Methods and Limitations
A1c levels among adults with diabetes are described using data from the National Health and Nutrition Examination Survey (NHANES). The NHANES is a nationally representative survey conducted by National Centers for Health Statistics, the Centers for Disease Control and Prevention. The NHANES program began in the early 1960s as a series of surveys. Since 1999, the survey became a continuous program. It collects information on the health and nutrition of the US noninstitutionalized civilian population. Participants were interviewed at home, and subsequently received a medical and laboratory examination in a mobile examination center. Survey instruments and physical examination and laboratory measurements are [http://www.cdc.gov/nchs/about/major/nhanes/datalink.htm].
Our study included adults aged 20 years or older with diagnosed diabetes with valid A1c values. Individuals were classified as having diagnosed diabetes based on the question of whether, other than during pregnancy, a doctor or health care professional had ever told them that they have diabetes. A1c was measured using whole blood at a central laboratory by a high-performance liquid chromatographic assay and standardized according to the method of the Diabetes Control and Complications Trial. Because there were changes to the equipment and laboratory that measured A1c and glucose in 2005–2006, values from 2005–2006 were converted via a linear transformation to make them comparable to values from NHANES 1988–1994 and NHANES 1999–20041. Percentages were age-adjusted using the 2000 U.S. Standard Population using age groups 20–44 years, 45–64 years, and 65 years or older. Data preparations were conducted using SAS 9.1.3 (SAS institute, Inc., Cary, North Carolina) and statistical analyses were performed using SUDAAN 9.0 (Research Triangle Institute, Research Triangle Park, North Carolina) to account for complex sample design of NHANES. We used t-tests to assess differences in means and rates between time periods or between subgroups. P-value < 0.05 was considered statistically significant.
Persons residing in nursing homes or other institutions are not included in this survey; therefore, these results cannot be generalized to those segments of the population. Due to limited sample size, we may not have enough statistical power to detect changes in A1c levels for some groups.
- Centers for Disease Control and Prevention National Center for Health Statistics: National Health and Nutrition Examination Survey 2005–2006. 2008. http://www.cdc.gov/nchs/nhanes/nhanes2005-2006/GHB_D.htm