title: Serum C-peptide concentrations poorly phenotype type 2 diabetic end-stage renal disease patients.
abstract: BACKGROUND
  A homogeneous patient population is necessary to identify genetic factors that regulate complex disease pathogenesis. In this study, we evaluated clinical and biochemical phenotyping criteria for type 2 diabetes in end-stage renal disease (ESRD) probands of families in which nephropathy is clustered. C-peptide concentrations accurately discriminate type 1 from type 2 diabetic patients with normal renal function, but have not been extensively evaluated in ESRD patients. We hypothesized that C-peptide concentrations may not accurately reflect insulin synthesis in ESRD subjects, since the kidney is the major site of C-peptide catabolism and would poorly correlate with accepted clinical criteria used to classify diabetics as types 1 and 2.
  
  
  METHODS
  Consenting diabetic ESRD patients (N = 341) from northeastern Ohio were enrolled. Clinical history was obtained by questionnaire, and predialysis blood samples were collected for C-peptide levels from subjects with at least one living diabetic sibling (N = 127, 48% males, 59% African Americans).
  
  
  RESULTS
  Using clinical criteria, 79% of the study population were categorized as type 1 (10%) or type 2 diabetics (69%), while 21% of diabetic ESRD patients could not be classified. In contrast, 98% of the patients were classified as type 2 diabetics when stratified by C-peptide concentrations using criteria derived from the Diabetes Control and Complications Trial Research Group (DCCT) and UREMIDIAB studies. Categorization was concordant in only 70% of ESRD probands when C-peptide concentration and clinical classification algorithms were compared. Using clinical phenotyping criteria as the standard for comparison, C-peptide concentrations classified diabetic ESRD patients with 100% sensitivity, but only 5% specificity. The mean C-peptide concentrations were similar in diabetic ESRD patients (3.2 +/- 1.9 nmol/L) and nondiabetic ESRD subjects (3.5 +/- 1.7 nmol/L, N = 30, P = NS), but were 2.5-fold higher compared with diabetic siblings (1.3 +/- 0.7 nmol/L, N = 30, P < 0.05) with normal renal function and were indistinguishable between type 1 and type 2 diabetics. Although 10% of the diabetic ESRD study population was classified as type 1 diabetics using clinical criteria, only 1.5% of these patients had C-peptide levels less than 0.20 nmol/L, the standard cut-off used to discriminate type 1 from type 2 diabetes in patients with normal renal function. However, the criteria of C-peptide concentrations> 0.50 nmol/L and diabetes onset in patients who are more than 38 years old identify type 2 diabetes with a 97% positive predictive value in our ESRD population.
  
  
  CONCLUSIONS
  Accepted clinical criteria, used to discriminate type 1 and type 2 diabetes, failed to classify a significant proportion of diabetic ESRD patients. In contrast to previous reports, C-peptide levels were elevated in the majority of type 1 ESRD diabetic patients and did not improve the power of clinical parameters to separate them from type 2 diabetic or nondiabetic ESRD subjects. Accurate classification of diabetic ESRD patients for genetic epidemiological studies requires both clinical and biochemical criteria, which may differ from norms used in diabetic populations with normal renal function.
authors.affiliation: Departments of Medicine, Physiology and Biophysics, and Epidemiology and Biostatistics, School of Medicine, Case Western Reserve University, Cleveland, OH 44109-1998, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Covic
authors.forename: J R
authors.initials: JR
authors.lastname: Schelling
authors.forename: M
authors.initials: M
authors.lastname: Constantiner
authors.forename: S K
authors.initials: SK
authors.lastname: Iyengar
authors.forename: J R
authors.initials: JR
authors.lastname: Sedor
mesh_terms: D000328:Adult; D017668:Age of Onset; D000465:Algorithms; D002096:C-Peptide / Q000097:blood*; D003922:Diabetes Mellitus, Type 1 / Q000097:blood / Q000235:genetics; D003924:Diabetes Mellitus, Type 2 / Q000097:blood* / Q000235:genetics*; D003928:Diabetic Nephropathies / Q000097:blood / Q000145:classification / Q000235:genetics; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007676:Kidney Failure, Chronic / Q000097:blood* / Q000145:classification / Q000235:genetics*; D008297:Male; D008875:Middle Aged; D010641:Phenotype; D011237:Predictive Value of Tests; D012680:Sensitivity and Specificity
chemical_list: D002096:C-Peptide; D007328:Insulin