Class III Lupus Nephritis is Focal Proliferative
Class III lupus nephritis is classified as focal proliferative glomerulonephritis, not focal segmental disease. The term "focal" refers to the involvement of less than 50% of glomeruli, while "proliferative" describes the endocapillary hypercellularity and inflammatory cell infiltration that characterizes this class 1.
Key Terminology Clarification
The confusion often arises from the term "segmental," which has a different meaning in lupus nephritis classification:
- Focal = affects <50% of total glomeruli (distribution across the kidney)
- Segmental = affects only part of an individual glomerulus (pattern within a single glomerulus)
- Proliferative = endocapillary hypercellularity with leukocyte influx
Class III is defined as focal proliferative lupus nephritis because it involves active proliferative lesions (endocapillary hypercellularity, subendothelial immune deposits, leukocyte infiltration) affecting less than half of the glomeruli 1, 2.
Important Distinction from Focal Segmental Glomerulosclerosis (FSGS)
"Focal segmental" typically refers to focal segmental glomerulosclerosis (FSGS), which is an entirely different disease entity characterized by segmental sclerosis. This is not what Class III lupus nephritis represents. Class III is an immune complex-mediated proliferative disease with active inflammation, not a primary sclerosing process 1.
Evolution of Classification
The 2018 ISN/RPS revision actually eliminated the segmental (S) and global (G) subdivisions for Class IV due to poor interobserver agreement and weak clinical significance 3. However, the fundamental definition of Class III as focal proliferative disease remains unchanged. The distinction between segmental and global lesions within individual glomeruli is now retained only in microscopic descriptions rather than formal subclassification 3.
Clinical Implications
Understanding this terminology is critical because:
- Class III (focal proliferative) has a better prognosis than Class IV (diffuse proliferative) when properly defined 2, 4
- Treatment intensity may differ based on accurate classification 5
- The "focal" designation specifically means <50% glomerular involvement, which has prognostic significance 1
Bottom line: Class III lupus nephritis is focal proliferative glomerulonephritis—"focal" describes the distribution (<50% of glomeruli), and "proliferative" describes the inflammatory nature of the lesions.