We observed from birth 7,410 children for positivity for DAAs and progression to type 1 diabetes over a median follow-up time of 9.2 years (range 0.9–14.2). The median follow-up time for subjects remaining unaffected by type 1 diabetes was 9.3 years (5.4–14.2), and the median age at diagnosis among 180 progressors (93 male subjects [2.4%]) was 5.0 years (0.9–12.5). The median age at the initial seroconversion was 4.2 years (0.2–13.7) among unaffected ICA-positive subjects, whereas among progressors, it was 1.5 years (0.3–9.6; P < 0.001). Progressors reached the maximal ICA-based autoantibody status by the median age of 2.2 years (0.5–10.1), while in unaffected subjects, the maximal autoantibody status was observed at the age of 5.1 years (0.5–13.7; P < 0.001). Delay from the initial seroconversion to maximal autoantibody positivity varied between 0.0 and 11.0 years, and among progressors and unaffected subjects, the median delays were 0.5 and 0.0 years (P < 0.001), respectively. The high-risk genotype (DQB1*02/0302) was carried by 1,575 children (21.3%) and the moderate-risk genotypes (DQB1*0302/x; x ≠ *02 or a protective allele) by 5,835 children (78.7%). The high-risk genotype was associated with higher risks for developing β-cell autoimmunity, positivity for multiple autoantibodies, persistent positivity, and type 1 diabetes (P < 0.001 for all comparisons) (supplementary Table 1). The corresponding ORs for the high- versus moderate-risk groups were 1.7 (95% CI 1.4–2.0), 2.4 (1.9–3.1), 1.7 (1.4–2.0), and 3.2 (2.1–4.8), respectively.
Altogether, 1,173 subjects (15.8%), 155 of whom were progressors, tested positive for ICAs during the follow-up. A majority (n = 967; 82.4%) of the first autoantibody-positive samples were ICA positive, while positivity for either ICAs or IAAs was seen in 93.3% (1,094 of 1,173) of the initially positive samples. Seventeen (68.0%) of 25 progressors who had remained ICA-negative during the pre-diabetic phase were positive for IAAs either before or at the time of the diagnosis (), and, in all, among the ICA-positive children, those who tested positive also for IAAs had a higher cumulative disease risk (59.6% [95% CI 49.9–69.3]) than those who remained IAA negative (10.8% [6.8–14.9]; P < 0.001). Twelve of 15 (80%) progressors without any signs of pre-diabetic β-cell autoimmunity did not adhere to the follow-up schedule of the DIPP study. Among these subjects, the median delay from the last sampling to diagnosis was 3.8 years (range 1.9–6.2). All previously seronegative progressors having samples available at diagnosis had developed β-cell autoimmunity by that time, and all but one tested positive for multiple autoantibodies.
Progressors remaining ICA negative during the pre-diabetic phase
The age at which subjects seroconverted had a predictive role regarding the risk of type 1 diabetes. Among all ICA-positive subjects, those with seroconversion before the age of 2 years had the highest cumulative disease risk (36.9% [95% CI 28.5–45.3]) (A), and for that age-group, the OR for type 1 diabetes was 5.0 (95% CI 3.5–7.1) when compared with those who had seroconverted after the age of 2 years. Subjects seroconverting under the age of 2 years were also more often positive for multiple autoantibodies at first positive sampling (18.3 vs. 12.1% in subjects with seroconversion at or after the age of 2 years, P = 0.006). The median delay from the initial seroconversion to diagnosis was 2.8 years (range 0.02–10.9) among the ICA-positive progressors, and this delay did not correlate with the age at seroconversion (rs = 0.005, P = 0.95).
Effect of the seroconversion age on the diabetes-free survival (A) and progression to type 1 diabetes in relation to initial ICA titer (JDFU) (B). *P < 0.001; †P = 0.003; ‡P = 0.006. T1D, type 1 diabetes.
