Impact of Case Ascertainment on Recent Trends in Diabetes Incidence in Rochester, Minnesota

American Journal of Epidemiology Copyright © 2002 by the Johns Hopkins Bloomberg School of Public Health All rights reserved

Vol. 155, No. 9 Printed in U.S.A.

Diabetes Incidence in Rochester, Minnesota Burke et al.

Impact of Case Ascertainment on Recent Trends in Diabetes Incidence in Rochester, Minnesota

James P. Burke,1 Peter O’Brien,1 Jeanine Ransom,1 Pasquale J. Palumbo,2 Eva Lydick,3 Barbara P. Yawn,4 L. Joseph Melton III,1 and Cynthia L. Leibson1

diabetes mellitus, non-insulin-dependent; incidence

itations of self-reporting and changes in clinical criteria over time. To address these limitations, researchers in the San Antonio Heart Study obtained data on glucose levels at baseline and follow-up for Mexican Americans and nonHispanic Whites residing in San Antonio, Texas. They found a threefold increase in the incidence of type 2 diabetes over the subsequent 7–8 years between 1979 and 1988, although the trend reached statistical significance only among Mexican Americans (4). Several limitations of the NHIS data were also addressed with retrospective review of laboratory glucose values in the predominately White population of Rochester, Minnesota. The Rochester study applied standardized glycemic criteria for diabetes and found that the age- and sex-adjusted incidence of diabetes increased significantly between 1970–1974 and 1985–1989 (5). Although a comparison of patient characteristics as of the incidence date suggested that the rise in diabetes incidence was not an artifact of earlier detection of milder cases (5, 6), the question of whether observed trends in incidence were accompanied by increases in diabetes ascertainment and/or detection remains unanswered. In the present study, we updated the Rochester incidence rates from 1970–1989 through the year 1994 to test whether the previously observed increase in diabetes incidence has continued at the same rate or has accelerated, leveled off, or declined. In addition, we examined trends in passive sur-

A number of studies have shown that the prevalence of diabetes is increasing in the United States (1, 2), but trends in prevalence fail to distinguish between increases in relative survival and increases in the rate at which new cases of disease develop. For purposes of devising and implementing appropriate intervention strategies, it is necessary to make such a distinction. Unfortunately, there are few opportunities to evaluate trends in diabetes incidence. Data from the National Health Interview Survey (NHIS) indicated no change in the incidence of diabetes between 1968 and 1992 (2), but a subsequent report found a 48 percent increase between 1980 and 1994 (3). The NHIS identifies incident cases annually by asking respondents whether they were first assigned a clinical diagnosis of diabetes in the previous year. This approach is problematic because of lim-

Received for publication April 18, 2001, and accepted for publication November 27, 2001. Abbreviations: NDDG, National Diabetes Data Group; NHIS, National Health Interview Survey. 1 Department of Health Sciences Research, Mayo Clinic, Rochester, MN. 2 Department of Medicine, Mayo Clinic, Scottsdale, AZ. 3 SmithKline Beecham Pharmaceuticals, Philadelphia, PA. 4 Olmsted Medical Center, Rochester, MN. Reprint requests to Dr. James P. Burke, Department of Health Sciences Research, Harwick 6, Mayo Clinic, 200 First Street SW, Rochester, MN 55905 (e-mail: [email protected]).

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The authors previously reported an increase in the incidence of diabetes mellitus among Rochester, Minnesota, residents during the period 1970–1989. This study provides updated rates from data collected through 1994. Trends in diabetes surveillance, i.e., the proportion of residents who had a blood glucose measurement in each year between 1987 and 1994, are also provided. The authors reviewed medical records to identify residents aged 30 years or more who first met National Diabetes Data Group criteria for diabetes between January 1, 1970, and December 31, 1994. Age- and sex-adjusted incidence rates were calculated for successive quinquennia (5-year periods), and Poisson regression was used to test for an effect of calendar year; calendar period (1970–1989 vs. 1990–1994) was added to assess whether the association with calendar year varied in the most recent quinquennium. Altogether, 1,992 Rochester residents first met National Diabetes Data Group criteria for diabetes between 1970 and 1994. The age-adjusted incidence per 100,000 person-years increased 67% for males (267.0 vs. 444.8) and 42% for females (225.4 vs. 319.1) between 1970–1974 and 1990–1994. Calendar year (p < 0.001) and calendar period (p = 0.026) were significant, suggesting that rates accelerated during 1990–1994. The proportion of residents with at least one blood glucose measurement per year was unchanged (p = 0.181) from 1987 to 1994, while the incidence of diabetes increased (p = 0.033). Thus, the authors conclude that the increase in diabetes incidence accelerated over the last quinquennium and was not due to increased surveillance. Am J Epidemiol 2002;155:859–65.

