<qa>  What is the best parameter to define the kidney function?Obesity promotes chronic kidney disease and is accelerating its shift to the stage of renal replacement. Bariatric surgery is a treatment for severe or morbid obesity whose renal benefit is currently unknown. The glomerular filtration rate GFR is the best parameter to define the kidney function. It can be estimated using formulas by assaying endogenous markers creatinine, cystatin C or measured with an exogenous tracer glomerular filtration 51 Cr-EDTA. 