Basal glomerular membrane represents mechanical and electrical obstacle for passing of plasma proteins. In physiological conditions only plasma proteins of low molecule weight are completely filtered through basal membrane. Due to damages of basal glomerular membrane there is increase in filtration of plasma protein of middle and high molecular weight. Depending on etiology of proteinuria it can be prerenal, renal and postrenal. By analyzing albumin a1-microglobulin, immunoglobulin G and a2-macroglobulin, together with total protein in urine, it is possible to detect and differentiate causes of prerenal, glomerular, tubular and postrenal proteinuria. The adequate and early differentiation of proteinuria is of an immense diagnostic and therapeutic importance.