Straight creatine will not knock you out of Ketosis. But using a creatine that is pre-mixed with a bunch of dextrose and other carbs(EAS Fruit Punch Flavor) will definitely keep you from staying in ketosis.

What should you do? Simple, buy some normal, non pre-mixed creatine. Like a jug of 500 grams and then just use that. You could use it every day if you wanted to and it wouldnt effect your state of ketosis.