Pauci-immune disease--a common cause of glomerulonephritis--refers to the histologic finding that there is an absence of glomerular immune staining (e.g., no immune complex or anti-GBM staining). Usually this is accompanied by positive ANCA titers, but not necessarily. When this occurs, the disease may be termed "idiopathic RPGN" or "ANCA-negative vasculitis."
The overall outcome and prognosis ends up being about the same as ANCA-positive disease.
I saw a patient today with ANCA-negative Wegener's Disease--without renal involvement. She has a history of long-standing sinus disease with multiple sinus surgeries, an ESR in the 80s, biopsies of a non-healing nasal ulcer consisting with a Wegener's vasculitis, and repeatedly negative ANCA titers. She is currently being managed with an ANCA-type regimen consisting of Cytoxan and prednisone--we will see how she responds.