The average age at diagnosis was 9.0 years: in 16% (n = 434) of the individuals diabetes was diagnosed when they were <5 years of age, 38% (n = 1,057) were between ages 5 and 10, 37% (n = 1,028) were between ages 11 and 15, and 10% (n = 270) were between 15 and 19 years of age at diagnosis. Average duration of diabetes was 3 years, with 33% (n = 925) having duration of <1 year, 24% (n = 661) having duration of 1–2 years, and 43% (n = 1,202) having duration of ≥years.
Among individuals with duration of diabetes of <1 year, 82.9% had a fasting C-peptide level ≥0.23 ng/ml, 31.2% had fasting C-peptide at or above the 5th percentile of the NHANES population and 7.2% had fasting C-peptide at or above the NHANES 50th percentile level. As expected, the proportion of individuals with preserved C-peptide diminished with increasing disease duration. This finding is illustrated in , which shows the proportion with preserved C-peptide in quarterly intervals from time from diagnosis. Among those with duration of ≥5 years, 10.7% had preserved C-peptide at the DCCT cut point, 1.0% at NHANES 5th percentile, and 0% at NHANES 50th percentile.
Figure 1 Percentage of participants with preserved C-peptide by duration of diabetes in 3-month intervals according to the DCCT definition (fasting C-peptide <0.23 ng/ml) (♦), NHANES 5th percentile definition (fasting C-peptide <1.0 ng/ml) (more ...)
We then explored various characteristics of participants with and without preserved fasting C-peptide (). In the unadjusted analysis, the proportion of non-Hispanic whites who had preserved fasting C-peptide was smaller than that of other race/ethnicities. More individuals with preserved C-peptide had two antibodies and a significantly higher percentage of HLA DR15. In addition, those with preserved fasting C-peptide were older at diagnosis, had a shorter duration of diabetes, and had a lower A1C at the time of the study visit. Those with C-peptide above the NHANES 5th percentile also had a higher BMI Z score than those with C-peptide below this level.
Characteristics of 2,789 SEARCH antibody-positive participants by preserved β-cell function status as defined by fasting C-peptide levels
presents the results of the multiple logistic regression models with the dichotomous outcome of preserved fasting C-peptide status based on the DCCT definition stratified by diabetes duration. Variables found to be independently associated with preserved β-cell function in all the models included older age at diagnosis and lower A1C at time of study visit. Race/ethnicity other than non-Hispanic white was also significant for those participants with diabetes duration of 1–2 years and ≥2 years; higher current BMI Z score was also significant only for those participants with duration of 1–2 years ().
Figure 2 Odds ratios for having preserved C-peptide according to the DCCT definition for individuals <1 year (), 1–2 years (■), and >2 years (●) from diagnosis. Incremental units are 1 mg/dl glucose, age 1 year, (more ...)
Among those with preserved fasting C-peptide (DCCT definition), older age at diagnosis, higher current BMI Z score, race/ethnicity other than non-Hispanic white, female sex, lower A1C, and shorter duration of diabetes were independently related to higher fasting C-peptide levels. The association of age at diagnosis and diabetes duration with fasting C-peptide is illustrated in , demonstrating that lower fasting C-peptide is seen in participants with diagnosis of diabetes at a younger age and those with longer duration of disease. The relationship between weight categories at the time of the study visit and prevalence of preserved β-cell function at various times from diagnosis is shown in . Although a greater percentage of obese/overweight compared with underweight subjects had preserved function, the association was significant (P < 0.0001) only in those with duration of diabetes of <2 years (P = 0.48, 0.41, and 0.58 for 2–4, 4–6, and >6 years from diagnosis, respectively).
Fasting C-peptide by duration and age at diagnosis among those with preserved C-peptide by DCCT definition. Cells with less than five subjects are not reported.
Figure 4 Percentage of participants with preserved C-peptide by duration of diabetes according to DCCT definition (fasting C-peptide <0.23 ng/ml) stratified by BMI classification (black bar, obese; dark gray bar, overweight; light gray bar, normal weight; (more ...)