What tests should be ordered when sending spinal fluid to the lab?

Standard CSF Laboratory Testing Panel

When sending cerebrospinal fluid to the laboratory, order the following core tests: cell count with differential, glucose, protein, Gram stain, and bacterial culture. 1 These five basic tests form the foundation of CSF analysis and should be performed on every sample when CNS infection is suspected.

Essential Core Tests (Order on Every Sample)

Basic Chemistry and Cell Analysis

• Cell count with differential - distinguishes bacterial from viral/aseptic meningitis based on neutrophil predominance 1
• Glucose concentration - bacterial meningitis typically shows CSF glucose <35-40 mg/dL or CSF/blood glucose ratio <0.23-0.36 1, 2, 3
• Protein concentration - elevated in bacterial meningitis (typically >220 mg/dL) 1, 2
• Opening pressure (if measured in lateral decubitus position, NOT sitting) - normal is 10-20 cm H₂O; >25 cm H₂O suggests meningitis or increased intracranial pressure 4, 5

Microbiological Studies

• Gram stain - identifies bacteria in 60-90% of untreated bacterial meningitis, though sensitivity drops to 40-60% after antibiotic pretreatment 1, 5
• Bacterial culture - definitive diagnosis, though may be negative after antibiotic administration 1

Additional Tests Based on Clinical Context

Viral Pathogens (Common Scenarios)

• HSV-1/2 PCR - for suspected encephalitis (sensitivity >95%) 1, 5
• Enterovirus PCR - for aseptic meningitis, especially in children 1
• VZV PCR - for varicella-zoster meningoencephalitis 1
• Parechovirus PCR - particularly in young infants with sepsis-like presentation 1

Immunocompromised Patients (Expanded Panel)

• CMV PCR - for cytomegalovirus encephalitis in HIV/transplant patients 1
• HHV-6/7 PCR - for human herpesvirus encephalitis 1
• Cryptococcal antigen - replaces India ink stain for fungal meningitis 1
• Toxoplasma PCR or serology - for toxoplasma encephalitis 1
• JC virus PCR - for progressive multifocal leukoencephalopathy 1
• Fungal stains and cultures - for suspected fungal meningitis 1

Chronic/Subacute Meningitis

• AFB smear and mycobacterial culture - for tuberculous meningitis (requires large volume ≥5 mL for optimal sensitivity) 1, 2
• VDRL - for neurosyphilis 1
• Oligoclonal bands and IgG index - for multiple sclerosis or chronic inflammatory conditions 1

Special Circumstances

• CSF lactate - levels ≥2 mmol/L support bacterial meningitis; <2 mmol/L helps exclude bacterial disease 2, 5
• West Nile virus IgM - for arboviral encephalitis during summer/fall 1
• Bartonella antibodies - with cat exposure and seizures 1
• Anti-NMDAR antibodies (CSF and serum) - for autoimmune encephalitis with psychotic features or movement disorders 1
• PCR multiplex panels - particularly valuable in patients who received antibiotics before lumbar puncture, as four of five S. pneumoniae cases were detected only by PCR after antibiotic pretreatment 6

Critical Volume and Transport Considerations

Sample Volume Requirements

• Minimum 1 mL for basic bacterial testing 2
• Collect 20 mL if possible and freeze 5-10 mL for additional testing 1
• Large volumes (≥5 mL) significantly increase sensitivity for mycobacteria and fungi 1, 2

Transport and Handling

• Room temperature transport within 2 hours for most tests 1
• Never refrigerate CSF - this kills fastidious organisms 1
• Closed sterile container for all specimens 1

Common Pitfalls to Avoid

Testing Errors

• Do NOT order extensive additional tests if opening pressure, cell count, and protein are all normal - among 334 such cases, 1385 additional tests were performed but useful in only 0.9% of patients 7
• Exception to above rule: immunocompromised patients, suspected multiple sclerosis, or childhood bacterial meningitis may require additional testing despite normal basic parameters 1, 7
• Samples with >500 RBCs/μL should be excluded from biomarker studies due to blood contamination 2

Interpretation Errors

• CSF/blood glucose ratio <0.36 is more precise than absolute CSF glucose for diagnosing bacterial meningitis (sensitivity 92.9%, specificity 92.9%) even after antibiotic pretreatment 3
• Antibiotic pretreatment ≥12 hours significantly alters CSF chemistry - increases glucose (48 vs 29 mg/dL) and decreases protein (121 vs 178 mg/dL) - but does NOT significantly change cell count or differential 8
• PCR multiplex testing has limited value in samples with normal cell count - only 2 positive results obtained, both clinically irrelevant HHV-6 6