Renal biopsy (RB) is the reference diagnostic procedure for Idiopathic Nephrotic Syndrome (INS). It is an invasive procedure, particularly risky during the acute phase of nephrotic syndrome when anticoagulation therapy is required. By identifying serum antibodies that allow for the non-invasive diagnosis of the main differential diagnoses of INS, (Membranous Nephropathy), the development of a non-invasive diagnostic tool for INS could help reduce the morbidity associated with the invasive renal biopsy procedure, minimizing the risk of incorrect diagnosis. We developed a diagnostic score, the SNIT, from routine clinical and biological variables to allow INS diagnoses in patients for whom detection of blood antibodies of Membranous Nephropathy were negative. The main goal of this study is to validate this diagnosis score in a prospective study, to allow INS diagnosis without renal biopsy.