Weirdly so I was reading about this last night as i was looking up how quickly we go out of ketosis if we eat carbs. Anyway, the answers i found included someone using these strips and an answer to hers was that the strips are not accurate as they only measure the ketones in our bodies not actually that our bodies are still in ketosis. that it takes a few hours to show anyway.
I found that it says that if you have an average of 30g carbs and 150g protein your body will stay in ketosis, I don't know how much we eat in carbs but am sure we are under this.