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veillance, i.e., the proportion of all local residents aged 30 years or more who had blood glucose values measured over this time period, which might explain temporal changes in the observed incidence of diabetes. MATERIALS AND METHODS

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Epidemiologic studies of diabetes are possible in Rochester (which had a population of 85,806 in the 2000 US Census) because the community is relatively isolated from other metropolitan areas and is home to few medical care providers, primarily only the Olmsted Medical Center and the Mayo Clinic, one of the world’s largest private medical centers. Therefore, nearly all of the medical care received by local residents is provided by either the Mayo Clinic, together with its two affiliated hospitals, or the Olmsted Medical Center, with its one affiliated hospital. In addition, every Mayo patient since 1907 has been assigned a unique identifier, and all information obtained from every Mayo contact is contained within a single dossier for each patient (7). This includes hospital inpatient and outpatient care, emergency room visits, office and nursing home visits, and all laboratory results, as well as death certificate and autopsy information. The diagnoses assigned at each visit are coded and entered into continuously updated computer files. Under the auspices of the Rochester Epidemiology Project, the diagnostic index and medical-record linkage were expanded to include the small number of other providers of medical care to local residents, primarily the Olmsted Medical Center. The result was linkage of all inpatient and outpatient medical records from all sources of medical care available to and utilized by members of this geographically defined population (8). The resources of the Rochester Epidemiology Project were used to construct the Rochester incident diabetes cohort. As described elsewhere (5, 9), case assignment was determined through retrospective review of each patient’s complete (hospital and ambulatory) medical records by trained nurse abstractors, under the direction of an endocrinologist (P. J. P.). The records were reviewed for all laboratory glucose values (available from 1930 to the present) and for evidence of any antidiabetes medication taken over the entire duration of residence in the community. Methods of ascertainment were consistent throughout the study period. Standardized case criteria were applied using National Diabetes Data Group (NDDG) recommendations—i.e., two consecutive fasting plasma glucose measurements of ≥140 mg/dl (7.8 mmol/liter) or both 1- and 2-hour glucose measurements of ≥200 mg/dl (11.1 mmol/liter) obtained during a standard oral glucose tolerance test (10). Using the method of West (11), adjustments were made for temporal changes in laboratory methods. Persons who failed to meet the above glycemic criteria but used oral antidiabetes agents or insulin for at least 2 weeks or until death also qualified as cases. The abstractor determined the date on which each individual first met the criteria and verified Rochester residency on that date. Case subjects were required to have been a Rochester resident 1 year prior to diagnosis. The study was approved by the institutional review boards of the Mayo Clinic and

the Olmsted Medical Center, and in accordance with a Minnesota state statute (8), individuals who refused authorization for use of their medical records in research (n  72) were excluded. Although laboratory glucose values have been available in the medical records since 1930, they were not available electronically prior to 1987. Because it was not feasible to manually review the hard-copy records of all Rochester residents, the review for all years was limited to potential cases. Potential cases were identified by generating a list from the Rochester Epidemiology Project diagnostic index of all Rochester residents ever assigned any diabetes-related diagnosis (e.g., elevated glucose level, hyperglycemia, impaired glucose tolerance, “rule out diabetes,” diabetic retinopathy, etc.) since 1945 (9). This preliminary step recognized that diabetes is a chronic progressive condition and was based on the assumption that individuals whose laboratory glucose values met NDDG criteria would, at some point in their medical history, receive a diagnosis of diabetes or a diabetes-like condition, e.g., “elevated glucose.” This assumption was examined in a recent study that identified all 7,192 Rochester residents who died on or after age 30 years during the time period 1979–1994 (12); the median duration of medical history available for review was 43 years (interquartile range, 24–58 years), and 25 percent (n  1,788) had been assigned at least one of the diabetes diagnoses under consideration. The likelihood of such a diagnosis did not change as a function of calendar year of death, providing confidence that the proportion of residents considered as potential cases in constructing the Rochester incident diabetes cohort was high and stable over time. Analysis of temporal trends in the present study was limited to adult members of the 1970–1994 incident cohort—i.e., all identified individuals who first met NDDG criteria between January 1, 1970, and December 31, 1994, and who were aged 30 years or more and residing in Rochester at the time they first met the criteria. Characteristics of the incident cases were summarized for successive quinquennia (1970–1974, 1975–1979, 1980–1984, 1985–1989, and 1990–1994) using descriptive statistics. Incidence rates were calculated for each sex and age group (30–44, 45–54, 55–64, 65–74, 75–84, and ≥85 years) for successive quinquennia, with newly diagnosed cases used as the numerator and Rochester person-years at risk as the denominator. The denominator was based on decennial census data with linear interpolation for intercensus years (13). Incidence rates were age- and sex-adjusted by the direct method to the White population of the United States as of 1980. Poisson regression was used to evaluate associations between incidence and age, sex, and calendar year of incidence. The significance of two-way interaction terms and higher-order polynomials was examined. To determine whether incidence of diabetes in the latest quinquennium increased at a faster rate, continued to increase at the same rate, leveled off, or declined, we added calendar period (1970–1989 vs. 1990–1994) to the final model as an independent variable. If the period variable contributed significantly to the model, this would indicate that there was a change in the incidence trend in the last quinquennium compared with the earlier time period.

