Creatinine measures kidney function. You'd have to look at the Physician's Prescribing info for each drug.

Metformin can't be used with some contrast dyes and it takes a bit getting used to gastrointestinal wise. You have to start very very slow.

It would not surprise me if metformin was not indicated for those with poor kidney function. As part of my sick plan, I dump it and most oral diabetic meds when I'm sick and just use insulin only.