Data come from the SEARCH for Diabetes in Youth study (17
). SEARCH is a multicenter population-based ascertainment of youth with diabetes beginning in 2001 and continuing through the present. SEARCH sought to identify all cases of nongestational diabetes in youth <20 years of age in 2001 and all newly diagnosed cases of nongestational diabetes in youth <20 years of age in subsequent calendar years.
SEARCH has six centers, with diabetes cases being identified in geographically defined populations in Ohio, Washington, South Carolina, and Colorado, among health plan enrollees in Hawaii and southern California, among members of the Gila River Indian Community participating in the National Institute of Diabetes and Digestive and Kidney Diseases Pima Indian Diabetes Study, and among health service beneficiary rolls in three other Native American populations. Cases of diabetes were considered valid if they were diagnosed by a health care provider.
The study was reviewed and approved by local institutional review boards with jurisdiction over local study populations. Following Health Insurance Portability and Accountability Act privacy rule compliant procedures, youth with diabetes identified by the SEARCH recruiting network were asked to complete a short survey that collected information on date of birth, date of diabetes diagnosis, age at diagnosis, race/ethnicity, and other items to confirm eligibility. Youth completing this survey, excluding youth whose diabetes was due to other conditions, were invited to a study visit. Before the visit, written informed consent was obtained according to guidelines established by the local institutional review board from subjects aged ≥18 years or from a subject's parent or guardian if the subject was aged <18 years. Written assent was also obtained from subjects aged <18 years, as governed by local institutional review board instructions. Survey instruments were translated into Spanish, and interviews and surveys were administered in Spanish if that was the subject's preferred language.
Diabetes type for SEARCH participants was based on the clinical diagnosis by health care providers, as reported to SEARCH. This information was collected at the time of the case ascertainment or validation from medical records or direct provider reports. Age at diagnosis of diabetes was ascertained by parental report or self-report among subjects aged ≥18 years.
During the study visit, height and weight were measured using a standardized protocol, and survey data, including family history of diabetes, were collected. A BMI z
score was calculated using growth charts with an SAS program available from the Centers for Disease Control and Prevention (18
Data were collected on the presence or absence of a history of diabetes for each biological parent and, if reported, the parent's age at which diabetes was diagnosed. Information on the parents’ diabetes type was not obtained. Study participants with a parental history of diabetes were categorized by the timing of that parent's diagnosis relative to the subject's birth, i.e., parental diabetes diagnosed before the child's birth, after the child's birth, or, if uncertain, indeterminate.
This article includes data from SEARCH participants who completed the baseline study visit and had type 1or type 2 diabetes diagnosed in calendar years 2001 through 2005. Of the 11,181 subjects with registered cases of diabetes, 4,509 participated in the study visit and had data available on family history of diabetes. Subjects whose diabetes type was missing or reported as other than type 1 or type 2 and those with diabetes diagnosed before 2001 were excluded. Two participants were excluded for missing date of birth, 43 for diabetes types other than type 1 or type 2, and 1,791 because they had diabetes diagnosed before 2001, resulting in a sample size of 2,673 subjects (2,342 with type 1 and 331 with type 2 diabetes). Nonparticipation was associated with older age, type 2 diabetes, and African American race (19
). Subsequent analyses were further restricted to the subset of subjects who have at least one parent with diabetes.
All analyses were performed using SAS (version 9.1; SAS Institute, Cary, NC). To test the significance of differences between sexes within each diabetes type, t tests were used for continuous variables (child's age at diagnosis of diabetes and BMI z scores) and χ2 analyses were used for categorical variables (race/ethnicity, history of maternal diabetes, history of paternal diabetes, and history of both parents with diabetes). Age of diagnosis for both type 1 and type 2 diabetes was examined within the three different timing categories of maternal and paternal diabetes diagnosis (before or after the child's birth or indeterminate).
A multivariate regression model was fitted for age of child's diabetes diagnosis for each diabetes type using PROC GLM (general linear model), the SAS procedure for analysis of variance. For this model, data were restricted to participants whose mothers also had diabetes either before or after the child's birth. The model was adjusted for timing of maternal diabetes (before birth or after birth), mother's age of diabetes diagnosis (as a continuous variable), history of paternal diabetes (yes or no), sex, and race/ethnicity (non-Hispanic White or racial/ethnic minority). The model was checked for linearity and potential influence of outliers and found to meet the assumptions of a linear regression. Results were considered to be significant at P < 0.05.