Sample Sources

Cerebrospinal fluid (CSF) infections, particularly meningitis and ventriculitis, are critical medical emergencies. Rapid and accurate diagnosis of these infections is essential for preventing severe neurological damage and death. The analytical procedures for CSF bacteriology must be carefully performed

Sources

Overview
- Normally Sterile: CSF, which bathes the brain and spinal cord, is normally sterile
- Infection Indication: The presence of microorganisms in CSF indicates a serious infection
- Specimen Source: CSF is obtained through a procedure to insert a needle into the spinal canal
Specific Specimen Sources
- Lumbar Puncture (LP)
  - Most Common Source: Performed to collect CSF from the lumbar cistern (space surrounding the spinal cord in the lower back)
  - Indications
    - Suspicion of meningitis (inflammation of the meninges, the membranes surrounding the brain and spinal cord)
    - Suspicion of encephalitis (inflammation of the brain)
    - Fever of unknown origin with neurologic symptoms
    - Subarachnoid hemorrhage (bleeding in the space surrounding the brain)
    - Certain neurologic disorders
  - Procedure
    - Performed by a physician
    - Patient positioned either in a lateral recumbent (lying on their side) or sitting position
    - Site preparation: Meticulous aseptic technique is critical
    - Needle is inserted between the lumbar vertebrae (usually L3-L4 or L4-L5)
    - CSF is collected into sterile tubes
    - Usually 3-4 tubes of CSF are collected
    - 1st tube: Routine chemistries
    - 2nd tube: Microbiology
    - 3rd tube: Cell counts
    - 4th tube: Additional studies, if needed
  - Collection Precautions
    - Aseptic technique to prevent contamination
    - Proper tube order to minimize contamination (e.g., microbiology before other tests)
    - Appropriate volume (1-3 mL per tube)
    - Immediate transport to the laboratory
    - Prevent leaks of fluid
- Shunt and Reservoir
  - Alternative Collection: Used to obtain CSF from patients with implanted devices
    - Shunt: A tube placed to drain excess CSF from the ventricles (fluid-filled cavities in the brain) to another body cavity (usually the peritoneum)
    - Reservoir: A subcutaneous device that allows for direct access to the ventricles for CSF sampling or medication administration
  - Indications
    - Suspicion of ventriculitis (inflammation of the ventricles), often associated with shunt infections
    - Fever in a patient with a shunt or reservoir
    - Neurologic symptoms in a patient with a shunt or reservoir
  - Procedure
    - Performed by a physician, often with surgical scrub and sterile technique
    - Palpate the shunt or reservoir to locate the device
    - Clean the skin over the device with antiseptic
    - Insert a needle into the shunt or reservoir to aspirate CSF. Sometimes the shunt has a special port to make this easier
    - Collect CSF into sterile tubes
  - Collection Precautions
    - Strict aseptic technique is crucial to prevent infection of the shunt or reservoir
    - Proper disinfection of the insertion site and device is critical
    - Collect an adequate volume of CSF for testing
    - Immediately transport the sample to the laboratory

Factors Affecting Specimen Quality

Aseptic Technique
- Contamination: Contamination of the CSF sample during collection is a serious problem
- Sources of Contamination: Skin flora
- Consequences: Can lead to false-positive results, incorrect diagnosis, and inappropriate treatment
- Prevention: Strict aseptic technique is essential
Traumatic Tap
- Definition: Blood entering the CSF sample during the LP procedure
- Causes: Accidental puncture of a blood vessel during the LP procedure
- Consequences
  - Cell Count Interference: Can artificially elevate the white blood cell (WBC) count and red blood cell (RBC) count, making interpretation of results difficult
  - Culture Interference: Blood in the CSF can inhibit bacterial growth and interfere with the detection of organisms
- Differentiation from Subarachnoid Hemorrhage: The clinical team must differentiate between a traumatic tap and a true subarachnoid hemorrhage
  - Appearance: A traumatic tap will often have decreasing red blood cell counts from tube 1 to tube 4
  - Laboratory tests: The lab may perform a xanthochromia test (measuring the presence of bilirubin in the CSF). Xanthochromia is a yellow discoloration of the CSF that indicates the presence of bilirubin, which is a breakdown product of hemoglobin. The presence of xanthochromia suggests that blood has been present in the CSF for a period of time, which is consistent with subarachnoid hemorrhage, not a traumatic tap
Volume of CSF Collected
- Adequate Volume: Sufficient volume is required for all necessary tests, especially for microbiology
- Too Little Volume: May compromise the sensitivity of the culture and other tests
- Laboratory guidelines
  - The lab should have guidelines regarding the volume of CSF required for the different tests (e.g., cell counts, chemistry, culture)
  - Generally, at least 1-2 mL of CSF is required for culture
Transport and Storage
- Prompt Transport: CSF should be transported to the laboratory immediately after collection
- Delays: Delays can affect the viability of organisms and lead to false-negative culture results
- Transport Conditions
  - Transport at room temperature
  - Protect from extreme temperatures
- Storage
  - If processing is delayed, store CSF at 4°C
  - Prioritize culturing
  - CSF is never frozen
Tube Order
- Importance: Correct tube order is crucial for minimizing contamination and optimizing testing
- Recommendations
  - Tube 1: Usually for chemistry tests and protein
  - Tube 2: Microbiology (culture and Gram stain). This is the priority
  - Tube 3: Cell counts (WBC, RBC)
  - Tube 4: Additional tests, if needed (e.g., molecular tests)
- Rationale: Minimizes the risk of introducing skin flora into the specimen for microbiology

Key Terms

Cerebrospinal Fluid (CSF): The clear, colorless fluid that surrounds the brain and spinal cord
Lumbar Puncture (LP): The procedure to collect CSF from the lumbar cistern
Meningitis: Inflammation of the meninges, the membranes surrounding the brain and spinal cord
Encephalitis: Inflammation of the brain
Ventriculitis: Inflammation of the ventricles (fluid-filled cavities in the brain)
Shunt: A tube placed to drain excess CSF from the ventricles to another body cavity
Reservoir: A subcutaneous device that allows for direct access to the ventricles
Aseptic Technique: A set of procedures used to prevent contamination of a specimen with microorganisms from the environment
Traumatic Tap: Blood entering the CSF sample during the LP procedure
Xanthochromia: Yellow discoloration of the CSF, indicating the presence of bilirubin
Gram Stain: A staining technique used to differentiate bacteria based on cell wall characteristics
Culture: The growth of microorganisms in a laboratory setting for identification and susceptibility testing

Cerebrospinal Fluid

Colony Morphology & ID