IgA Nephropathy & Post-Strep Glomerulonephritis (PSGN) are both relatively common causes of glomerulonephritis which can both be associated with upper respiratory infections. How can you tell them apart?
One of the most important distinctions can be made in taking a good history: In PSGN, glomerulonephritis typically does not set in until several weeks after the initial infection. In contrast, IgA Nephropathy may present with so-called "synpharyngitic glomerulonephritis"--pharyngitis and glomerulonephritis at the same time.
In addition, PSGN classically presents with hypocomplementemia, and if the patient undergoes a renal biopsy there is evidence of an immune complex-mediated process. In contrast, IgA Nephropathy shows normal serologic values (though IgA levels may be elevated in about a third of patients) and the renal biopsy will show mesangial IgA deposition.