Body Fluids CSF PP

Chapter 10: Cerebrospinal Fluid (CSF)

Learning Outcomes

Upon completing this chapter, the reader will be able to:

  1. Describe the formation and major functions of cerebrospinal fluid (CSF).
  2. Correctly distribute CSF specimen tubes numbered 1, 2, 3, and possibly 4 to their appropriate laboratory sections and preserve them correctly.
  3. Describe the appearance of normal CSF and the causes of abnormal appearances.
  4. Define xanthochromia and state its significance.
  5. Differentiate between CSF specimens caused by intracranial hemorrhage and a traumatic tap.
  6. Calculate CSF total cell count and counts for white blood cells (WBCs) and red blood cells (RBCs) based on given cell counts, specimen dilution, and numerical counts in the Neubauer chamber.
  7. Describe the leukocyte content of the CSF in different forms of meningitis including bacterial, viral, tubercular, and fungal.
  8. Discuss the significance of macrophages in CSF.
  9. State the reference values for CSF total protein and enumerate three pathological conditions leading to elevated CSF protein levels.
  10. Determine if increased CSF albumin or immunoglobulin results from damage to the blood-brain barrier or central nervous system production.
  11. Discuss CSF electrophoresis, immunophoresis, and isoelectric focusing in diagnosing multiple sclerosis and identifying CSF.
  12. State the reference values for CSF glucose and describe the pathological significance of decreased glucose levels.
  13. Discuss the diagnostic value of CSF lactate and glutamine determinations.
  14. Identify the microorganism associated with a positive India ink preparation.
  15. Discuss the importance of bacterial and cryptococcal antigen tests.
  16. Explain the advantages of molecular diagnostic methods for identifying the causative organisms in meningitis.
  17. Evaluate CSF data to determine if meningitis is bacterial, viral, fungal, or tubercular in nature when presented with relevant laboratory information.
  18. Describe the Venereal Disease Research Laboratories (VDRL) test and the fluorescent treponemal antibody-absorption test for syphilis in CSF testing.

Definition and Function of CSF

Cerebrospinal fluid (CSF) is a clear, colorless fluid that forms a protective cushion around and within the central nervous system (CNS). Its physiological system serves several critical functions:

  • Supplies essential nutrients to the nervous tissue (brain)
  • Removes metabolic wastes from the CNS
  • Maintains intracranial pressure
  • Provides a mechanical barrier, cushioning the brain and spinal cord against potential trauma

Key Points and Purpose of Testing CSF

  • Proper distribution of CSF tubes is critical, with a sequence for lab departments:
      1. Chemistry/Serology
      2. Microbiology (it is essential to avoid skin contamination)
      3. Hematology (to avoid cellular contamination from the tap)
      4. An optional tube for additional tests may be utilized.
  • Normal appearance and values for CSF cell count, protein, and glucose are benchmarks for a healthy CNS.
  • Critical differentiation between traumatic taps and hemorrhages, and preparations for cultures, are vital in diagnosing conditions.
  • Expected results for bacterial vs. viral meningitis help clinicians decide on treatment modalities.
  • The relationship between multiple sclerosis (MS) and oligoclonal bands in CSF testing is significant in diagnosing this demyelinating disorder.

Formation and Physiology of CSF

CSF is produced in the choroid plexuses, which are specialized structures made up of capillary masses located in each of the four ventricles of the brain. The production of CSF occurs through:

  • Selective filtration from plasma
  • Hydrostatic pressure and active transport secretion
    The production rate and volume of CSF vary by the patient's size and age:
  • Adult production rates are approximately 20 mL/hr.
  • Total CSF volume is 90-150 mL in adults and 10-60 mL in neonates.
  • An essential characteristic is that the rate of reabsorption equals the rate of CSF production.
Structure of Meninges

The brain and spinal cord are surrounded by three protective coverings known as meninges:

  1. Dura mater (outer layer)
  2. Arachnoid mater (middle layer)
  3. Pia mater (innermost layer, adhering directly to the brain and spinal cord)
  • CSF flows between the arachnoid mater and pia mater, filling support spaces in the ventricles.
Characteristics of the Blood-Brain Barrier
  • The blood-brain barrier plays a crucial role in maintaining the unique environment of the CNS by restricting the passage of certain substances:
      - It prevents harmful chemicals and substances from entering the CNS.
      - Antibodies and certain medications cannot cross this barrier.
  • The composition of CSF differs significantly from plasma due to this barrier:
      - CSF is NOT simply an ultrafiltrate of plasma (as urine is).

Symptoms Indicating the Need for Lumbar Puncture

Certain clinical symptoms suggest an immediate need for lumbar puncture (spinal tap) to analyze CSF:

  • Weakness of appendages
  • Fever of unknown origin (FUO)
  • Confusion
  • Severe headaches
  • Stiff neck
  • Coma
  • Imaging studies (CT and MRIs) have refined the diagnostic approach for brain disorders significantly.

CSF Collection and Handling

CSF is collected between the 3rd and 5th vertebrae, typically at L3/L4 in adults and L4/L5 in infants.

  • The procedure is referred to as a spinal fluid tap using a lumbar kit in a sterile environment, often with the patient in a fetal position.
  • The volume collected is about 10-15 cc in adults with 2-4 cc distributed into three to four sterile tubes.
Order of Tube Collection and Distribution
  1. Tube for Chemistry/Chemical Tests
  2. Tube for Microbiology
  3. Tube for Hematology
  4. An optional tube for additional tests
  • Proper handling is crucial to ensure accuracy; preservation conditions vary by test type:
      - Hematology specimens should be refrigerated for up to 4 hours.
      - Microbiology specimens can remain at room temperature.
      - Chemistry/serology specimens should be frozen.

Reporting Macroscopic Findings

When evaluating CSF, specific aspects must be reported:

  • Color
  • Clarity
  • Total Volume Received
  • Presence of Clots
Appearance of CSF: Clarity and Color

Normal CSF:

  • Crystal clear appearance
  • Cloudy, milky, or hemolyzed indicative of different pathological conditions such as:
      - Cloudy = infection
      - Milky = lipid or protein content
  • Specific color designates particular conditions:
      - Bright red indicates recent hemorrhage or a traumatic tap.
      - Brown signifies old hemorrhage.
      - Xanthochromic indicates RBC degradation products, or other conditions such as jaundice and elevated protein levels.
Differentiating Traumatic Tap from Hemorrhage
  • A traumatic tap occurs if a blood vessel is punctured during collection and can be distinguished by uneven blood distributions in tubes.
  • Clot formation may occur in traumatic taps, whereas in hemorrhage, the CSF lacks enough fibrinogen to create cl