Transdermal blood sampling for C-peptide is a minimally invasive, reliable alternative to venous sampling in children and adults with type 1 diabetes

Objective: C-peptide and islet autoantibodies are key type 1 diabetes biomarkers, typically requiring venous sampling, which limit their utility. We assessed transdermal capillary blood (TCB) collection as a practical alternative. Research Design and methods: Ninety-one individuals (71 type 1 diabetes, 20 controls; type 1 diabetes: aged median 14.8 years[interquartile range 9.1-17.1]; diabetes duration 4.0 years[1.5-7.7]; controls 42.2 years[38.0-52.1]) underwent contemporaneous venous and TCB sampling for measurement of plasma C-peptide. Type 1 diabetes participants also provided venous serum and plasma, and TCB plasma for measurement of autoantibodies to glutamate decarboxylase, islet antigen-2, and zinc transporter 8. The ability of TCB plasma to detect significant endogenous insulin secretion (venous C-peptide ≥200pmol/L) was compared along with agreement in levels using Bland-Altman. Venous serum was compared with venous and TCB plasma for detection of autoantibodies using established thresholds. Acceptability was assessed by age-appropriate questionnaire.Results: Transdermal sampling took a mean of 2.35minutes (SD 1.49). Median sample volume was 50 µl(IQR 40-50) with 3/91(3.3%) failures, and 13/88(14.7%) <35 µL). TCB C-peptide showed good agreement to venous plasma (mean venous ln(C-peptide) – TCB ln(C-peptide) = 0.008, 95% CI(-0.23, 0.29), with 100%(36/36) sensitivity/100%(50/50) specificity to detect venous C-peptide ≥ 200pmol/L. Where venous serum in multiple autoantibody positive TCB plasma agreed in 22/32 (sensitivity 69%), comparative specificity was 35/36 (97%). TCB was preferred to venous sampling (type 1 diabetes: 63% vs 7%; 30% undecided). Conclusions: Transdermal capillary testing for C-peptide is a sensitive, specific, and acceptable alternative to venous sampling, TCB sampling for islet autoantibodies needs further assessment.