The lumbar puncture is a diagnostic, and very occasionally therapeutic, procedure involving the sampling of cerebrospinal fluid from the lumbar subarachnoid space.
Indications
- Diagnosis of infection (meningitis, encephalitis).
- Diagnosis of immune conditions such as Guillian-Barre syndrome and multiple sclerosis.
- Investigation of CNS involvement in oncology/haematology (mostly lymphoma).
- Other subspecialty nonsense.
- Administration of intrathecal medication (the most common of which would be spinal anaesthesia).
- Symptomatic relief for idiopathic intracranial hypertension.
The cerebrospinal fluid
Normal CSF is a clear Newtonian fluid with a viscosity about the same as pure water.
If it is pink it could indicate subarachnoid haemorrhage, but it is probably a traumatic tap.
If it is yellow and clear it is almost certainly infected.1 If it is yellow and turbid it is infected with bacteria.
Routine tests
Cell count
Occasional monocytes are normally present in the CSF, more than that is aptly named pleocytosis. The presence of granulocytes is never normal and may indicate bacterial meningitis. Viral infection is associated with lymphocyte (and/or monocyte) predominant pleocytosis. There are various other reasons for pleocytosis; one worth remembering is epileptic seizure.
Traumatic lumbar puncture leading to peripheral blood contamination will produce a cell count with the same differential as the peripheral blood. The presence of erythrocytes in the context of erythrophagocytosis, however, indicates that the blood was present in the CSF prior to LP and is not a result of traumatic tap.
Chemistry
Normal CSF glucose concentration is greater that 2/3 that of the serum concentration (or just 2.5-4.5mmol/L). Significant decreases are usually indicative of bacterial infection (including tuberculous meningitis). In viral of fungal infection no change (or mild decrease) is more common.
Normal protein is 0.15-0.45g/L, a significant increase could be consistent with bacterial meningitis. Lots of other things can explain an increase in protein though. The zebras to know here are multiple sclerosis and Guillian-Barre.
CSF in the microbiology lab
The humble gram stain may well make the diagnosis of bacterial meningitis. Other organisms can be seen with the help of a microscope (and various stains). But, culture is usually better.
Even better and faster than culture is polymerase chain reaction testing but this is more expensive and not available to every man and his dog. Check with the lab…
Further reading
Footnotes
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Xanthochromia is usually indicative of a previous subarachnoid bleed. Carotenaemia and rifampicin could also produce yellow CSF. ↩