Lynn, The ideal range is below 1% phosphorous, prescription foods tend to be 0.5% or less. The highest on the list is Hi-life seafood at 6.8% http://www.felinecrf.org/tinned_food.htm
Is that dry matter analysis Mark? Just to confuse things below is from the CRF website:
Cat foods vary in how much moisture they contain, which makes it difficult to compare them to each other. It is very hard, for example, to compare a tinned cat food to a dry cat food because the former naturally contains much more water; and this affects all the percentages of the different nutrients. Dry matter analysis is a way of comparing foods assuming all the moisture content has been removed: this makes it easier to compare different products. Whenever this site mentions levels of the various components of foods, it is talking about them on the basis of dry matter analysis, which is not necessarily the same as what it says on the tin.
Let's take an example. Let's say:
you give your cat a food with 80% moisture, a typical level for many tinned foods;
the food apparently has phosphorus of 0.25%;
your cat eats 100g of the food.
It therefore appears that your cat is eating 0.25g of phosphorus (100g x 0.25%).
However, the food is 80% water. So of the 100g your cat has just eaten, 80g (80%) of it was simply water, and only 20% was actual food, or dry matter. So the amount of phosphorus is actually higher - in percentage terms - than it first appeared, i.e. your cat has eaten 0.25% divided by (100%-80%) or 1.25% phosphorus.
Another way of looking at it is to say that your cat food initially had 1.25% phosphorus. Then the manufacturers added 80% water. There is still the same total amount of phosphorus in the food, but at first the percentage appears lower because of the diluting effect of the water. So in order to understand exactly how much phosphorus your cat is eating, you need to discount the water in the food.