Cerebrospinal Fluid Analysis
Formation and Physiology
The brain and spinal chord are lined by a protective membrane called the meninges
There are 3 layers of meninges:
Dura mater - outer layer
Arachnoid mater - middle layer
Pia mater - surfaces of brain and spinal chord
CSF is produced in the choroid plexus by ultrafiltration of plasma and active secretion
Choroid plexus - a network of capillaries in the ventricle of the brain
20 mL/min of CSF produced in adults
Total volume of CSF in adults is 90-150 mL
Total volume for neonates is 10-60 mL
The blood-brain complex is a barrier of endothelial cells between the brain tissue and capillary blood that selectively filters components from the blood into CSF
Functions of CSF
Provides barrier/lubrication for CNS
Removes metabolic waste
Supplies nutrients to nervous tissue
Specimen Collection and Storage
CSF analysis are STAT
Do NOT discard extra CSF after testing
Samples are collected by a lumbar puncture between 3rd, 4th, and 5th vertebrae
Usually 10-20 mL collected
Sample is distributed into 3 tubes after collection
Preservation of specimens depends on the purpose of test:
Hematology: refrigeration
Microbiology: room temperature
Chemistry/serology: refrigeration/frozen
Appearance of CSF
The normal of appearance of CSF is clear and colorless
Milky, cloudy, or turbid
Meningitis - inflammation of the meninges, often caused by microbial infection; usually cloudy/turbid CSF
Cloudy/turbid - a cloudy appearance of CSF indicates infection
Milky - a milky appearance of CSF indicates the presence of lipids or proteins
Hemolyzed/bloody - intact RBCs
Xanthochromic - pink, orange, or yellow
RBC degradation
Increased bilirubin due to jaundice
Cerebral hemorrhage
Traumatic Collection vs. Hemorrhage
A traumatic collection is when a blood vessel is punctured during the tap
Uneven distribution of blood in tubes (tube #1 will appear the most red)
Clotting occurs
Supernatant is clear (no xanthochromia)
A cerebral hemorrhage is bleeding that has been present in the body and already has RBC degradation
Blood will be evenly distributed in all 3 tubes
No clotting occurs
Supernatant has xanthochromia
Routine CSF Testing
Tube #1 - Chemistry
Centrifuge and use supernatant
CSF protein
Normal range is 15-45 mg/dL
Decreased levels = fluid leakage
Increased levels = blood-brain barrier damage, IG production, decreased CSF clearance, degeneration of neural tissue
Meningitis/hemorrhage most common cause for blood-brain barrier damage
Measured by turbiditry, automation (nephelometry), or dye-binding using Coomassie blue
CSF glucose
60-70% of plasma glucose value = 40-70 mg/dL
Normal CSF glucose + increased lymphocytes = viral/fungal meningitis
Decreased CSF glucose can be independent of plasma glucose
Decreased CSF glucose + increased neutrophils = bacterial meningitis
Decreased CSF glucose + increased lymphocytes = tubercular meningitis
Increased CSF glucose is ALWAYS a result of increased plasma glucose; not clinically significant
Method of measurement is the SAME as serum glucose
CSF electrophoresis
Detects oligoclonal bands (IgG) in the gamma region that indicate inflammation within CNS (specifically MS)
Serum electrophoresis is simultaneously performed
No bands in serum + bands in CSF = multiple sclerosis (MS)
Bands in both serum and CSF = leukemia, lymphoma, HIV
Myelin basic protein (MBS) is present in CSF when the myelin sheath around axons and neurons breaks down
Used to monitor MS and effectiveness of treatments
CSF lactate
Used in diagnosing and managing bacterial/viral meningitis
Bacterial/Fungal meningitis or TB: > 25 mg/dL
Viral meningitis: < 25 mg/dL
Is also for diagnosis and management of severe head injuries
CSF glutamine
Produced by brain cells from ammonia
Increased CSF glutamine = coma, Reye’s syndrome, liver disease
Normal value is 8-18 mg/dL
Tube #2 - Microbiology
Gram stain and culture - uses cytocentrifuged/centrifuged sediment
TB smears - tuberculosis is a serious cause of meningitis
Latex agglutination tests - latex beads coated with antibodies specific to bacterial antigens
fungal cultures - India Ink used to stain cryptococcus neoformans
parasite cultures - naegleria fowleri
Serological testing - primary test for neurosyphilis, third stage
Tube #3 - Hematology
Cell counts
Uses a whole well mixed specimen of CSF
Performed using automation or hemocytometer
Neubauer counting chamber
Total cell count (cells/uL) = (# of cells counted x dilutions) / (# of squares x 0.1)
Normal CSF contains 0-5 WBCs/uL and an occasional RBC
RBC counts are performed when there was a traumatic collection to correct the WBC count
Differential counts
Smear uses cytocentrifuged/centrifuged sediment
Lymphocytes/Monocytes are normally found in CSF
Neutrophils - bacterial meningitis
nRBCs - bone marrow contamination
Increased lymphocytes and monocytes - viral/tubercular/fungal meningitis
Increased monocytes = viral meningitis or MS
Eosinophils - parasitic/fungal infections
Macrophages - appear in CSF 2-4 hours after hemorrhage
Differential Diagnosis of Meningitis
Bacterial
Predominant WBCs - neutrophils
CSF protein - large increase
CSF glucose - decreased
CSF lactate - increased
Misc. - positive gram stain and bacterial antigen test
Viral
Predominant WBCs - lymphocytes
CSF protein - increased
CSF glucose - normal
CSF lactate - normal
Tubercular
Predominant WBCs - lymphocytes and monocytes
CSF protein - increased to marked increase
CSF glucose - decreased
CSF lactate - increased
Misc. - pellicle formation (thin layer of film on CSF)
Fungal
Predominant WBCs - lymphocytes and monocytes
CSF protein - increased to marked increase
CSF glucose - normal to decreased
CSF lactate - increased
Misc. - positive India ink for cryptococcus neoformans