What actually works to get pee smell out of mattress?

enzyme cleaner, the pet-stain kind. the smell is uric acid crystals and ordinary cleaners don't break them down, which is why it comes back every time the room gets humid. blot up what you can, then use enough spray to reach as deep as the pee went - a light mist only treats the surface. cover the patch with cling film for a few hours so it stays wet and keeps working, then uncover and let it air dry completely. baking soda over the top once dry, leave it overnight, hoover. how old is it? a fresh one comes out first go, an old one can take two or three rounds.