Extent of endocapillary proliferation and its clinicopathological associations in pediatric IgA vasculitis nephritis: a single-center retrospective study

The E lesion in the Oxford classification records only the presence or absence of endocapillary proliferation (EP; E0/E1) and does not capture the extent of glomerular involvement. This study aimed to characterize the extent of EP in children with IgA vasculitis nephritis (IgAVN) and evaluate its associations with clinical and pathological features at kidney biopsy. We retrospectively included 847 children with biopsy-proven IgAVN and compared clinicopathological characteristics between Oxford E0 and E1. Among E1 biopsies, we further quantified the proportion of glomeruli affected by EP and evaluated its clinicopathological associations. Among the 847 children, 330 (39.0%) had Oxford E1. Compared with E0, children with E1 had higher 24-hour urinary protein excretion and lower serum albumin levels. Among the 286 E1 biopsies with quantifiable EP extent, the median proportion of glomeruli affected by EP was 9.60% (IQR 5.56%–18.36%), and 83.9% had EP involving no more than 25% of sampled glomeruli. EP extent was positively associated with 24-hour urinary protein excretion. After adjustment for age, sex, and time from the first urinary abnormality to kidney biopsy, each 10-percentage-point increase in EP extent was associated with a 24.0% higher geometric mean 24-hour urinary protein excretion (GMR 1.240, 95% CI 1.153–1.334; P < 0.001). Pathologically, EP extent showed no clear correlation with the proportion of glomeruli with mesangial hypercellularity or with the proportions of active or total crescents. EP extent showed substantial heterogeneity among children with Oxford E1, and more extensive EP was associated with higher proteinuria at kidney biopsy. Continuous quantification of EP extent may provide clinicopathological information complementary to the binary Oxford E0/E1 classification.