Proteinuria selectivity index based upon alpha 2-macroglobulin or IgM is superior to the IgG based index in differentiating glomerular diseases. Technical note.

BACKGROUND: The proteinuria selectivity index (SI) may be used to describe changes of the glomerular permeability for macromolecules in glomerular diseases. Proteins the size of alpha 2-macroglobulin (alpha 2 M) or IgM cannot normally pass the glomerular barrier, whereas IgG can pass through the large pores of glomerular basement membrane. Comparison of the clearance of the three high-molecular-weight proteins to that of albumin may be useful in characterization and diagnosis of different glomerular diseases as well as in understanding of the permeability characteristics of the glomerular filter.
METHODS: Three types of SI, each calculated as a ratio of clearance of either IgG, alpha 2M or IgM to that of albumin, were measured in 199 proteinuric patients. The patients were subdivided into eight different biopsy-verified glomerular diseases.
RESULTS: Two diagnoses could be clearly distinguished using SI based on alpha 2M (alpha 2 M SI) or IgM (IgM SI). Both alpha 2M SI and IgM SI were significantly lower in minimal change nephropathy and higher in crescentic necrotizing glomerulonephritis than in all the other diagnoses. The SI based on IgG (IgG SI) was less useful in determining specific diagnoses, since patients with minimal change nephropathy could not be distinguished from those with other types of primary glomerulonephritis and patients with crescentic necrotizing glomerulonephritis did not differ from those with diabetic nephropathy.
CONCLUSIONS: The findings of this study indicate that alpha 2M SI and IgM SI are superior to IgG SI in characterization of glomerular disorders and might replace the IgG SI for this purpose.