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Clinical Nephrology |




* Division of Nephrology and Hypertension;
Department of Pathology;
Division of Epidemiology; and
Division of Biostatistics, Department of Health Sciences Research, Mayo Clinic College of Medicine, Rochester, Minnesota
Address correspondence to: Dr. Fernando C. Fervenza, Division of Nephrology and Hypertension, Mayo Clinic, 200 First Street SW, Rochester, MN 55905. Phone: 507-266-7961; Fax: 507-266-7891; fervenza.fernando{at}mayo.edu
| Abstract |
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| Introduction |
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| Materials and Methods |
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Biopsies were categorized as (1) FSGS, (2) MN, (3) minimal change (MC), (4) lupus nephritis, (5) membranoproliferative glomerulonephritis (MPGN), (6) IgA nephropathy (IgAN), (7) crescentic/necrotizing GN, and (8) other/secondary GN. The overall crude biopsy rate per 100,000 population per year was calculated using the total number of renal biopsies done in each of the three time periods: 1974 to 1983, 1984 to 1993, and 1994 to 2003.
The relative frequencies of each type of GN were calculated for three 10-yr intervals. In calculating incidence rates, the entire population of Olmsted County was considered to be at risk. Denominator age- and gender-specific person-years were estimated from decennial census data (7). To obtain some sense of variability, we assumed that, given a fixed number of person-years, the number of GN cases follows a Poisson distribution. This allowed estimation of standard errors and the calculation of 95% confidence intervals (CI) for the incidence rates. Rates were directly age and gender adjusted to the population distribution of the United States in 2000. Even though the age and gender distribution of the Olmsted County is known, the distribution has changed over time. Because the current Olmsted County age distribution is similar to US age distributions, to facilitate comparisons, we used the more widely available 2000 US census age distribution. Comparison of the age- and gender-adjusted incidence of GN and specific GN subtypes among the three 10-yr periods was done using the z test. Summary quantitative data are reported as mean ± SD or median (25th, 75th percentiles) as appropriate. All analyses were carried out in SAS (SAS Institute, Cary, NC), and P < 0.05 was considered significant.
| Results |
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GN was diagnosed in a total of 208 of these biopsies in the past 30 yr. Thirteen patients refused access to their records for research, leaving 195 for analysis. GN was seen in 35 biopsies in 1974 to 1983 compared with 56 in 1984 to 1993 and 104 in 1994 to 2003, but the population grew in the interval from 92,006 (98% white) in 1980 to 106,470 (96% white) in 1990 to 124,277 (90% white) in 2000. The percentage of renal biopsies with GN was 52% overall, with the rate relatively stable during the three time periods (47, 62, and 49%; P = 0.08). Overall, the most frequent primary GN diagnoses were IgAN (n = 42), FSGS (n = 33), lupus (n = 25), and MN (n = 20), followed by MPGN (n = 13), necrotizing (n = 12) and crescentic (n = 10) GN, and MC (n = 8). GN was secondary to other conditions in 32 patients.
The mean ± SD age at the time of GN biopsy was 44 ± 20 yr, and 111 (56.9%) were male. Median (25th, 75th percentiles) serum creatinine values at time of GN biopsy increased slightly over time (P = 0.10), being 1.2 (0.9, 2.2), 1.4 (0.9, 2.1), and 1.5 (1.1, 2.6) for years 1974 to 1983, 1984 to 1993, and 1994 to 2003, respectively. Median 24-h urine protein was somewhat higher (P = 0.052) in later years, being 1.7 (1.2, 3.4), 3.7 (1.9, 7.0), and 2.8 g (1.2, 7.3) for the periods 1974 to 1983, 1984 to 1993, and 1994 to 2003, respectively.
The overall annual incidence of GN in this population was 6.6 per 100,000, directly age and gender adjusted to the population structure of the United States in 2000. As noted in Table 1 the GN incidence rate was 7.9 per 100,000 person-years in male individuals compared with 5.4 per 100,000 person-years in female individuals (P = 0.006). There was a 2.3-fold increase in the incidence of any GN from the period 1974 to 1983 to the period 1994 to 2003 (P < 0.001), as the annual incidence of GN increased from 3.9 per 100,000 (95% CI 2.6 to 5.3) in 1974 to 1983 to 9.0 per 100,000 (95% CI 7.3 to 10.7) in 1994 to 2003 (Table 1). The age-specific incidence generally increased for both genders, peaking at 60 to 79 yr then decreasing after age 80.
