Prediction of Impending Type 1 Diabetes through Automated Dual-Label Measurement of Proinsulin:C-Peptide Ratio

Abstract

Background: The hyperglycemic clamp test, the gold standard of beta cell function, predicts impending type 1 diabetes in islet autoantibody-positive individuals, but the latter may benefit from less invasive function tests such as the proinsulin:C-peptide ratio (PI:C). The present study aims to optimize precision of PI:C measurements by automating a dual-label trefoil-type time-resolved fluorescence immunoassay (TT-TRFIA), and to compare its diagnostic performance for predicting type 1 diabetes with that of clamp-derived C-peptide release.

Methods: Between-day imprecision (n = 20) and split-sample analysis (n = 95) were used to compare TT-TRFIA (AutoDelfia, Perkin-Elmer) with separate methods for proinsulin (in-house TRFIA) and C-peptide (Elecsys, Roche). High-risk multiple autoantibody-positive first-degree relatives (n = 49; age 5-39) were tested for fasting PI:C, HOMA2-IR and hyperglycemic clamp and followed for 20-57 months (interquartile range).

Results: TT-TRFIA values for proinsulin, C-peptide and PI:C correlated significantly (r2 = 0.96-0.99; P<0.001) with results obtained with separate methods. TT-TRFIA achieved better between-day %CV for PI:C at three different levels (4.5-7.1 vs 6.7-9.5 for separate methods). In high-risk relatives fasting PI:C was significantly and inversely correlated (rs = -0.596; P<0.001) with first-phase C-peptide release during clamp (also with second phase release, only available for age 12-39 years; n = 31), but only after normalization for HOMA2-IR. In ROC- and Cox regression analysis, HOMA2-IR-corrected PI:C predicted 2-year progression to diabetes equally well as clamp-derived C-peptide release.

Conclusions: The reproducibility of PI:C benefits from the automated simultaneous determination of both hormones. HOMA2-IR-corrected PI:C may serve as a minimally invasive alternative to the more tedious hyperglycemic clamp test.

Fig 1

Evolution of proinsulin (A), C-peptide (B), glucose (C) and PI:C ratio (D) during OGTT in relatives at high autoantibody-inferred risk; *P = 0.003; **P<0.001. The number of available samples tested is indicated above each time point.

Fig 2

Relation between fasting PI:C and AUC_5-10min C-peptide (A), fasting PI:C and HOMA2-IR (B) and fasting PI:C corrected for HOMA2-IR and AUC_5-10min C-peptide (C) in relatives at high autoantibody-inferred risk (HR) (IA-2A⁺ or ZnT8A⁺ plus ≥ 1 other autoantibody) (9). Filled triangles = progressors within 2 years (n = 10), open triangles = slow-/non-progressors (n = 39). AUC_5-10min C-peptide: first-phase AUC C-peptide release during hyperglycemic clamp test (min 5–10); HOMA2-IR: homeostatic model assessment for insulin resistance; r_s: Spearman's rank correlation coefficient; NS: not significant