What are the best tests to accurately measure renal function and to rule out kub ultrasound finding "renal parenchymal disease"? thank you.

Serum creatinine : Is helpful blood test to evaluate renal function. Renal parenchymal disease means scarring in the kidney however the ultrasound is highly operator dependent and results may vary on subsequent tests. My advice if your blood test are normal than ultrasound findings can be ignored. See a nephrologist if you are overly concerned for reassurance. Thanks.

Renal disease: Is evaluated by urine analysis and calculation of GFR with that information combined with the history, physical, and understanding of a patient's potential for underlying conditions which predispose to renal disease. Imaging (ultrasound, nuclear medicine, ct, mri) is used not to make the diagnosis but to add further information in those patients in whom it is appropriate to do so.