Cerebrospinal fluid (CSF) is found in the subarachnoid space of the brain (within the ventricles) and spinal canal. It is produced by the choroid plexus in the ventricles of the brain and the cerebral vessels at the rate of 500 ml/day. Production matches reabsorption so at any one time the in an adult the average volume of CSF is about 150 ml.

Indications for lumbar puncture

For information on performing a lumbar puncture and sampling see the Lumbar Puncture article.

1. Diagnostic lumbar puncture:
   • Infection, e.g. meningitis, viral encephalitis, tuberculosis.
   • Demyelinating disorders (including multiple sclerosis).
   • Benign intracranial hypertension.
   • Subarachnoid haemorrhage.
2. Therapeutic lumbar puncture:
   • Benign intracranial hypertension.

Analysis

It is helpful to note the appearance of CSF and the opening pressure (normal 10-20 cm H₂O). Samples are usually sent for:

Biochemistry

• Protein - high (>0.4 g/L) levels seen in infection and infiltration disorders (falsely high results are seen if the sample is contaminated with blood).
• Glucose - CSF glucose is usually 60-80% of plasma glucose. A blood sample for glucose should be taken at the same time as the lumbar puncture. Implies there is increased uptake of glucose in the CNS.

Microscopy, culture and sensitivity

• Cell count - white cells with differential (neutrophils and lymphocytes) and red cells. When performing a lumbar puncture, red cells may be present as a result of damage to a blood vessel during the procedure (commonly called a “bloody tap”). To confirm whether this represents a subarachnoid haemorrhage the CSF is centrifuged: clear supernatant fluid confirms a bloody tap, whereas the supernatant fluid is stained yellow in a subarachnoid haemorrhage (known as xanthochromic).
• Gram stain - for bacterial organisms.
• Culture - if appropriate.

Additional investigations

• Xanthochromia - yellow appearance of CSF after centrifugation. This represents breakdown of haemoglobin. Seen in subarachnoid haemorrhage.
• Oligoclonal bands - seen in multiple sclerosis and neurosyphilis.
• Virology.
• Polymerase chain reaction (PCR), e.g. for tuberculosis, viral and partially treated bacterial meningitis.
• Bacterial antigen testing - may be useful if PCR not available and patient partially treated.
• India ink staining for cryptococcus.