Index60 Identifies Individuals at Appreciable Risk for Stage 3 Among an Autoantibody-Positive Population With Normal 2-Hour Glucose Levels: Implications for Current Staging Criteria of Type 1 Diabetes

Objective: We assessed whether Index60, a composite measure of fasting C-peptide, 60-minute C-peptide, and 60-minute glucose, could improve the metabolic staging of type 1 diabetes for progression to clinical disease (stage 3) among autoantibody positive (Ab+) individuals with normal 2-hour glucose values (<140 mg/dL).

Research Design and Methods: We analyzed 3058 Type 1 Diabetes TrialNet Pathway to Prevention participants, with 2-hour glucose<140 mg/dL and Index60<1.00 values from baseline OGTTs. Characteristics associated with type 1 diabetes (younger age, greater autoantibody positivity [Ab+], higher HLA DR3-DQ2/DR4-DQ8 prevalence, lower C-peptide) were compared among four mutually exclusive groups: top 2-hour glucose quartile only [HI-2HGLU], top Index60 quartile only [HI-IND60], both top quartiles [HI-BOTH], neither top quartile [LO-BOTH]. Additionally, within the 2-hour glucose distribution of <140 mg/dL, and separately within the Index60<1.00 distribution, comparisons were made between those above or below the medians.

Results: HI-IND60 and HI-BOTH were younger, with greater frequency of >2 Ab+, and lower C-peptide levels than either HI-2HGLU or LO-BOTH (all p<0.001). The cumulative incidence for stage 3 was greater for HI-IND60 and HI-BOTH than either HI-2HGLU or LO-BOTH (all p<0.001). Those with Index60 values above the median were younger, had higher ≥2Ab+ (p<0.001) and DR3-DQ2/DR4-DQ8 prevalence (p<0.001), and lower AUC C-peptide levels (p<0.001) than those below. Those above the 2-hour glucose median had higher AUC C-peptide levels (p<0.001), but otherwise did not differ from those below.

Conclusion: Index60 identifies individuals with characteristics of type 1 diabetes at appreciable risk for progression who would otherwise be missed by 2-hour glucose staging criteria.