CHAPTER 9 Urinalysis and Body Fluids Analysis
XIV. Cerebrospinal Fluid (CSF) Analysis
A. CSF location, formation and functions
1. The central nervous system (CNS) is bathed by a network of CSF-filled reservoirs internally and externally. CSF is synthesized by the choroid plexus in the ventricles and circulates out to the subarachnoid spaces surrounding the CNS (brain and spinal cord).
2. The CSF functions to protect and cushion the CNS, provide nutrients to neural tissue, and remove metabolic waste. Approximately 500 mL/day of CSF is produced, with a total volume of 140 to 170 mL in adults.
B. Specimen collection is only for diagnosis or for the treatment of disease.
1. Ventricular puncture to obtain CSF can be performed by a physician in special circum- stances. Typically, sterile lumbar puncture between L2 and L3 vertebrae or between the L3 and L4 vertebrae is the most common collection area. This collection taps CSF in the lumbar cistern of the subarachnoid space around the base of the spinal cord where there is no chance of piercing the cord itself.
2. Three specimen tubes are aseptically collected and labeled as 1, 2, and 3. Each tube is uniquely used for specific testing.
a. Tube 1 is used for CSF chemistry analysis. This tube may contain tissue juice and some cells from the spinal tap.
b. Tube 2 is used for microbiology studies.
c. Tube 3 is used for a hemocytometer cell count and cytospin differential. This tube should contain no contaminating cells from surrounding tissue; therefore, any cells found in this tube are intrinsic to the CSF. Physical examination of the CSF sample may be done using this tube.
3. All testing on CSF specimens should be performed immediately upon receipt of spec- imen because of rapid cellular degeneration.
C. Pathologic diseases detected by and involving the CSF include the following conditions:
1. Subarachnoid or intracerebral hemorrhages (i.e., strokes or trauma)
2. Infections such as meningitis (e.g., bacterial, fungal, parasitic, or viral); abscesses and encephalitis
3. Malignant processes such as primary brain tumors, metastatic tumors with a primary site elsewhere, or leukemias and lymphomas.
4. Multiple sclerosis.
D. Routine CSF analysis typically includes a physical, chemical, and microscopic analysis.
1. Gross examination includes a report of color and clarity of the specimen.
a. Normal CSF is clear and colorless.
b. Turbidity is most often produced by the increased presence of WBCs (>200 cells per milliliter), by increased RBCs (>400 RBCs per milliliter), or by microorgan- isms. Cellular turbidity is known as pleocytosis.
c. Abnormal specimen color is most commonly caused by a disease process.
(1) Pink or red CSF is the result of RBC lysis and can be seen 4–10 hours after a subarachnoid hemorrhage. This can also be caused by a traumatic tap.
(2) Xanthochromic (i.e., yellow) specimens result from the following condi- tions:
(a) After pathologic bleeding caused by breakdown of hemoglobin and biliru- bin formation in CSF or due to traumatic tap
(b) When CSF protein is increased>250 mg/dL
(c) Liver disease caused by increased total bilirubin levels (3) Brown CSF specimens are the result of the following conditions:
(a) Presence of methemoglobin (b) Subdural or intracerebral hematoma
(c) Presence of melanin caused by a melanoma
d. Distinguishing between pathologic bleeding and traumatic tap is often necessary.
The following criteria are used to distinguish between the two:
(1) A serial decrease in RBCs in tubes 1 to 3 is seen with a traumatic tap.
(2) A clotted specimen or clumped RBCs on microscopic examination indicates traumatic tap.
(3) The color of the supernatant of a bloody specimen after centrifugation can be suggestive. A clear supernatant indicates a traumatic tap, whereas a pink, yel- low, or brown supernatant indicates pathologic bleeding. However, if a sample contains bilirubin from lysed RBCs that have come from a traumatic tap, it may have a yellowish color.
(4) A ratio of>500 RBCs for every WBC indicates traumatic tap.
2. Chemical examination can include many analytes, but only a few have any diagnostic value on a routine basis.
a. Total protein in CSF represents a combination of prealbumin, albumin, transferrin, and trace amounts of immunoglobulin G (IgG).
(1) Normally, CSF protein ranges between 20 and 50 mg/dL, with albumin repre- senting 50% to 70% of the total.
(2) A CSF protein level provides information as to the integrity of the blood-brain barrier since large protein molecules are kept out of CSF.