Diabetes Incidence in Rochester, Minnesota

To address whether temporal trends in diabetes incidence were accompanied by changes in surveillance, we used Poisson regression to test for a significant effect of calendar year on the percentage of local residents who had a blood glucose measurement in each year during the period 1987–1994. As we noted above, laboratory glucose values were not available electronically for the entire population prior to 1987. For comparison purposes, we determined whether there was an increase in diabetes incidence over this same time period by limiting the Poisson regression analysis of incidence described above to the calendar years 1987–1994 and testing for a significant effect of calendar year. RESULTS

100,000 person-years in 1970–1974 to 371.8 per 100,000 person-years in 1990–1994. The results of Poisson regression analysis of incidence rates for the period 1970–1994 revealed that diabetes incidence rates differed between males and females ( p  0.001) and increased with increasing age ( p < 0.001). However, the effect of age was nonlinear ( p < 0.001 for age squared), and it differed between the sexes ( p < 0.001 for the interaction between sex and age and p  0.001 for the interaction between sex and age squared). There was also a significant increase in diabetes incidence with increasing calendar year ( p < 0.001). The absence of any significant interaction between calendar year and sex suggests that temporal trends were similar for males and females. However, consistent with the findings from table 2, the temporal trends were not constant across all age groups ( p < 0.001 for the interaction between age and calendar year). To facilitate interpretation of this complex model, we show in figure 1 the model-predicted incidence of diabetes for males and females at three ages (50, 60, and 70 years) and for the calendar years 1970 and 1990. Within each calendar year, incidence rates were higher for males than for females, and the between-sex difference was greater at older ages. Incidence rates increased with each decade of age, but the difference in incidence was greater between ages 50 and 60 years than between ages 60 and 70 years. At each age and for both sexes, the incidence rates were higher in 1990 than in 1970. The increase in incidence between 1990 and 1970 was greatest at younger ages. To determine whether the incidence of diabetes in the last quinquennium had increased at a faster rate, increased at the same rate, leveled off, or decreased, we included calendar period (1970–1989 vs. 1990–1994) as an independent factor in the previous Poisson regression model. The effect of calendar period was positive and significant ( p  0.026), indicating that the increase in incidence occurred at a faster rate during the latest quinquennium.

TABLE 1. Characteristics of diabetic residents of Rochester, Minnesota, aged ≥30 years at the time they met National Diabetes Data Group criteria for diabetes mellitus, by 5-year calendar period, 1970–1994 Quinquennium 1970–1974 (n = 273) Mean or %

SD†

1975–1979 (n = 290) Mean or %

SD

1980–1984 (n = 336) Mean or %

1985–1989 (n = 460) SD

Mean or %

1990–1994 (n = 633) SD

Mean or %

SD

Male sex (%)

43.2

Mean age (years) Males* Females*

61.6 64.2

13.8 13.3

59.0 65.8

11.7 13.1

62.0 63.2

12.8 14.4

59.6 62.2

12.6 15.2

58.6 59.4

13.5 15.9

Mean glucose level (mg/dl) 215.7 Males 215.2 Females

92.6 87.0

217.5 217.9

95.3 106.2

216.2 221.2

82.3 92.3

220.3 211.7

84.4 82.6

214.9 197.1

75.3 65.0

* p < 0.05 for trend over the five quinquennia. † SD, standard deviation.

