The World Health Organization (WHO) of lupus nephritis has provided a useful and simple framework for understanding this complex disease. In 2003, a committe of the International Society of Nephrology (ISN) & Renal Pathologic Society (RPS) instituted some subtle but important changes to the existing criteria, which rely predominantly on histopathologic inspection of renal biopsy.
To review, the simplistic take on the classification scheme is the following (I'm ignoring many of the existing subdivisions):
WHO Class I: minimal mesangial lupus nephritis
WHO Class II: mesangial proliferative lupus nephritis
WHO Class III: focal proliferative lupus nephritis (<50%>50% of glomeruli involved)
WHO Class V: membranous lupus nephritis
WHO Class VI: advanced sclerotic lupus nephritis (e.g., too much fibrosis to tell)
One of the main additions to this classification scheme was the subdivision into WHO Class IV into SEGMENTAL (Class IV-S) and GLOBAL (Class IV-G), based on whether there is less than 50% capillary tuft involvement (IV-S) versus more than 50% capillary tuft involvement (IV-G). Several researchers had found that there was a slightly improved prognosis in the IV-G group compared to the IV-S group which might justify such a division; however, this difference has not been reproducibly demonstrated in follow-up studies. Thus, whether or not there really are distinct subpopulations of IV-G and IV-S individuals with different prognoses remains controversial.