Reassessment of Selectivity Index in Nephrotic Syndrome with Special Reference to the Clinical Significance of a Small-Molecular IgG Fraction in the Urine

: The selective index O and the transR IgG have been valuable in clinical use, but they do not appear to always correctly reflect the GBM sieving effect on plasma protein and thus do not correctly predict the response to steroids. We applied a new technique to 22 nephrotic patients with the following results. (a) Fragments of IgG molecules (M-IgG, MW 5-10 X 10(4); S-IgG, MW 1-5 X 10(4)) were present in the urine in addition to normal IgG (L-IgG). These fragments were immunologically identical with the intact IgG molecule and therefore, could falsely portray GMB permeability. (b) The data strongly suggest that the S-IgG fragment found in the nephrotic urine is the IgG-Fc fragment present in increased amounts in such urine. (c) IgG L/S shows a positive correlation with O and transR IgG and can be used as a new and better selectivity index of GBM permeability. This, in turn, may provide a more accurate means of predicting the response to steroid therapy. For example, in the cases we analyzed, the transR IgG and O were incorrect in 31.9% and 22.7%, respectively, whereas with our new method they were incorrect in only 13.6%.