Based on your description of the situation, I am not convinced that anything is wrong with your cats. And I sincerely doubt that the food you are using is harming them.
Young, healthy cats have very powerful kidneys. As cats age, the kidneys typically weaken. Veterinarians can run a variety of blood and urine tests to assess the function of the kidneys. These tests are extremely valuable. However, it is sometimes tempting to over-interpret their results. This is especially true when the results are in the “normal” range.
I put the word normal in quotes because there is no universally accepted definition of normal test results. When laboratories report test results, they provide a range of values that are considered normal. They determine that range by averaging the results from a large number of apparently healthy animals and applying a statistical formula to determine the highest and lowest values that should regularly be encountered in healthy animals.
This method is not without pitfalls. First, animals that seem healthy but actually aren’t may be included in the sample. Also, because of the statistical method used, some completely healthy animals will have test results that are not in the “normal” range. What this means is that the normal range of values for a test result is a very useful guide, but it is most definitely not the word of God.
I suspect that your cats’ kidneys are not weakening. It is more probable that they are healthy, and their test results simply do not conform to the range that is “normal” for young animals.
The way to sort this out is to repeat the tests periodically, as you are already planning. If the results do not show a continuous, worsening trend, there is probably nothing to worry about.
I also doubt that the diet you are feeding is causing any harm to your pets. Premium cat foods are designed to be easy on the kidneys. And, unless they are contaminated (like the foods affected by the Menu Foods recall), good cat foods should not cause kidney problems.
If you are not comfortable with the food you’re using, there is nothing wrong with switching. For your situation, a low-protein diet will be most effective at preventing weakening of the kidneys. Before you switch, talk to your vet to make sure that the new diet is appropriate for your cats.
However, you may want to wait until the next set of test results are in before switching foods. Those results may show that your worries were unfounded.