Question: Opinion on options re: kidney function

Hello,
I'm writing about my father's left kidney condition. He's a medical doctor, ~59 yrs (height-175 cm, weight-77 kg) and has had a lower functioning left kidney for many years. He first discovered it ~6-7 yrs ago, but may have had it from an early age. He has had dull but persistent pain in his stomach as a result. He recently consulted with a urologist, got a CT scan.
Urologist suggested a nephrectomy; my parents also discussed more conservative procedures like pyeloplasty/pyelotomy, vs. removing the left kidney altogether. Doctor’s opinion is that since the kidney function is <20% ( 7yrs ago it was 19%, now it is 18%), it is better to remove the kidney, since it is almost not functional anyway. Just going for a pyeloplasty/pyelotomy might present the possibility of a recurrence in this condition and additional surgery, which in turn might result in scarring and the possibility of left kidney removal again at a later stage.
My mother (medical doctor) & I are of the opinion that if a pyeloplasty/pyelotomy might relieve his pain and condition (and changes of recurrence of the condition are low after this procedure), why go for kidney removal, especially when it is still functioning and presents no issue?
Specifically, I'd really like to know:
a) What % kidney function classifies it as functioning kidney? My mother referred to journals saying 10-15%
b) What are the chances of cure for my father’s condition if we go for a conservative surgery like pyeloplasty, pyelotomy etc?

CT SCAN RESULTS: liver diffusely hypodense; no focal parenchymal lesions identified. Right kidney normal.
Marked left hydronephrosis w/ marked thinning of left renal parenchyma to a depth of only 8mm. Marked dilatation of left renal pelvis to a diameter of 5.2cm. Left ureter is normal in caliber; suspect the patient’s marked left hydronephrosis is secondary to a UP junction obstruction.
No other renal abnormality. Right kidney ~12.3cm in sagittal length; the left kidney ~9.1cm.

Thank you.

Hello Dr. XXXXXXX Goel,

My father underwent a percutaneous nephrostomy a few weeks ago to alleviate his UPJ obstruction. He is contemplating further surgery. He is otherwise healthy and continuing to go to work. Investigations of his blood and urine are normal. However, he has been complaining about excessive tiredness after routine daily work, since his nephrostomy procedure.

Could you kindly advise on whether his tiredness is related to the nephrostomy procedure in any way?

Thanks.