An improved simple method for determination of selectivity index has been described. This new index (Creteil index) has been compared to Cameron index. The correlation between both indexes has been 0.949 (R-squared). Creteil index is a less expensive method because only two determinations are requisited in serum and urine: albumin and IgG. In Cameron index, transferrin and IgG are detected and albumin has to be measured in addition in the blood to appreciate the nephrotic syndrome.