Cerebrospinal fluid examination

Keywords: CSF, Glucose CSF

Specimen:

5-15 mL divided between three sterile plain tubes.

At least 2 mL required for immunoglobulin studies.

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 by NAA;
viral detection by NAA;
India ink preparation and cryptococcal antigen detection;
specific serological testing (eg, syphilis);
cytological examination and/or immunophenotyping;
stain for eosinophils;
glucose (with simultaneous measurement of plasma glucose);
xanthochromia or CSF bilirubin
lactate;
total protein, albumin;
IgG/albumin ratio (with simultaneous assays of albumin and IgG on serum); and
electrophoresis or IEF with immunofixation of IgG to detect oligoclonal bands.

Specific NAA for meningitis, ie, meningococcal, pneumococcal and Haemophilus influenzae.

Reference Interval:

Normal CSF is clear and colourless.

Cell count: <5 x 106/L mononuclears; no neutrophils or red cells

Glucose: 60-70% of plasma levels (usually) 2.8-4.4 mmol/L

Lactate: 1.2-2.8 mmol/L

Protein:

Full term neonate: 0.1-1.2 g/L

Adult: 0.15-0.45 g/L

IgG/albumin ratio: <0.2

Oligoclonal bands: qualitative interpretation.

Application:

Diagnosis of:

  1. Bacterial, viral or fungal meningitis;

  2. Encephalitis;

  3. Malignant infiltrates eg, acute leukaemia, lymphoma;

  4. Disorders with local immunoglobulin production in the CNS (eg, multiple sclerosis, SSPE);

  5. Subarachnoid haemorrhage; or

  6. Spinal canal blockage.

Interpretation:

Macroscopic appearance:

Coloured 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 colouring of all three tubes suggests recent bleeding into the subarachnoid space.

Xanthochromia (increased CSF bilirubin) may be seen within 12 hours of subarachnoid haemorrhage and may persist for up to three weeks.

It may also be seen in any condition in which CSF protein is very high and in patients with jaundice.

In the neonate, normal CSF may appear xanthochromic.

Cell count:

The white cell count is increased when there is inflammation of the central nervous system, particularly the meninges.

Bacterial infections (eg, meningitis, cerebral abscess, early tuberculous meningitis, septicaemia) are usually associated with the presence of neutrophils in the CSF.

Viral infections (eg, 'aseptic meningitis', encephalitis) are associated with an increase in mononuclear cells, although in some (eg, Coxsackievirus, poliovirus infection) there may be an early increase in neutrophils.

An increase in mononuclear cells may also be seen with cerebral abscess, acute leukaemia, lymphoma, intracranial vein thrombosis, cerebral tumour or multiple sclerosis.

An increase in both neutrophils and mononuclear cells occurs in cerebral abscess, tuberculous meningitis and early viral meningitis.

Eosinophils may be seen in meningitis caused by Angiostrongylus cantonensis and in cysticercosis and coccidioidomycosis.

Red cells are present after subarachnoid haemorrhage and trauma and with haemorrhagic inflammation (eg, HSV encephalitis).

If the CSF cell count is >5 x 106/L, or if the specimen is from an HIV antibody-positive 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 fowleri or Acanthamoeba spp).

Cytological examination:

The detection of malignant cells indicates meningeal involvement with carcinoma, lymphoma or leukaemia.

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 or longer to become positive.

Bacterial antigen detection:

Antigen testing for Neisseria meningitidis, Haemophilus influenzae type b, Streptococcus pneumoniae or, in infants, Group B streptococcus, may be useful for the diagnosis of meningitis which has been partially treated.

DNA detection:

PCR amplification, followed by specific nucleic acid probes, provides a sensitive technique for detection of Mycobacterium tuberculosis, herpes simplex virus, enteroviruses, cytomegalovirus and Toxoplasma gondii and other organisms.

Glucose:

Low glucose levels, as compared with 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.

Xanthochromia or CSF bilirubin:

Xanthochromia (increased CSF bilirubin) may be seen within 12 hours of subarachnoid haemorrhage and may persist for up to three weeks.

May be indicated when it is suspected that xanthochromia is due to elevated CSF protein or is being masked by free haemoglobin.

The recommended method, spectrophotometric scanning allows quantitation of haemoglobin (recent bleed, traumatic tap), methaemoglobin and bilirubin (bleeding several hours previously).

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 genetic lactic acidosis.

Protein:

High protein levels are found:

in conditions where CSF circulation is impeded (eg, spinal tumour);

with meningeal inflammation (eg, purulent or tuberculous meningitis);

with increased vascular (blood-brain) permeability (eg, Encephalitis, Guillain-Barré syndrome);

with local immunoglobulin production (eg, Multiple sclerosis); and

whenever there is pus or blood in the CSF.

Oligoclonal bands; IgG/albumin ratio:

Adjunctive tests in the investigation of demyelinating disorders, especially multiple sclerosis, SSPE.

The presence of oligoclonal bands in CSF, but not serum, is indicative of local immunoglobulin synthesis and occurs most commonly in multiple sclerosis (>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 Guillain-Barré syndrome, CNS infections (including neurosyphilis and HIV infection), after cerebrovascular accidents and in other CNS disorders.

Reference:

Gray LD and Fedorko DP. Clin Microbiol Rev 1992; 5: 130-145.

Hayward RA et al. Lancet 1987; 1: 1-4.