Elevated levels of creatine in two cats.

Hi guys, first time posting in this group so bear with me.

I have 4 rescue cats, 2 Persian mixes, 1 Scottish fold and an Arabian Mau. I feed them all the same food: Royal Canin wet and dry food. I alternative between them, but they largely stick to a dry food diet. My husband and I both work long hours and so we cannot make our own pet food. We do our best to feed them good brands. They have a running fountain next to their bowls and all 4 drink regularly.

Recently, 2 of the cats (one Persian and the Scottish fold) went for dental scaling and cleaning. The blood tests have shown elevated creatine levels. The vet is saying it’s a sign of early onset kidney disease. I’m at a loss. They have no signs of any illness. They play, sleep, cuddle - no changes in bathroom habits or behaviour. One cat is almost 4 years old and the other is around 6-7.

What could be causing this? Is there anything more preventative I can do? Do we need to switch food brands? The vet is insisting to do further tests, which we will do once payday hits again. Any advice please?