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50.6

52.4

48.2

55.9

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A total of 1,992 Rochester residents aged 30 years or more were identified as first meeting NDDG criteria for diabetes between January 1, 1970, and December 31, 1994. Characteristics of the cases at the time of identification are provided for successive quinquennia in table 1. In particular, glucose values at the time of diabetes identification did not change significantly over time. In addition, the proportion of cases who were male did not change significantly over time, but there was a significant decline in age at identification among both males ( p  0.028) and females ( p < 0.001) that was most apparent in the latest quinquennium. The incidence of diabetes in Rochester during the period 1979–1994 is presented by sex and age group in table 2. There was an increase in diabetes incidence over time that was most apparent at younger ages. In males, the ageadjusted incidence rate increased 67 percent, from 267.0 per 100,000 person-years in 1970–1974 to 444.8 per 100,000 person-years in 1990–1994. In females, the age-adjusted incidence rate increased 42 percent, from 225.4 per 100,00 person-years to 319.1 per 100,000 person-years, over the same time period. The overall age- and sex-adjusted incidence of diabetes increased 54 percent, from 240.4 per

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341.9, 401.7 371.8 315.9 460 336 * p < 0.05 for temporal trend. † Per 100,000 person-years. ‡ CI, confidence interval. § Directly age-adjusted to the 1980 US White population. ¶ Directly age- and sex-adjusted to the 1980 US White population.

243.1 290 211.7, 269.0 240.4 273 Total¶

225.4 155 Subtotal§

DISCUSSION

256.7

228.9, 284.5

286.4, 345.3

319.1

633

255.5 219 174

214.9, 271.2

128 189.6, 261.3

177.6

146.1, 209.0

222.2

188.3, 256.0

220.4, 290.6

313

282.1, 356.1

165.8 500.3 532.9 635.6 582.8 99.2, 312.6, 305.1, 367.6, 358.1, 129.3 398.2 407.4 488.2 460.3 84.0 191.9 402.7 499.7 392.2 34 30 50 54 51 73.6 190.0 262.5 517.9 346.7 24 26 31 54 39 42.1 259.4 430.7 418.8 513.5 5.9, 107.4, 217.8, 195.4, 259.7, 18.1 171.4 311.2 291.7 371.0 5 22 36 29 36 104.9 299.6 329.7 691.1 630.3 36.9, 133.5, 147.0, 386.9, 325.0, 64.6 204.5 225.1 522.8 459.0 16 26 26 49 38 Women 30–44* 45–54* 55–64* 65–74 ≥75

267.0 118 Subtotal§

47.2, 124.0, 178.5, 388.9, 246.5,

109.6 278.3 372.5 675.8 473.9

58.2, 129.5, 298.8, 375.3, 292.2,

117.4 274.1 528.8 652.1 515.8

62 74 53 55 69

394.7, 495.0 444.8 393.4 162 162 218.1, 315.9

329.1

277.7, 380.5

312.6

263.8, 361.5

241

342.7, 444.0

320

86.7, 362.9, 428.4, 787.9, 472.9, 115.8 457.7 554.9 988.6 665.2 53 80 65 83 39 78.1 352.4 649.0 862.5 480.8 13 34 41 49 25 14 45 57 27 19 53.1 158.8 431.8 489.9 665.5 Men 30–44* 45–54* 55–64 65–74* ≥75

13 18 37 26 24

28.3, 94.2, 304.0, 319.9, 426.6,

90.8 251.0 595.1 717.7 990.4

52.9 381.8 610.5 478.4 493.9

28.9, 278.3, 462.1, 315.0, 297.3,

88.8 510.9 791.3 696.0 771.5

42.3 265.1 405.2 778.9 585.2

22.5, 183.7, 290.9, 576.4, 378.6,

72.7 370.6 549.9 1,029.7 863.8

30 52 71 64 24

52.7, 263.3, 506.9, 663.1, 308.2,

111.5 462.4 819.1 1101.5 715.4

1990–1994

Rate† No. 95% CI

1985–1989

Rate† No. 95% CI

1980–1984

Quinquennium

Rate† No. 95% CI

1975–1979

Rate† 95% CI‡

No. 1970–1974

Rate† No.

