OBJECTIVES : Economic evaluations require target population estimates. Heterogeneity of published RWE introduces challenges estimating incidence, especially in rare diseases. We sought to describe challenges encountered while developing a literature-based incidence estimate for IgAN, a rare autoimmune proliferative glomerulonephritis.

METHODS : Using PRISMA guidelines and PICOS criteria (Population, Intervention, Comparators, Outcomes, Study design), EMBASE, MEDLINE, and Cochrane databases (2010−6/26/2020) and congress abstracts (2017 to 2020) were searched for incidence of IgAN. Meta-analysis was performed on selected studies to calculate the incident population of IgAN among patients undergoing renal biopsy for any reason. We used the reported rate of renal biopsies in the US to determine annual incidence of IgAN.

RESULTS : Of 74 studies reporting IgAN rates, 16 were US-based; 58 were Western Europe, Canada or Australia-based (ex-US). Since IgAN diagnosis requires renal biopsy, only 5 studies reporting IgAN rates in 38,887 patients undergoing renal biopsy were included, with average IgAN rate of 8.2% (95% CI 6.1 to 10.9), which was lower than the 16.3% average in 15 selected ex-US studies (95% CI 13.5 to 19.5). We identified one study reporting US renal biopsy rates (15.8/100,000), which was higher than the 6.1 to 10.9/100,000 reported in 5 ex-US studies. These led to an estimated US IgAN annual incidence of 1.29/100,000 and an ex-US rate of 0.99 to 1.78/100,000, in line with previously published rates (US: 1.6 to 2.3/100,000; ex-US: 1.2 to 1.9/100,000).

CONCLUSIONS : Careful selection of relevant high-quality studies for epidemiology estimates is extremely important. In our research, multiple studies were excluded, as the populations in these studied were not well-defined (eg, glomerular nephropathy diagnosed with or without biopsy), which, together with very limited data on biopsy rates, can impact the robustness of results. Triangulation of US estimates with ex-US and previously published analyses improves the validity of epidemiology estimates for rare diseases such as IgAN.