Investigation of choice for confirming Henoch Schonlein Purpura is

Biopsy of the kidney may be performed both to establish the diagnosis or to assess the severity of already suspected kidney disease. The main findings on kidney biopsy are increased cells and IgA deposition in the mesangium (pa of the glomerulus, where blood is filtered), white blood cells, and the development of crescents.(Refer: Nelson's Textbook of Pediatrics, SAE, 1st edition, pg no. 1217 - 1218)