Oncologist's office just called.  At first I thought they were just confirming my appointment next week, but it turned out that they'd gotten the lab results from yesterday and my creatinine levels (which have always been fine) were considerably elevated.  She wants me to hydrate today, tomorrow, and Sunday, and then have the test redone on Monday just to make sure that it wasn't a mistake.

Creatinine is produced by creatine, a molecule used for energy production in the muscles. The kidneys filter out and dispose most creatinine through urination. If kidneys become impaired in any way, the creatinine level in the blood rises since it is not being cleared properly by the kidneys.

I'm not really worried, but if there is an issue it's always better to find out sooner rather than later in order to (hopefully!) nip the problem in the bud.