Incidence of biopsy-proven primary glomerulonephritis in an Italian province.

Between January 1, 1970, and December 31, 1994, 1,926 cases of biopsy-proven primary glomerulonephritis (PGN) were diagnosed in an adult population (> 15 years of age) in a northwestern region of Italy with approximately 3.5 million inhabitants. The principal long-term changes were an increase in the absolute number of biopsies per year, an increase in the mean age of patients undergoing biopsy (from 29.3 +/- 12.2 years to 47.0 +/- 17.8 years), an increase in the percentage of patients older than 65 years (from 1.7% to 20.4%), and an increase in the percentage of isolated urinary abnormalities as an indication for biopsy (from 3.5% to 29.6%). In the total biopsy material, immunoglobulin A glomerulonephritis (IgA-GN) is the most frequent type (26%), followed by membranous glomerulonephritis (MGN; 20%). An incidence study was begun in 1990; this survey was restricted to the population of the province of Torino (approximately 2 million inhabitants) as only this area completely refers to the nephrologic centers that entered patients into this study. The overall incidence of PGN is 4.68 new cases/yr/10(5) population with a predominance of males (> 2:1); IgA-GN is the most common type (1.47/yr/10(5) population [34.5%]) in the overall population. In the elderly, cases of PGN are twice as high as in adults (8.19/yr/10(5) population v 4.02/yr/10(5) population in the 65 to 74 year and 45 to 54 year age groups, respectively); MGN mainly accounts for this high incidence (3.4/yr/10(5) population), while the nephrotic syndrome is the most common indication for biopsy (53.8%). A comparison with the incidence in the same area in the early 1970s is evaluable only for PGN, which was mainly registered in the age groups for which an unrestricted biopsy policy was already in place (15 to 35 years). In contrast with a misleading increase of all types of PGN, which is in reality due to the extension of the biopsy policy to older and asymptomatic patients, membranoproliferative glomerulonephritis type I shows a countercurrent decrease from 0.43 to 0.13/yr/10(5) population. Evidence of a simultaneous decrease in severe cardiac valvulopathy, due to rheumatic fever, is also provided. We feel that before epidemiologic conclusions can be reached, a clear understanding of one's own biopsy policy is essential. An apparent change in the PGN rate in our region over the last 25 years mainly depends on modifications in our biopsy policy, most probably coupled with a change in the threshold of detection of symptoms in the general population. At present, according to our experience, IgA-GN is the most common type of PGN in the total bioptic material, as demonstrated in other European countries, while the elderly show a peculiar pattern with a higher PGN incidence, mainly represented by MGN and heralded by the nephrotic syndrome. We also confirm that membranoproliferative glomerulonephritis type I is indeed decreasing in parallel with changes in the microbiologic environment.