Urine C-peptide creatinine ratio can be used to assess insulin resistance and insulin production in people without diabetes: an observational study

Objectives: The current assessment of insulin resistance (IR) in epidemiology studies relies on the blood measurement of C-peptide or insulin. A urine C-peptide creatinine ratio (UCPCR) can be posted from home unaided. It is validated against serum measures of the insulin in people with diabetes. We tested whether UCPCR could be a surrogate measure of IR by examining the correlation of UCPCR with serum insulin, C-peptide and HOMA2 (Homeostasis Model Assessment 2)-IR in participants without diabetes and with chronic kidney disease (CKD). Design: Observational study. Setting: Single-centre clinical research facility. Participants: 37 healthy volunteers and 30 patients with CKD (glomerular filtration rate 15-60) were recruited. Primary and secondary endpoints: Serum insulin, C-peptide and glucose at fasting (0), 30, 60, 90 and 120 min were measured during an oral glucose tolerance test (OGTT). Second-void fasting UCPCR and 120 min post-OGTT UCPCR were collected. HOMA2-IR was calculated using fasting insulin and glucose. The associations between UCPCR and serum measures were assessed using Spearman's correlations. Results: In healthy volunteers, fasting second-void UCPCR strongly correlated with serum insulin (r(s)=0.69, p<0.0001), C-peptide (r(s)=0.73, p<0.0001) and HOMA2-IR (r(s)=-0.69, p<0.0001). 120 min post-OGTT UCPCR correlated strongly with C-peptide and insulin area under the curve. In patients with CKD, UCPCR did not correlate with serum C-peptide, insulin or HOMA2-IR. Conclusions: In participants with normal renal function, UCPCR may be a simple, practical method for the assessment of IR in epidemiology studies.