If you’re doing a spinal tap or a spinal anaesthesia, and you’re not 100% sure if you’ve got CSF, one quick tip for checking is using yellow chlorhexidine, coloured by phenol red.
If the fluid from your needle changes the yellow to orange/red = CSF. No color change = something else. And here’s the background article with a bit of show and tell
Earlier, I’ve heard mentioned that the glucose in CSF induced the colour change in yellow disinfectant. This article shows the coloured disinfectant solution Coloured Chlorhexidine Alcohol contains phenol red, a weak acid that can be used as a pH indicator.
The colour of phenol red changes from yellow to red with increasing pH, meaning the colour is pH dependent, and a distinct colour change happens above a pH of 6.2.
As a set-up to see if this colour change could indeed be used in clinical practise to test for CSF, different fluids were dripped onto cotton swabs with yellow disinfectant. The different fluids tested were sodium chloride (90 mg/ml), glucose (50 mg/ml), lidocaine (10 mg/ml), mepivacaine (10 mg/ml), bupivacaine (50 mg/ml), ropivacaine (20 mg/ml), albumin and CSF.
Apart from CSF, none of the fluids tested induced a colour change. They’re too acidic. The other fluids could lead to a light yellow/white colour change by a wash-out reaction as seen on some of the test swabs, and after this, the cotton swab is of course useless for further indicator use.
The authors say “the test may be used to identify CSF in other medical conditions, such as traumatic head injuries with rhinorrhoea.” I don’t know how good it would be for testing for CSF from ear/nose in TBI when the CSF will often be mixed with other fluids, but it would be an interesting test.
It’s a great little trick that can be useful during spinal taps or regional anaesthesia as it helps to identify CSF and is a rapid, simple, cheap and – according to the authors – sensitive way of doing so.