Methods and Limitations

Methodology

We estimated the age distribution at diagnosis of diabetes and the mean and median age at diagnosis of 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 on the health of the United States population, including information on 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

Age at diagnosis of diabetes was calculated among incident cases for adults aged 18–79 years, because age at diagnosis of diabetes was not collected among persons aged 17 years and younger and there were limited data available on age of diagnosis among adults aged 80 years and older from 1997.

Medians of age at diagnosis are presented by 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. Boxplots were constructed to display the distribution of age at diagnosis among incident cases for each year. 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.

1980–1996

Each year, a one-sixth sub-sample 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. For participants reporting that they had been diagnosed in the past 12 months, their actual ages were used to estimate age at diagnosis. Assuming dates of birth are normally distributed throughout the year, half of those reporting that they had been diagnosed in the past 12 months may have been a year younger than their current reported age. To eliminate this possible bias, 6 months were subtracted from each age.

1997–2008

Adult respondents are asked whether a health professional had ever told them they had diabetes. To exclude gestational diabetes, women are 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.

Data Limitations

From 1980 to 1996, there was no "age at diagnosis" variable available; therefore, we used respondents’ actual ages to estimate age at diagnosis. Hispanic information was only available from 1997. Sample sizes for incident cases of diagnosed diabetes in the NHIS are not large enough to generate reliable median age at diagnosis of diabetes for race-sex groups before 1997.

References

1. Botman SL, Moore TF, Moriarity CL, Parsons VL. Design and estimation for the National Health Interview Survey, 1995–2004. National Center for Health Statistics, CDC. Vital and Health Statistics 2000;2(13).