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| Discussion |
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DAgati and colleagues (9,10) noted a seven-fold increase in the incidence of FSGS in their renal biopsy database from 1974 to 1993. Subsequently, Haas et al. (3) confirmed the increasing incidence of FSGS among adult nephropathies, observing a three-fold increase from 1974 to 1993. More recently, Kitiyakara et al. (11) showed that ESRD as a result of FSGS has increased 11-fold in the past 2 decades and observed that the incidence rate for FSGS related ESRD was four-fold greater in black than white patients. Even though these studies showed an increasing incidence of FSGS in white as well as black adults, clearly the findings were more impressive for the black population, and collapsing GN (seen most often in black patients) constituted up to 4.7% of all patients with FSGS. Furthermore, they included referral cases from metropolitan areas with different racial backgrounds (3,4,8). This may restrict extension of these observations to the general population of the United States, which is predominantly white. However, the findings in our study confirm these observations in the white population and suggest that factors other than race are responsible for the increasing incidence of FSGS. Significantly, collapsing FSGS was not observed in any patient in our study and does not explain the increasing incidence. Urbanization, a suggested explanation for the increase in the incidence of other GN, such as MC, and the progressive increase in the frequency of obesity may play a role in the increasing incidence of FSGS. However, as noted from the findings of our study, in which there was no increase in incidence of obesity in patients with FSGS during the three periods, obesity is unlikely to be the major factor to account for increasing incidence of FSGS (12,13), and involvement of yet-unidentified new environmental factors in the causation of FSGS remains an intriguing possibility.
Korbet et al. (14) noted a decrease in the incidence of MN in black individuals compared with an increase of approximately 10% over 10 yr in the white individuals. Later, Braden et al. (4) also observed a similar decrease in the incidence of MN in the US population, but they noted a decrease in incidence in both white and black individuals. The findings in our study, in which the incidence of MN did not change significantly during the past 30 yr, contrasts with these observations in the past and confirms the findings from a study by Haas et al. (3). Note, however, that we could not assess trends in nonwhite individuals, who represented only 2% of the population in 1980 and 10% in 2000 (0.4 versus 2% black) and who, moreover, were much younger on average than the majority population.
Although rare in black individuals, IgAN remains the most common biopsy-proven GN in adults, and especially in Hispanic individuals (14). The results of our study extend this finding to the white population also. It is interesting to note that the incidence of IgAN increased three-fold from the first 10-yr period to the last 10-yr period, and part of this increase could be attributed to a two-fold increase in renal biopsy rate in the same time frame. Haas et al. (3,8) observed a similar increasing trend for IgAN, but the cause for this increase in incidence also remains unexplained. It is interesting to note that the overall incidence of GN was higher in male than in female individuals, and the incidence of GN progressively increased for both genders during the three periods.
Although the results of our study need to be interpreted in the context of an increasingly aggressive diagnostic approach to glomerular diseases, this is unlikely to be the main reason for the increasing incidence of FSGS because of the following reasons. First, the degree of elevation in serum creatinine and the severity of proteinuria at the time of renal biopsy were not less in the later time periods. Second, the percentage of all biopsies that were GN remained fairly stable during the three time periods between 1974 to 2003, and the incidence of other GN-like MN did not show a similar trend in the same population. Third, the overall annual incidence of GN in Olmsted County in the most recent time period was in fact slightly less than that reported from Victoria, Australia (9.0 per 100,000 here versus 12.4 per 100,000 in Australia, both age and gender adjusted to the US 2000 population), and the renal biopsy rate in this community (12 per 100,000 per year in 1974 to 2003) was significantly lower than that reported by Briganti et al. (2) (21.5 per 100,000; 1147 per 2030 with 57% with GN). Also, the incidence of FSGS increased 13-fold between 1973 to 1983 and 1994 to 2003 compared with a corresponding increase of only 2.1-fold in population-adjusted annual crude renal biopsy rates. Last, the annual incidence of GN subtypes (age and gender adjusted to the US 2000 population) in Olmsted County (1994 to 2003) and Australia (1995 to 1997) are comparable (FSGS 2.1 versus 1.8; MN 1.3 versus 1.0; and IgAN 4.2 versus 2.1 per 100,000, respectively).
There are some limitations of our study. Because this is a renal biopsy-based study and not all patients with GN undergo renal biopsy, our results potentially could underestimate the true number of patients with primary glomerular disease in the population. Also, we cannot exclude the possibility that a few patients from this county were seen and biopsied at centers elsewhere. Despite these limitations, the findings in our study are unique and best represent the US white population. Our study confirms that the incidence of GN is growing overall, particularly so for FSGS, which in our data are second only to IgAN as the leading cause of GN in white adults and is the most common cause of NS.
| Acknowledgments |
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| Footnotes |
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Received August 15, 2005. Accepted March 1, 2006.
| References |
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