Addressing un-level floor when using ceramic tile
If you have a good solid base, you can use a modified thinset, the same material to lay the Ditra into. Use a straight edge to screed the thinset over the low spot of the floor, do not sweat the "rake" marks left by the screed. If any "tags" occur from using the screed, they can be scraped off and vacuumed the next day. Then set the Ditra. The Ditra otherwise may not sink into the depression of the floor and get a good bond to the substrate. Also, the flatter you get you floor to start, the easier it will be to get the tile to lay in plane with the surrounding tile.