Primary renal diseases associated with glomerulonephritis and low serum complement levels include acute postinfectious glomerulonephritis and idiopathic MPGN (IMPGN). The former has been most extensively studied after streptococcal infections, although the syndrome has been reported after other bacterial, viral, parasitic, rickettsial, and fungal infections.30 Although IMPGN remains an important cause of glomerular disease in children, the incidence of primary disease in adults has declined.31,32 Secondary forms of the disease may be associated with autoimmune diseases, chronic infections, microangiopathies, and paraprotein deposition diseases.33 Most patients with IMPGN have recurrent bouts of glomerulonephritis (and/or nephrotic syndrome). By contrast, with glomerulonephritis after streptococcal infections, recovery (lack of progression to end-stage renal disease) is the rule, especially in children (<2% progression to end-stage renal disease), and, thus the disease course is helpful in confirming the diagnosis. Nevertheless, persistent urinary abnormalities may last for years, and a small percentage of adults develop slowly progressive renal failure.30 For diagnosis, repeated evaluation of serum complement levels, determination of autoantibodies to complement pathway components, and renal biopsy findings are especially helpful in distinguishing glomerulonephritis after streptococcal infections from IMPGN (Table 3). These serologic determinations are especially useful in situations where the distinction between glomerulonephritis after streptococcal infections and MPGN is difficult on clinical grounds alone (eg, where there is persistent or recurrent disease).