becomes crucial in the scenario of partially treated bacterial meningitis 6, 7
• Non-viral infections, particularly TB and listeriosis, can present with lymphocytic CSF pleocytosis similar to viral infections 6, 7
• Clinical setting and other CSF parameters (low glucose ratio and higher protein) help distinguish these from viral causes 6, 7
• For suspected viral encephalitis, all patients must have CSF PCR for HSV-1, HSV-2, VZV, and enteroviruses, as this identifies 90% of known viral cases 6, 7
• Consider EBV/CMV PCR, especially if immunocompromised 6, 7
• Further testing should be directed by clinical features such as travel history and animal/insect contact 6, 7
• When to delay LP and obtain imaging first, coagulopathy (platelet count <100 × 10⁹/L) is a consideration 6, 7
• Local skin infection at puncture site is also a consideration for delaying LP 6, 7
• Clinical suspicion of spinal cord compression is another reason to delay LP 6, 7
• When blood contaminates the CSF sample, subtract 1 white cell for every 700 red blood cells 6, 7
• However, in HSV encephalitis, blood-stained CSF may reflect the hemorrhagic pathophysiology of the condition, especially if serial specimens are blood-stained 6, 7
• In patients with CNS infections receiving beta-lactam antibiotics, concomitant blood and CSF sampling at 24-48 hours after treatment onset may be useful to ensure adequate antibiotic exposure 8, 9
• Do not rely on a single CSF parameter—interpretation requires integration of all findings with clinical context 6, 7
• High CSF lactate may also indicate metabolic disorders, particularly mitochondrial encephalopathy 6, 7