Sex and age group (years)

Incidence of diabetes mellitus among Rochester, Minnesota, residents aged ≥30 years, by age group, sex, and time period, 1970–1994

Figure 2 shows the age-adjusted incidence of diabetes in both males and females for the time period 1987–1994, along with the age-adjusted proportions of local residents who had at least one blood glucose measurement in each year during the same time period. Over this time period, there was no significant effect of calendar year ( p  0.181), adjusted for age and sex, on the percentage of local residents with at least one laboratory glucose measurement taken at the Mayo Clinic or the Olmsted Medical Center. The percentage of local residents with at least one blood glucose measurement in each year averaged approximately 37 percent annually for males and 44 percent annually for females. In each year, 90 percent of individuals with at least one blood glucose determination had three or fewer measurements per year; the proportion with only one value was unchanged over time (i.e., 71 percent, 69 percent, 66 percent, 69 percent, 70 percent, 69 percent, 65 percent, and 70 percent, successively, in the years 1987–1994). However, during the same time period, there was a significant effect of calendar year ( p  0.033), adjusted for age and sex, on diabetes incidence. This indicates that while surveillance did not change over this time period, the incidence of diabetes increased significantly.

A continually rising trend in the incidence of diabetes was observed among individuals aged 30 years or more in the Rochester, Minnesota, population from 1970 through 1994. This trend was significant in both sexes and was greatest in younger age groups. Further analyses indicated that the trend in Rochester accelerated in the last quinquennium studied, 1990–1994, as compared with the previous time period, 1970–1989. The recent rise in diabetes incidence in Rochester between 1970 and 1994 may have two possible explanations. One is an increase in surveillance over this time period. A second explanation is that there was an actual increase in the rate of diabetes, probably due to changes in environmental factors (e.g., obesity). The first possibility is improbable, since the proportion of individuals receiving at least one blood glucose measurement each year did not increase. Thus, increased surveillance is an unlikely explanation for the observed increase in diabetes incidence. Instead, a possible explanation for the rise in diabetes incidence may be the marked increase in obesity in the general population, which has been observed in a number of studies (4, 14, 15). In a previous article (16), we provided data on body mass index (weight (kg)/height (m)2) from a prospective survey of Rochester residents aged ≥45 years in 1986. A subsequent survey of Rochester residents was performed in 1996. In both surveys, weight and height were measured in a sample of the population identified by random digit dialing. There was a significant increase in mean body mass index between 1986 and 1996 for both male and female residents. The difference was most apparent in the younger age groups (Dr. Irene Meissner, Mayo Clinic, personal communication, 2001). These observations regarding temporal increases in body mass index within the Rochester populaAm J Epidemiol Vol. 155, No. 9, 2002

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TABLE 2.

151.5 569.9 707.5 1225.8 909.3

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95% CI

862

Diabetes Incidence in Rochester, Minnesota

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tion generally are consistent with our recent report of temporal increases in body mass index among members of the 1970–1989 Rochester diabetes cohort, as well as with the age-specific temporal increases in the rate of new cases of diabetes observed in this population (see figure 1). Other than the studies in Rochester, there have been few opportunities, using standardized glycemic criteria, to examine trends in the incidence of diabetes in this country. One such study in San Antonio, Texas, found an increase in the incidence of type 2 diabetes from 1979 through 1988 in Mexican Americans and non-Hispanic Whites who participated in the San Antonio Heart Study (4). Participants in the

study were aged 25–64 years and were randomly selected from low-, middle-, and high-income neighborhoods in San Antonio. Participants who were nondiabetic at baseline and returned for follow-up 7–8 years later were examined for diabetes. Standardized criteria for identifying cases of diabetes were used at both examinations. The percentage of individuals who developed type 2 diabetes in the subsequent 7- to 8year follow-up period increased significantly, from 5.7 percent for Mexican Americans first enrolled in 1979 to 15.7 percent for those first enrolled in 1988. In non-Hispanic Whites, there was a borderline-significant increase from 2.6 percent to 9.4 percent over the same time period. This trend

FIGURE 2. Age-adjusted incidence of adult-onset diabetes mellitus and age-adjusted proportion of the population with at least one blood glucose measurement in each year, Rochester, Minnesota, 1987–1994.

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FIGURE 1. Predicted incidence of adult-onset diabetes mellitus among Rochester, Minnesota, males and females aged 50, 60, and 70 years for 1970 and 1990.

