title: Urine C-peptide creatinine ratio is an alternative to stimulated serum C-peptide measurement in late-onset, insulin-treated diabetes.
abstract: OBJECTIVE
  Serum C-peptide measurement can assist clinical management of diabetes, but practicalities of collection limit widespread use. Urine C-peptide creatinine ratio may be a non-invasive practical alternative. The stability of C-peptide in urine allows outpatient or community testing. We aimed to assess how urine C-peptide creatinine ratio compared with serum C-peptide measurement during a mixed-meal tolerance test in individuals with late-onset, insulin-treated diabetes.
  
  
  METHODS
  We correlated the gold standard of a stimulated serum C-peptide in a mixed-meal tolerance test with fasting and stimulated (mixed-meal tolerance test, standard home meal and largest home meal) urine C-peptide creatinine ratio in 51 subjects with insulin-treated diabetes (diagnosis after age 30 years, median age 66 years, median age at diagnosis 54, 42 with Type 2 diabetes, estimated glomerular filtration rate > 60 ml min(-1) 1.73 m(-2) ).
  
  
  RESULTS
  Ninety-minute mixed-meal tolerance test serum C-peptide is correlated with mixed-meal tolerance test-stimulated urine C-peptide creatinine ratio (r = 0.82), urine C-peptide creatinine ratio after a standard breakfast at home (r = 0.73) and urine C-peptide creatinine ratio after largest home meal (r = 0.71). A stimulated (largest home meal) urine C-peptide creatinine ratio cut-off of 0.3 nmol/mmol had a 100% sensitivity and 96% specificity (area under receiver operating characteristic curve = 0.99) in identifying subjects without clinically significant endogenous insulin secretion (mixed-meal tolerance test-stimulated C-peptide < 0.2 nmol/l). In detecting a proposed serum C-peptide threshold for insulin requirement (stimulated serum C-peptide < 0.6 nmol/l), a stimulated (largest home meal) urine C-peptide creatinine ratio cut-off of 0.6 nmol/mmol had a sensitivity and specificity of 92%.
  
  
  CONCLUSIONS
  In patients with insulin-treated diabetes diagnosed after age 30 years, urine C-peptide creatinine ratio is well correlated with serum C-peptide and may provide a practical alternative measure to detect insulin deficiency for use in routine clinical practice.
authors.affiliation: Peninsula NIHR Clinical Research Facility, Peninsula Medical School, University of Exeter, Exeter, UK.
authors.forename: A G
authors.initials: AG
authors.lastname: Jones
authors.forename: R E J
authors.initials: RE
authors.lastname: Besser
authors.forename: T J
authors.initials: TJ
authors.lastname: McDonald
authors.forename: B M
authors.initials: BM
authors.lastname: Shields
authors.forename: S V
authors.initials: SV
authors.lastname: Hope
authors.forename: P
authors.initials: P
authors.lastname: Bowman
authors.forename: R A
authors.initials: RA
authors.lastname: Oram
authors.forename: B A
authors.initials: BA
authors.lastname: Knight
authors.forename: A T
authors.initials: AT
authors.lastname: Hattersley
mesh_terms: D017668:Age of Onset; D000368:Aged; D002096:C-Peptide / Q000097:blood / Q000652:urine*; D003404:Creatinine / Q000652:urine*; D003922:Diabetes Mellitus, Type 1 / Q000097:blood / Q000652:urine*; D003924:Diabetes Mellitus, Type 2 / Q000097:blood / Q000652:urine*; D005215:Fasting; D005260:Female; D005934:Glucagon / Q000097:blood / Q000652:urine*; D005951:Glucose Tolerance Test; D006442:Glycated Hemoglobin / Q000652:urine*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012680:Sensitivity and Specificity
chemical_list: D002096:C-Peptide; D003404:Creatinine; D005934:Glucagon; D006442:Glycated Hemoglobin; C517652:hemoglobin A1c protein, human