Each protocol was approved by a human research ethics committee and was carried out in accordance with the Declaration of Helsinki as revised in 2008.
The TrialNet TN-01 Pathway to Prevention Study (NCT00097292) is an islet autoantibody screening and metabolic monitoring programme that has operated in North America, Europe, Australia and New Zealand since 2004. Individuals aged up to 45 years with a first-degree relative with type 1 diabetes and those aged up to 20 years with a first/second/third-degree relative with type 1 diabetes are screened for IAA, GADA, IA-2A and ZnT8A. TN-01 data current to December 2019 were downloaded in January 2020 and erroneous outlier values removed. Eligibility required fasting glucose <7.0 mmol/l (126 mg/dl), 120 min glucose <11.1 mmol/l (200 mg/dl), HbA1c <48 mmol/mol (6.5%), age at least 2 years and BMI between 12 and 40 kg/m2. To enter the TrialNet multiple-autoantibody training population, participants needed complete data for the input measures of interest (electronic supplementary material [ESM] Table 1) and had an OGTT either at the same time that they first tested positive to multiple autoantibodies or at their next study visit (median [Q1, Q3] time between screening and OGTT 1.8 [0.3, 3.0] months). The TrialNet multiple-autoantibody validation population comprised individuals who met the same glucose, HbA1c, age and BMI criteria, and who had all measures required to calculate the DPTRS and the newer risk scores. These participants were not included in the training population because they underwent OGTT testing two or more visits after screening positive to multiple autoantibodies, lacked data for HLA genotype or did not have data for ZnT8A, which was only introduced into TrialNet in 2012. The median [Q1, Q3] time between screening and OGTT in the training population was 3.2 [1.5, 9.8] months. The TrialNet single-antibody population comprised 612 participants who tested positive to only one autoantibody and who underwent an OGTT that returned normal or impaired glucose tolerance and HbA1c <48 mmol/mol (6.5%).
Data for the DPT-1 (NCT00004984) and The Environmental Determinants of Diabetes in the Young (TEDDY) study (NCT00279318) were obtained from the National Institute of Diabetes and Digestive and Kidney Diseases data repository in March and April 2020. DPT-1 recruited relatives with stage 1 or 2 type 1 diabetes between 1994 and 2003 and showed that neither parenteral nor oral insulin delayed progression to stage 3 [21, 22]. DPT-1 participants were positive for islet cell antibodies by indirect immunofluorescence assay and negative for the protective HLA-DQA1*01:02-DQB1*06:02 haplotype. Some assays for IAA were performed during the study whereas other IAA measurements, and all GADA and IA-2A measurements, were performed retrospectively on stored samples. TEDDY is a birth cohort study that enrolled 8668 North American and European newborns whose HLA genotype or family history conferred an increased risk of type 1 diabetes . Data for multiple-autoantibody-positive children who had undergone a limited OGTT (blood sampling at 120 min) were extracted. For both DPT-1 and TEDDY, participants who had 120 min glucose of 11.1 mmol/l (200 mg/dl) or more, those who were missing data needed to calculate risk scores and those who had not been followed beyond their first OGTT were excluded.
The Fr1da study (NCT04039945) enrolled children aged 2 to 6 years from the general Bavarian population . Children who screened positive for two or more islet autoantibodies were invited to undergo an OGTT with blood sampling at 0, 30, 60, 90 and 120 min. Participants with missing results for BMI, HbA1c and IA-2A were excluded. Data were current to March 2020.
Stage 2 type 1 diabetes was defined as a fasting glucose of 5.6 to 7.0 mmol/l (100 to 125 mg/dl), a glucose at 30–90 min greater than 11.1 mol/l (200 mg/dl), a 120 min glucose of 7.8 to 11.1 mmol/l (140 to 199 mg/dl) and/or HbA1c of 39 to 46 mmol/mol (5.7% to 6.4%), inclusive . Stage 3 type 1 diabetes was defined using ADA criteria for diabetes mellitus . The dose of glucose used in OGTTs was 1.75 g/kg to a maximum of 75 g. C-peptide was measured by radioimmunoassay in DPT-1 and for other studies using the TOSOH autoanalyser (TOSOH, South San Francisco, CA, USA). In TrialNet, DPT-1 and TEDDY, HbA1c was measured using ion-exchange high-performance liquid chromatography on TOSOH autoanalysers and standardised using the Diabetes Control and Complications Trial reference method. HbA1c measurements for Fr1da were performed at the participant’s local clinical laboratory.
The glmulti (v1.0.8)  and survival (v3.1-12)  packages of R software (v3.6.3; www.r-project.org) were used to build all possible single OGTT time point Cox proportional hazards regression models to predict progression from stage 1/2 to stage 3 type 1 diabetes using all possible combinations of the inputs listed in ESM Table 1. Models were then ranked by their Akaike’s information criterion (AIC) score. For each OGTT time point, the simplest model that was within 2 AIC units of the model with the lowest AIC score was selected for further testing. Coefficients for these models, named M0, M30, M60, M90 and M120, are presented in ESM Table 2. Model calibration testing was performed with the Greenwood–D’Agostino–Nam test using the GND.calib R function , where deciles with few events were integrated into the next decile, as appropriate, and p > 0.05 considered no evidence of poor fit.
Equations for the DPTRS, DPTRS60, Index60 and M120 risk tools are provided below, where the units for BMI, age, glucose, C-peptide and HbA1c are, respectively, kg/m2, years, mg/dl, ng/ml and percentage units. Sex was assigned a score of 1 for male and 2 for female, and IA-2A status assigned 0 for absent and 1 for present. Glucose is converted from mmol/l to mg/dl by multiplying by 18; C-peptide is converted from nmol/l to ng/ml by dividing by 3.00; and HbA1c is converted from mmol/mol to percentage units by adding 23.5 and then dividing by 10.93.
DPTRS = 1.569×loge(BMI) − 0.056×(age) + 0.00813×(sum of glucose from 30 to 120 min) − 0.0848×(sum of C-peptide from 30 to 120 min) + 0.476×loge(fasting C-peptide) 
DPTRS60 = 1.364×loge(BMI) − 0.065×(age) + 0.465×loge(fasting C-peptide) + 0.019×(60 min glucose) − 0.311×(60 min C-peptide) 
Index60 = 0.3695×loge(fasting C-peptide) + 0.0165×(60 min glucose) − 0.3644×(60 min C-peptide) 
M120 = 0.448×(sex) + 0.631×(IA-2A) − 0.0302×(age) + 0.0605×(BMI) + 1.380×(HbA1c) + 0.0265×(120 min glucose) − 0.191×(120 min C-peptide)
Prism software (v8.3.1g for Mac; GraphPad, San Diego, CA, USA) was used to perform Mann–Whitney tests for inter-group comparisons, to chart Kaplan–Meier survival curves of groups above and below the median value, and to compare the curves using the logrank (Mantel–Cox) test. AUC analysis of receiver operating characteristic (ROC) plots and comparisons of different prediction models were performed using the pROC package in R . Calculations for sensitivity (TP/[TP + FN]), specificity (TN/[TN + FP]) and accuracy ([TP + TN]/N) used the median value as the risk threshold, where TP, TN, FP, FN and N are true-positives, true-negatives, false-positives, false-negatives and total number of participants, respectively.
Statement of informed consent
Informed consent was obtained from all individual participants and, for children, their parents or legal guardians.