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for each update of the Rochester diabetes cohort are identified from the list of residents with any prior clinical mention of diabetes or diabetes-like conditions through the end of the study period (see Materials and Methods). Thus, with each update, some individuals who have qualified as diabetes cases on the basis of glycemic criteria but have not yet been assigned a clinical diagnosis are missing from the list of potential cases. For example, in the most recent update prior to this one (5), we reported that the numbers of cases identified for the time periods 1970–1974, 1975–1979, 1980–1984, and 1985–1989 were 270, 288, 334, and 437, respectively. While the present update detected a relatively small number of previously unidentified cases (i.e., three, two, and two) for the periods 1970–1974, 1975–1979, and 1980–1984, respectively, 23 new cases were identified for the time period 1985–1989. The 1985–1989 age- and sex-adjusted annual incidence rate reported in the previous update was 303 per 100,000 population (95 percent confidence interval: 274, 332), as compared with the 316 per 100,000 (95 percent confidence interval: 286, 345) reported here. Thus, it is likely that the next update of the cohort through 1999 will identify additional cases for the time period 1990–1994 and that the apparent downturn from 1993 to 1994 (figure 2) is partly an artifact of our method of case identification. In conclusion, we observed a significant increase in the incidence of diabetes among residents of Rochester, Minnesota, aged 30 years or more during the period 1970–1994. This increase accelerated in the last quinquennium and was seen in both sexes, most dramatically in younger adults. An increase in diabetes surveillance is an unlikely explanation for this increased incidence. These results, combined with those of other studies (4, 5), provide further (albeit indirect) evidence that the epidemic of obesity in the United States is contributing to an epidemic of diabetes. The implications of a further increase in diabetes incidence are likely to be substantial, including increases in the incidence of cardiovascular disease and other complications related to diabetes.

ACKNOWLEDGMENTS

This study was supported by grants AG08729 and AR30582 from the National Institutes of Health and by SmithKline Beecham Pharmaceuticals (Philadelphia, Pennsylvania). The authors acknowledge the contributions of the nurse abstractors on this project, Kristine Otto-Higgins and Sue Helling.

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remained significant for Mexican Americans and borderlinesignificant for non-Hispanic Whites after adjustment for a number of baseline risk factors, including obesity. The diabetes case criteria in both the San Antonio Heart Study and the present investigation were standardized over time, although we did not prospectively examine the population as was done in the San Antonio study. These two studies differ from almost all other reports of secular trends in diabetes incidence, in which case identification is based on a self-report of having received a diagnosis of diabetes from a clinician in the past year. Data from the sequential NHIS surveys revealed little change between 1968 and 1992 in the annual rate of noninstitutionalized US adults aged ≥18 years reporting having received a first diagnosis of diabetes in the previous year (2). These trends differ from a subsequent NHIS report that revealed a significant increase in both crude and age-adjusted diabetes incidence rates for the time period 1980–1994 (17). The disparate trends could reflect either a sudden increase in diabetes prevalence in 1992–1994 or a failure to detect an increase in the earlier NHIS because of the 1979 introduction of more stringent NDDG recommendations for assigning a clinical diagnosis of diabetes (9, 19). The extent to which the present study findings are generalizable to the entire US population is limited by the fact that the Rochester population was 95 percent White during the years of the study. Our study was also limited by the fact that, to minimize misclassification and to ensure a standardized definition throughout the study period, the criteria we employed for ascertaining incident diabetes cases were intentionally strict. As a result, individuals whose glucose values never met NDDG criteria but did meet more recently introduced American Diabetes Association criteria (18) would have been excluded. Individuals who never had a diagnosis suggestive of diabetes in the Rochester Epidemiology Project diagnostic index during their entire period of residence in the community but who would have qualified if prospectively screened also were not included. Cross-sectional studies that prospectively screen communitydwelling persons suggest that a substantial proportion of such individuals qualify as diabetic on the basis of oral glucose tolerance testing or fasting glucose testing, even though they report no prior diagnosis of diabetes (1). However, because diabetes is a chronic, progressive disease, many such individuals ultimately come to clinical attention, as was demonstrated by our study of the proportion of deceased Rochester residents with any mention of diabetes or diabetes-like conditions in their medical history (12). In addition, a significant (35–45 percent) and consistent proportion of Rochester residents had at least one blood glucose determination each year. While a large proportion of Rochester residents have a blood glucose measurement every few years, the increase in diabetes incidence observed in the most recent quinquennium, 1990–1994, could still be an underestimate. With each update of the Rochester diabetes cohort, we discover new cases that, on the basis of review of the patient’s glucose values, actually qualified as cases in earlier time periods. This discrepancy results from the fact that potential cases

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