If a food has zero carbs, as many proteins do, and you are already in ketosis, then you will stay in ketosis.
Our bodies use glycogen from carbs as energy, when we deprive our bodies of carbs there is no glycogen so our bodies produce ketones and use fat for its source of energy (long and short of it) so as long as your carb intake stays low, you will stay in ketosis. (There are a few things that can potentially kick you out of ketosis, citric acid/juice is one) You must have lean protein though, if you have high fat protein while in ketosis then you are giving your body extra fuel which can interfere with your weight loss.
That's how I understand it from all the research I've done. Hope it helps!