One kidney left: Any tips on diet? Testing required?

I would recommend Cystatin C in addition to eGFR. This test, from what I am told by my nephrologist, confirms eGFR and provides a better marker for predictability of future possible cardiac or CKD damage. Stay away from herbs/supplements unless given the green light by your doctor. Please follow up with a team of docs like your PCP, nephrologist, dietitian, and others. Get blood tests as frequently as you can, perhaps every 6 to 12 weeks by your PCP. I am not sure if having only one kidney after a transplant has more restrictions so your transplant team and nephrologist can give you more guidance. You may also wish to review the comments at the kidney/bladder group at Mayo Connect as we cover most topics on kidney health. If you gave a kidney to another, thank you. If you received a new kidney as a result of disease, this forum along with your doc will help you manage your long new life ahead.

Lastly, in addition to eGFR, Cystatin C tests, and whatever else your your doc recommends, try to establish a trend with your lab values. Any one lab test can cause happiness or dismay, but it is the trend of those lab values for the long run that really matters.

@sade96, I have located some basic information about living with one kidney from the National Kidney Foundation.
- Living With One Kidney
https://www.kidney.org/atoz/content/onekidney

@sade96 - Have you discussed your dietary needs or the safety of herbs / supplements with your doctor?