Reproducibility for pathological prognostic parameters of the Oxford classification of IgA nephropathy: a Japanese cohort study of the Ministry of Health, Labor and Welfare.

Background/aims: The Oxford classification of IgA nephropathy (IgAN) was proposed by international working group in 2009. Interobserver reproducibility of each pathological definition was already evaluated, but that of four pathological prognostic parameters score has not yet been assessed. We first assess the reproducibility of each pathological definition in Japanese patients. Our study is aimed to assess that of four pathological prognostic parameters score among the five Japanese pathologists.

Methods: The renal specimens from 411 Japanese patients, aged 3-85 years, with biopsied proven primary IgAN were collected from 50 facilities between 2006 and 2012. The reproducibility of pathological definitions was assessed by the intraclass correlation coefficient (ICC) and that of four pathological prognostic parameters score (mesangial hypercellularity (M), endocapillary hypercellularity (E), segmental glomerulosclerosis (S), and tubular atrophy/interstitial fibrosis (T)) was assessed by kappa statistics.

Results: The ICC of M, E, S, T, global sclerosis and cellular crescents and/or fibrocellular crescents were good or moderate agreement among the five pathologists and were well agreed with results of the Oxford study. Kappa statistics was moderate agreement for M and T score assessed with the semi-quantitative method by the Oxford group, but that was poor agreement for S and E score based on a simple "present" or "absent" assessment.

Conclusion: This is the first report to assess the reproducibility of pathological prognostic parameters score in the Oxford classification. Our study supports the utilization of the pathological lesions in routine diagnosis. The methodological assessment of pathological prognostic parameters score should be reconsidered.