My son has vur and hydronephrosis. his doctor is saying his right kidney is only functioning at 20% but he's not having kidney failure or at risk for ckd at this time. how is that possible if his kidney function is declining?

The human system as capable of functioning normally with only one kidney. Some kids are even born with only one. It is worth trying to get any issues corrected so the impaired kidney can work better. This may be confusing, but I do not consider it so. The long-term plan would include making life choices to protect the functioning kidney, like not taking up riding horses or contact sports.

VUR is the abnormal backflow of some urine from the bladder into the kidney and over a period of time can destroy renal tissue and function depending on its duration and severity. Properly corrected now it will prevent further loss of function. You should consult a pediatric Urologist and he will fix things up. Good luck