Correct identification, description and quantification of glomerular lesions is essential, not only for diagnosis but for accurate classification that is required to provide prognostic information and guide patient management. Clear and precise lesion definitions play an important role in assisting pathologists classify glomerular disease in a consistent and reproducible manner. Many lesions are common to several different diseases and included in various classification schema. However, the working groups that generated these schema frequently defined the same lesion in different ways, making it difficult for pathologists to apply the definitions in reporting renal biopsies.

In recognition of this issue, in 2020 a Renal Pathology Society (RPS) working group, containing representatives from the various disease-specific working groups, published a set of consensus definitions with illustrations [1]. This was subsequently demonstrated to have a positive impact on improving interobserver agreement between pathologists [2]. The definitions were based on interpretation of standard histochemical stains that are routinely used for medical renal biopsies. However, despite these definitions, the interobserver variation for assessment of some glomerular lesions remains high, with resultant poor reproducibility in the classification of some glomerulonephritides.

Difficulties arise in part from sub-optimal laboratory handling of the biopsies, with overly thick sections and poor stains. However, it appears that some RPS lesion definitions are inadequate for pathologists to apply in a reproducible manner. Certain lesions cause particular difficulty, including the quantification of mesangial hypercellularity, identifying endocapillary hypercellularity, distinguishing inflammatory crescents from pseudocrescents, and assessing endocapillary and extracapillary lesions that comprise both cells and matrix. Some of these difficulties can be overcome by following detailed guidance on which stains to use for particular lesions, and also by the routine application of immunohistochemistry to facilitate quantification of glomerular inflammation. This will be an interactive presentation, challenging attendees to define and classify illustrated difficult lesions.