This document provides information about cerebrospinal fluid (CSF) and interpreting lumbar puncture results. It discusses how CSF is produced and circulated in the brain and spinal cord. Key functions of CSF include preventing ischemia, maintaining chemical stability, protecting the brain, and providing buoyancy. The document outlines normal CSF constituents and indicates when lumbar punctures are used. Abnormal protein, glucose, white blood cell counts, or blood-stained CSF can indicate infections or other conditions.

1. Cerebrospinal fluid – interpreting
lumbar punctures
By JP Byass, 4th year, HYMS

2. CSF Facts
• Clear, colourless fluid
• Found in sub-arachnoid space within the brain
(ventricles) and spinal cord
• Produced by choroid plexus and cerebral
vessels at rate of 500ml/day
• Reabsorbed into venous sinus blood via
arachnoid granulations
• Choroid creates, arachnoid absorbs!
• Production matches reabsorption so total
circulating CSF at any time is around 130ml

3. CSF Functions – remember ICP-B
• Ischaemia prevention – by decreasing CSF and
ICP (intracranial pressure) and allowing adequate
blood perfusion to brain
• Chemical stability – homeostatic regulation of
neuroendocrine factors & removal of waste
metabolic products from neural tissue into blood
• Protection – protects brain from trauma
• Buoyancy – brain in suspended in CSF and
therefore not impeded by its own weight, which
would cut off blood supply and impede neurons.

4. • About 60% produced at
choroid plexus by modified
ependymal cells (which line
ventricles)
• Constituents from fenestrated
capillaries
• Choroidal epithelium has tight
junctions, preventing
macromolecules leaving blood
• One choroid plexus in each
ventricle (i.e. 4)
Anatomy of CSF production

5. CSF circulation

6. Indications for lumbar puncture
• Suspected meningitis (normal in 8%), viral
encephalitis, TB
• Suspected intra-cranial bleeding (but CT more
valuable)
• Demyelinating disorders e.g. MS
• Check for sub-arachnoid haemorrhage
(performed after normal CT as a minority cannot
pick up SAH)
• Occasionally used to treat idiopathic raised ICP

7. When not to do an LP
• Skin infection overlying puncture area
• Thrombocytopenia
• Any suspicion of cerebral herniation with raised
intracranial pressure:
 Glasgow Coma Scale (GCS <8)
 Abnormal pupil size and reaction
 Absent doll's eye movements
 Abnormal tone
 Tonic posturing
 Abnormal respiration (hyperventilation, Cheyne-Stokes
breathing, apnoea, respiratory arrest - in this instance, a CT scan
should be performed to look for evidence of cerebral herniation
before proceeding to LP.)

8. Normal Constituents
Component Value
Protein 0.2-0.4g/l with albumin:globulin ratio of
8:1
Glucose 3.3-4.4mmol/l (at least 60-80% of blood
glucose)
Chloride 122-128mmol/l
Lactate ≤2.8mmol/l
Opening pressure 10-20cm of water
WCC ≤4 per cubic mm
Polymorphs None!
pH 7.31

9. Interpreting CSF results

10. • Protein – high (>0.4g/l) levels seen in any infection,
Guillan Barre syndrome and MS
• Glucose – plasma glucose should be taken at same
time as LP. If CSF glucose < 60-80% blood glucose then
increased uptake in neural tissue i.e. think infection
• WCC – presence indicates infection & MS. Need to
gram stain and culture (if appropriate)
• Blood stained CSF – maybe present as a result of
damage to blood vessels during procedure (‘bloody
tap’). Initially red, but clearer fluid will follow.
Interpreting CSF results