Lumbar Puncture 4
Important Good patient positioning and an experienced assistant are the most important factors for success.
Interpreting the results
Cerebro-spinal fluid results can be difficult to interpret.
The following is a guide only: be aware of local variations and take advice from local experts.
There are two common indications for an lumbar puncture which need particularly careful interpretation:
? sub-arachnoid haemorrhage
? meningitis
? Sub-arachnoid haemorrhage
The differential red cell count between the first and third tube is important in distinguishing a traumatic tap - the red cell count should be significantly lower in the final tube if the tap was traumatic.
Xanthochromia
Xanthochromia, the yellow colour of the CSF caused by the breakdown of haem molecules (from the blood) into bilirubin. Prior to CT it was the only method of detecting SAH.
Xanthochromia should be present 6 hours after a bleed. Detecting small amounts of xanthochromia is difficult, and needs a colorimeter.
Many labs persist in believing that holding a tube up with a piece of white paper behind it is adequate. It is not, and this exposes the patient and the doctor ordering the test to unacceptable levels of risk.
? Meningitis
The ratio of WBC to RBC in the CSF should be no more than 1 WBC to 500 RBC. A higher ratio signifies an infective process.
