Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I personally fail every game I play AP Kat so I stick to this with a small margin of substitutes. This is exactly how I play Kat every game I select her. Any of the damage items here can be increased/decreased. I.E. Here: Bloodthirster x2 Inf Edge x1 Black Cleaver x1. Feel free to: Bloodthirster x5, or Inf Edge x4 or whatever makes your enemies explode and you smile.