Medical Fun Facts Podcast

MFF0056: Examining cerebrospinal fluid

It’s Thursday 13 April 2017 and tomorrow is Good Friday.

Tonight, I want to describe how we examine cerebrospinal fluid in the microbiology laboratory. I’m going to limit myself to a context of patients with acute meningitis.

Imagine you have a sudden onset of the worst headache ever and a high fever. It hurts to open your eyes in bright light and when you try to bend your head down to look at the ground it hurts. You feel like death warmed up. You may also have a funny blotchy rash forming on your skin. You have signs and symptoms suggestive of acute bacterial meningitis. You need urgent medical attention. In the old days, you’d be seen and a lumbar puncture would be performed as soon as possible. These days, you’ll be triaged urgently, assessed quickly, blood will be collected for culture, antimicrobials will be administered and then you’ll be sent to have a CT scan of your head to make sure you don’t have a space occupying lesion. If you do have a SOL whether it be a tumour or abscess you run the risk of coning. This coning is worse than the grand cone that pathologists in private practice in Australia complain about, this form of coning occurs when you have a space occupying lesion in your brain and when you insert a lumbar puncture needle the sudden pressure differential results in the hindbrain pushing the brainstem through the foramen magnum at the base of your skull which can then result in compromising your cardiorespiratory centres so you stop breathing. Coning, is never a good thing.

Assuming you don’t have a SOL in your head, a lumbar puncture can be safely performed. A long needle is inserted into the subarachnoid space at the L3/4 or L4/5 interspace. That is the space between the third and fourth lumbar vertebrae or between the fourth and fifth lumbar vertebrae.

Did you know that up to 40 mL of CSF can be safely removed? Most doctors have been incorrectly taught that 10 drops per tube in three tubes is sufficient for complete analysis. In most circumstances that will work but in an era where we see more tests being requested it’s often not enough. If we assume 1 drop equals 0.05 mL, then there are 20 drops per millilitre, that means you could safely collect 800 drops of CSF!

We advise referring doctors that three tubes should be collected. This is because it’s not uncommon for some blood to contaminate the first part of a collection if a capillary has been nicked. We will compare the erythrocyte count between the first and third tubes. If the first tube has more blood than the third, that’s usually okay and represents a traumatic tap. If the amount of blood is the same in tubes 1 and 3, this could represent blood in the CSF which maybe because of hæmorrhage in the subarachnoid space. The first tube can be used by chemical pathology to determine protein and glucose levels. If the protein is high and glucose is low, this is suggestive of a bacterial infection.

From the third tube, in microbiology, we put some CSF into a counting chamber and do a cell count and differential. That means we count the leucocytes (white blood cells) and erythrocytes (red blood corpuscles) and look to see any microorganisms in the chamber. We also have to differentiate between polymorphonuclear leucocytes (neutrophils) and mononuclear leucocytes (lymphocytes) because a higher neutrophil count tends to reflect bacterial rather than viral infection.

Gram’s stain is always performed to look for bacteria and agar plates are set up for culture. Back when I was a boy bacterial antigen detection (BAD) tests were performed to try to identify the common causes of acute bacterial meningitis, viz., Hæmophilus influenzæ type b, Streptococcus pneumoniæNeisseria meningitidisEscherichia coli and Streptococcus agalactiæ. These days, we have PCR assays for this and these include common viral causes of meningoencephalitis like Herpes simplex virus and enterovirus.

This has been a cursory description of what we do with CSF and hopefully, in future shows, I’ll expand on various elements.

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