(3) Increased protein levels in CSF can be the result of the following conditions:
(a) Contamination with peripheral blood on obtaining the specimen (b) Obstruction of CSF circulation (e.g., hydrocephalus or tumor)
(c) Tissue degeneration
(d) Increased permeability of the blood-brain membrane caused by drugs, tox- ins, or infection (meningitis)
(e) Intrathecal (by the brain) production of protein by tumors
(4) Recognizing increases in individual protein constituents in the CSF can be important.
(a) Prealbumin is uniquely found in CSF specimens.
(b) Any albumin present in a specimen is from the passage of plasma albumin across a damaged blood-brain barrier (formed by tight junctions between
endothelial cells). CSF albumin is increased when the permeability of the blood-brain barrier is compromised (i.e., normal CSF/serum albumin index
<9.0).
(c) IgG is present in trace amounts but can originate from intrathecal produc- tion in the CNS.
(i) A CSF/serum IgG index<0.77 is normal.
(ii) An IgG CSF/serum index>0.77 is highly indicative of multiple scle- rosis.
(d) CSF protein electrophoresis can be a useful tool to distinguish protein content. Abnormal oligoclonal bands in the gamma region of the trac- ing are comprised of IgG and are highly diagnostic of multiple sclero- sis.
(e) Myelin basic proteins may be present with multiple sclerosis.
b. CSF glucose is in equilibrium with plasma glucose.
(1) Normal values in the CSF range from 50 to 80 mg/dL or approximately 60%
of plasma glucose levels.
(2) Levels are decreased in bacterial meningitis and fungal infections.
(3) Levels are increased in hyperglycemia and in cases of a traumatic tap.
c. CSF enzyme levels can be detected and are elevated in a variety of pathologic conditions.
(1) LDH concentrations can be elevated in the following conditions:
(a) Bacterial and viral meningitis (b) Subarachnoid hemorrhage
(c) Lymphomas (d) Leukemias (e) Metastatic tumors
(2) Creatine kinase (CK) levels can be elevated in the following conditions:
(a) Stroke
(b) Multiple sclerosis (c) Degenerative disorders (d) Primary brain tumors (e) Viral and bacterial meningitis
(f) Epileptic seizure
(3) Aspartate aminotransferase (AST) levels can be elevated in the following con- ditions:
(a) Intracerebral hemorrhage (b) Subarachnoid hemorrhage
(c) Bacterial meningitis
d. CSF lactic acid concentrations are unrelated to plasma values.
(1) The normal concentration ranges from 10–22 mg/dL.
(2) Lactate is increased with any disorder associated with increased metabolism or ischemia in the CNS.
e. CSF electrolytes reflect basically the same values as in serum, and their measure- ment in CSF is of no diagnostic advantage.
3. Cell count in CSF normally reflects a low number of cells (see Web Color Image 9–43).
a. Normally, mononuclear cells (i.e., lymphocytes, monocytes) predominate in a low concentration of 0 to 10 cells per microliter.
(1) Neutrophils compose 0% to 6%.
(2) Monocytes compose 15% to 45%.
(3) Lymphocytes compose the majority of cell types: 40% to 80%.
b. RBCs should be absent. The presence of RBCs indicates either cerebral hemorrhage or traumatic tap.
c. Cell counts must be performed within 1 hour of collection.
d. If increased WBCs are found, a cytospin smear is prepared, and the smear undergoes Wright’s stain to determine a WBC differential count.
e. Any other cell type or a change in differential percent could indicate a serious disorder.
(1) An increase in neutrophils is indicative of bacterial meningitis.
(2) An increase in lymphocytes can be found in cases of viral, tubercular, or fungal meningitis.
(3) Plasma cells may be found in individuals who have multiple sclerosis or chronic inflammatory conditions.
(4) An eosinophilia may be associated with parasitic or fungal disorders (see Web Color Image 9–44).
(5) Macrophages can be found in CSF in association with hemorrhage. The pres- ence of ferritin granules in the cytoplasm of CSF macrophages may indicate an older or chronic hemorrhagic condition.
(6) Malignant cells can be present as a result of a CNS tumor, metastatic tumor or a leukemic process.
4. Microscopic examination consists of a Gram’s stain or acid-fast stain, as well as a microbiologic culture and sensitivity determination. Viral studies or fungal cultures can also be performed (see Chapter 7).
5. Meningitis-causing organisms include the following:
(1) Cryptococcus spp.
(2) Coccidioides immitis (3) Mycobacterium tuberculosis (4) Haemophilus influenzae (5) Neisseria meningitidis (6) Streptococcus pneumoniae (7) Staphylococcus aureus