Cerebrospinal
fluid examination
Caution: Make sure the patient does not take any anticoagulants and that the INR is <1.3 and platelet count >50,000/mm3.
Samples:
10-15 mL divided between 4 sterile plain tubes,
containing at least 5 mL required for immunological studies (OCBs).
Method: Macroscopic
inspection; microscopy of wet film including cell count. Additional tests, as
appropriate: Gram stain; special stains for AFB; phase contrast microscopy;
bacterial culture and antigen detection; nucleic acid probes after PCR
amplification; viral culture; India ink preparation and cryptococcal antigen
detection; specific serological testing (eg syphilis); cytological
examination; stain for eosinophils; glucose (with simultaneous measurement of
plasma glucose); lactate; total protein, albumin; IgG/albumin ratio (with
simultaneous assays of albumin and IgG on serum); electrophoresis or IEF with
immunofixation of IgG to detect OCBs. Chloride is no longer
performed, as it has poor sensitivity for tuberculous meningitis.
Reference values:
Normal CSF is clear and colorless. Opening pressure in
lateral
decubitus should be between 70-180 mm H20:
>200
mm H20 with patient relaxed and legs straightened indicates increased
intracranial pressure; <50 mm H20 indicates intracranial hypotension.
In sitting position the values are approximately double those of the lateral
decubitus and reach up to the cisterna magna.
| Cell count: |
<5 mm3 lymphocytes or
other mononuclear cells; no neutrophils or red cells
Bloody tap if: 1 WBC/500-1,000 RBC assuming the hematocrit is normal or 1 mg/dL protein increase/1,000 RBC
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| Glucose: | 2.8-4.4 mmol/L or 50-75 mg/dL, CSF/serum ratio =0.6 | |
| Lactate: | 1.2-2.8 mmol/L or 1.6 meq/L | |
| Protein: | ||
| 0.15-0.50 g/L | ||
| LDH (isoenzymes 4 and 5): | LDH is about 10% of plasma LDH | |
| IgG index: | 0.01-0.6 | |
| IgG/albumin ratio: | <0.2 | |
| Oligoclonal bands (OCBs): | absent | |
Indication: Diagnosis
of:
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Interpretation:
| Macroscopic appearance |
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Colored or turbid CSF is abnormal, except after a traumatic tap, in which case there is less blood staining in the second and third tubes. Uniform red coloring of all three tubes suggests recent bleeding into the subarachnoidal space. Xanthochromia (increased CSF bilirubin) may be seen within 12 hours of SAH and may persist for up to three weeks. It may also be seen in any condition where CSF protein is very high and in patients with jaundice. Centrifugation of bloody fluid from a bloody tap will yield a colorless supernatant. |
| Cell Count |
| The WBC is increased when there is inflammation of the CNS, particularly the meninges. Bacterial infections (e.g. meningitis, cerebral abscess, early TB meningitis, septicemia) are usually associated with the presence of polymorph nuclear cells in the CSF. Viral infections (e.g. aseptic meningitis, encephalitis) are associated with an increase in mononuclear cells, although in some (e.g. Coxsackievirus and poliovirus infection) there may be an early increase in neutrophils. An increase in mononuclear cells may also be seen with cerebral abscess, acute leukemia, lymphoma, intracranial CVT, cerebral tumor or MS. An increase in both neutrophils and mononuclear cells occurs in cerebral abscess, TB meningitis and early viral meningitis. Eosinophils are seen in meningitis caused by A. cantonensis and in cysticercosis and coccidioidomycosis. RBCs are present after SAH and trauma, and with hemorrhagic inflammation (e.g. HSE). The CSF rule for SAH corresponds to the rule of halves: 1/2 h after the SAH event RBCs, 1/2 day xanthochromia, 1/2 week RBCs disappear and 1/2 month xanthochromia disappears. If the CSF cell count is >5 mm3, or if the specimen is from an HIV patient, an India ink preparation should be examined and cryptococcal antigen detection should be considered. |
| Phase microscopy |
| Indicated for the detection of motile amoebae (Naegleria sp or Acanthamoeba sp). |
| Cytological examination |
| The detection of malignant cells indicates meningeal involvement with carcinoma, lymphoma or leukemia. |
| Gram stain and bacterial culture |
| The Gram stain is positive in approximately 70% of patients with acute bacterial meningitis. A negative Gram stain and/or bacterial culture does not exclude infection, particularly when the patient has received antibiotics. If an anaerobic organism is suspected, special cultures are required. Cultures for AFB may take four weeks to become positive. |
| Bacterial antigen detection |
| Bacterial antigen detection for N. meningitidis, H. influenzae type b, S. pneumoniae may be useful for the diagnosis of meningitis which has been partially treated. It may also be indicated for CSF specimens with a raised neutrophilic count but without organisms on Gram stain. |
| Syphilis serology |
| DNA detection |
| PCR amplification, followed by specific nucleic acid probes, provides a sensitive technique for detection of M. tuberculosis, HSV, enteroviruses, CMV and T. gondii. |
| Glucose |
| Low glucose levels, as compared to plasma levels, are seen in bacterial meningitis, cryptococcal meningitis, malignant involvement of the meninges and sarcoidosis. Glucose levels are usually normal in viral infections of the CNS. |
| Lactate |
| In bacterial and cryptococcal infection, an increased CSF lactate is found earlier than a reduced glucose. In viral meningitis, lactate levels remain normal, even when neutrophils are present in the CSF. Raised levels may also occur with severe cerebral hypoxia or mitochondrial disease. |
| Protein |
| High protein levels are found in conditions where CSF circulation is impeded (eg spinal tumor); with meningeal inflammation (eg purulent or TB meningitis); with increased vascular (blood-brain) permeability (eg encephalitis, GBS); with local immunoglobulin production (eg MS, SSPE); whenever there is pus or blood in the CSF. |
| Xanthochromia studies |
| May be indicated when it is suspected that xanthochromia is due to elevated CSF protein or is being masked by free hemoglobin. Spectrophotometric scanning allows quantification of hemoglobin (recent bleed, traumatic tap), methemoglobin and bilirubin (bleeding several hours previously). |
| Oligoclonal bands; IgG/albumin ratio |
| Adjunctive tests in the investigation of demyelinating disorders, esp MS, SSPE. The presence of oligoclonal bands in CSF, but not serum, is indicative of local immunoglobulin synthesis and occurs most commonly in MS (>90% of patients) and SSPE. The IgG/albumin ratio is also often increased in these patients, but the test is less sensitive than oligoclonal bands. The finding of oligoclonal bands is not specific as they may also be found in GBS, CNS infections (including neurosyphilis and HIV infection), after cerebrovascular accidents and in other CNS disorders. |
Adapted from http:www.rcpa.edu.au/pathman/cerebros.htm