Methods and Limitations
We estimated the incidence of diagnosed diabetes using data from the National Health Interview Survey (NHIS) of the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention (CDC). Conducted continuously since 1957, NHIS is a health survey of the civilian, noninstitutionalized, household population of the United States. The survey provides information about the health of the U.S. population, including information about the prevalence and incidence of disease, the extent of disability, and the use of health care services. The multistage probability design of the survey has been described elsewhere.1,2 The NHIS was redesigned in 1997.3
Beginning in 1997, age of diagnosis of diabetes was not collected among persons aged 17 years and younger, and there were limited data available on age of diagnosis of diabetes among adults aged 80 years and older. Therefore, incidence was calculated only among adults aged 18–79 years. Estimates are presented by age, sex, and race/ethnicity. Persons of Hispanic origin may be of any race. The race groups include persons of both Hispanic and non-Hispanic origin. Three-year averages were used to improve the precision of the annual estimates. The 2000 U.S. population was used as the standard to conduct age-adjusting.
Each year, a one-sixth subsample of survey participants was asked whether anyone in the family had diabetes during the past 12 months. If a person was reported as having diabetes, he or she was asked when the condition was first noticed. The incidence was calculated based on the weighted number of adults who reported that they were diagnosed with diabetes within the past year, divided by the weighted estimate of the survey sample size, excluding adults who had been diagnosed with diabetes for more than 1 year.
Adult respondents were asked whether a health professional had ever told them they had diabetes. To exclude gestational diabetes, women were asked whether they had been told they had diabetes other than during pregnancy. Adults who reported being diagnosed with diabetes were then asked at what age they were diagnosed. We calculated the number of years each person had been diagnosed with diabetes by subtracting the age at which they were diagnosed from their current age. Adults who had a value of zero were identified as having been diagnosed with diabetes within the last year. In addition, half of the adults who had a value of one were classified as having been diagnosed with diabetes within the last year. To calculate incidence, the numerator was the weighted number of adults who were diagnosed with diabetes within the last year and the denominator was the weighted estimate of adult population, excluding adults who had been diagnosed with diabetes for more than one year and adults who answered "refused," "don’t know," or had missing values on the diabetes status question.
Approximately one-third of persons with diabetes are unaware they have diabetes because their diabetes has not been diagnosed.4 Therefore, the NHIS may underestimate the true incidence of diabetes. Because data are cross-sectional, it is impossible to determine if an increase of new cases is due to a true increase in disease incidence, improved case ascertainment, or a combination of these factors. Sample sizes for incidence cases of diagnosed diabetes in the NHIS are not large enough to generate reliable incidence rates for racial or sex-age groups before 1997.1
- Massey JT, Moore TF, Parsons VL, Tadros W. Design and estimation for the National Health Interview Survey, 1985–1994. National Center for Health Statistics. Vital Health Stat 1989;2 (110).
- Botman SL, Moore TF, Moriarity CL, Parsons VL. Design and estimation for the National Health Interview Survey, 1995–2004. National Center for Health Statistics. Vital Health Stat 2000;2(130).
- Blackwell DL, Tonthat L. Summary health statistics for the U.S. Population: National Health Interview Survey, 1997. Vital Health Stat Stat 2002;10(204).
- Cowie CC, Rust KF, Byrd-Holt DD, Eberhardt MS, Flegal KM, Engelgau MM, Saydah SH, Williams DE, Geiss LS, Gregg EW. Prevalence of diabetes and impaired fasting glucose in adults in the U.S. population: National Health And Nutrition Examination Survey 1999–2002. Diabetes Care 2006;29(6):1263–8.