title: Stability and reproducibility of a single-sample urinary C-peptide/creatinine ratio and its correlation with 24-h urinary C-peptide.
abstract: BACKGROUND
  C-peptide measurement in blood or 24-h urine samples provides useful information regarding endogenous insulin secretion, but problems related to the rapid degradation of C-peptide in blood and difficulty of 24-h urine collection have limited widespread routine clinical use of this test. We assessed the feasibility of measuring urinary C-peptide (UCP) with correction for creatinine concentration in single urine samples.
  
  
  METHODS
  We analyzed UCP using a routine electrochemiluminescence immunoassay in samples from 21 healthy volunteers. We investigated the stability of UCP with different preservatives and storage conditions and compared the reproducibility of urinary C-peptide/creatinine ratio (UCPCR) in first- and second-void fasting urines, then assessed correlations with 24-h collections.
  
  
  RESULTS
  UCPCR was unchanged at room temperature for 24 h and at 4 degrees C for 72 h even in the absence of preservative. UCPCR collected in boric acid was stable at room temperature for 72 h. UCPCR remained stable after 7 freeze-thaw cycles but decreased with freezer storage time and dropped to 82%-84% of baseline by 90 days at -20 degrees C. Second-void fasting UCPCRs were lower than first-void (median 0.78 vs 1.31, P = 0.0003) and showed less variation (CV 33% vs 52%), as second-void UCPCRs were not influenced by evening food-related insulin secretion. Second-void fasting UCPCR was highly correlated with 24-h UCP (r = 0.8, P = 0.00006).
  
  
  CONCLUSIONS
  Second-void fasting UCPCR is a reproducible measure that correlates well with 24-h UCP in normal samples. The 3-day stability of UCPCR at room temperature greatly increases its potential clinical utility.
authors.affiliation: Peninsula NIHR Clinical Research Facility, Peninsula Medical School, Exeter EX2 5DW, UK.
authors.forename: Tim J
authors.initials: TJ
authors.lastname: McDonald
authors.forename: Bridget A
authors.initials: BA
authors.lastname: Knight
authors.forename: Beverley M
authors.initials: BM
authors.lastname: Shields
authors.forename: Pamela
authors.initials: P
authors.lastname: Bowman
authors.forename: Maurice B
authors.initials: MB
authors.lastname: Salzmann
authors.forename: Andrew T
authors.initials: AT
authors.lastname: Hattersley
mesh_terms: D000328:Adult; D002096:C-Peptide / Q000652:urine*; D003404:Creatinine / Q000652:urine*; D005215:Fasting; D005260:Female; D005615:Freezing; D006801:Humans; D007118:Immunoassay; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D013696:Temperature; D013997:Time Factors; D055815:Young Adult
chemical_list: D002096:C-Peptide; D003404:Creatinine