The median ICA level in the first ICA-positive samples was 5 JDFU (range 3–640) among nonprogressors and 15 JDFU (4–668) (P < 0.001) in progressors. The higher the initial ICA value, the higher the cumulative disease risk (B). The 5-year progression rate for those with a low initial ICA level (<10 JDFU) was 5.7% (95% CI 3.9–7.5), while the corresponding values for those with moderate (10–19 JDFU) and high (≥20 JDFU) ICA titers were 31.8% (21.8–41.8) and 61.2% (51.1–71.9), respectively. During the follow-up, the difference in ICA levels became more prominent between progressors and nonprogressors: the peak ICA titer among progressors reached 168 JDFU (range 5–2,620), while it remained at 5 JDFU (3–2,620) in nonprogressors (P < 0.001). Prospective observations from the time point at which the maximal ICA level was observed showed a 5-year cumulative disease risk of 2.3% (95% CI 0.3–4.3) among those with a low ICA (<10 JDFU) level at that time, whereas among those with moderate (10–19 JDFU) and high (≥20 JDFU) ICA levels, the risk estimates were 11.7% (2.9–20.5) and 76.5% (61.4–91.6) (P < 0.001 between all groups), respectively. The maximal ICA level correlated clearly with the number of detectable autoantibodies at sampling (rs = 0.68, P < 0.001), but the correlation between the ICA titer and type 1 diabetes remained significant, even after correcting for the number of positive autoantibodies (rs = 0.10, P < 0.001).
To further analyze the predictive role of ICAs in combination with the other three autoantibodies, ICA-positive subjects were categorized by their maximal autoantibody status. Frequencies and predictive characteristics (sensitivity, specificity, PPV, NPV, +LR, −LR, and cumulative disease risks) of the autoantibody combinations are presented in and and . Positivity for four antibodies was associated with the highest disease sensitivity (54.4%) and NPV (98.9%) and the lowest −LR (0.5). The combination of persistent ICA and IAA positivity resulted in the highest PPV (91.7%), +LR (441.8), cumulative disease risk (100%), and specificity (100%). The highest cumulative disease risks were associated with IAAs (B and C), whereas GADA positivity resulted in significantly lower disease risks. Especially, the combination of ICAs and GADAs resulted in a low progression rate, thus decreasing also the risk estimate of double positivity. Transient and persistent ICA positivity had different predictive characters, since only 1.7% (95% CI 0–4.0) of those with transient ICA-based positivity developed type 1 diabetes during follow-up, whereas among those with (at least) pICA positivity, the proportion was 41.2% (34.1–48.3) (P < 0.001). In the whole study population, the difference between the seronegative and transiently ICA-positive subjects remained small but significant (0.5% [95% CI 0.3–0.7] vs. 1.7%) (P = 0.008). The pICA-based combinations of multiple persistently positive autoantibodies had highly variable disease risks, the highest associating with the combination of pICAs and pIAAs (100%) and the lowest with pICAs and pGADAs (11.2%). Persistent IAA positivity seemed to distinguish those with a high disease risk and rapid progression to type 1 diabetes from those with a lower disease risk and slower progression rate. In this population, the pIAA positivity–associated 5-year disease risk was 70.7% (95% CI 62.2–79.1) compared with that of 11.9% (8.4–15.4) observed among the ICA-positive subjects lacking pIAA positivity. Since all children had been observed for at least 5 years, the 5-year predictive characteristics are presented in supplementary Table 2. The predictive characteristics in relation to HLA genotype are shown in supplementary Table 3, while the corresponding characteristics of those 7,077 children who had no affected first-degree relative are presented in supplementary Table 4.
Predictive characteristics (sensitivity, specificity, PPV, NPV, +LR, −LR, and cumulative disease risk) of the four diabetes-associated autoantibodies (DAAs, ICAs, IAAs, and GADAs, and IA-2A)
Progression to type 1 diabetes (T1D) in relation to number of positive autoantibodies (A) and combinations of double (B) and triple (C) positivity further defined. *P < 0.001; †P = 0.02; ‡P = 0.04.
FIG. 3. Progression to type 1 diabetes (T1D) in relation to number of persistently positive autoantibodies (A) and combinations of persistent double (B) and triple (C) positivity further defined. p, persistent positivity. *P < 0.001; †P = 0.006; (more ...)