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If you are writing notes on a particular case, where the anaesthetist has decided which to use, "...ml of anaesthetic" would be understandable. If the notes are general notes advising treatment, "...ml of an anaesthetic" tells the team that the specific anaesthetic is not confirmed.
How about "mL of anesthetic agent"?
Is it also OK to omit "an" if you are writing notes on a particular case, where the anaesthetist has decided which to use?
Can this rule be applied to other countable nouns too???
"anesthetic" 
Linear Mode
