[Concentration of C-peptide in correlation to kidney function (author's transl)]

Outcomes after simultaneous pancreas and kidney transplantation and the discriminative ability of the C-peptide measurement pretransplant among type 1 and type 2 diabetes mellitus

BACKGROUND

Earlier studies reporting outcomes after pancreas transplantation have included a combination of C-peptide cutoffs and clinical criteria to classify type 2 diabetes mellitus (T2DM). However, because the kidney is the major site for C-peptide catabolism, C-peptide is unreliable to discriminate the type of diabetes in patients with kidney disease.

METHODS

To improve the discriminative power and better classify the type of diabetes, we used a composite definition to identify T2DM: presence of C-peptide, negative glutamic acid decarboxylase antibody, absence of diabetic ketoacidosis, and use of oral hypoglycemics. Additionally among T2DM patients with end-stage renal disease (ESRD), body mass index of <30 kg/m(2) and use of <1 u/kg of insulin per day were selection criteria for suitablity for simultaneous pancreas and kidney transplantation (SPKT). We compared graft and patient survival between T1DM and T2DM after SPKT.

RESULTS

Our study cohort consisted of 80 patients, 10 of whom were assigned as T2DM based on our study criteria. Approximately 15% of patients with T1DM had detectable C-peptide. Cox regression survival analyses found no significant differences in allograft (pancreas and kidney) or patient survival between the 2 groups. The mean creatinine clearance at 1 year estimated by the modification of Diet in Renal Disease (MDRD) equation was not significantly different between the 2 groups. Among those with 1 year of follow-up, all patients with T2DM had glycosylate hemoglobin of <6.0 at 1 year versus 92% of those with T1DM.

CONCLUSION

SPKT should be considered in the therapeutic armamentarium for renal replacement in selected patients with T2DM and ESRD. Use of C-peptide measurements for ESRD patients can be misleading as the sole criterion to determine the type of diabetes.

Serum C-peptide concentrations poorly phenotype type 2 diabetic end-stage renal disease patients

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.

Insulin production rate following glucose ingestion estimated by splanchnic C-peptide output in normal man

Insulin production rate has been estimated in healthy male volunteers (n = 16), and evaluated with respect to splanchnic glucose exchange. Insulin production rate was calculated from splanchnic immunoreactive C-peptide output. C-peptide secretion was estimated by the hepatic venous catheter technique both in the basal state and for 2 h following ingestion of various glucose loads (0, 12.5, 25, 50, 75, and 100 g). The results demonstrate a basal insulin production rate of 0.017 +/- 0.002 U/min (mean +/- SEM) or 2.04 U/2 h. Values rose in a dose dependent manner from 2.6 +/- 1.1 U/2 h after ingestion of 12.5 g of glucose to 10.8 +/- 1.1 U/2 h following a glucose load of 100 g. Insulin retention by the liver was estimated at 0.012 +/- 0.001 U/min in the basal state, and ranged from 47-85% (70 +/- 2%) of production following an oral glucose load. It was also demonstrated 1) that the relative splanchnic glucose output was inversely related to the amount of ingested glucose, and reached a minimum when glucose in excess of 50 g was ingested; and 2) that hepatic glucose retention was directly proportional to insulin production rate (r = 0.83; p less than 0.001; n = 15). It is suggested that the adaptive capacity of the splanchnic bed to retain glucose depending on the amount of ingested glucose guarantees that splanchnic glucose output fluctuates in healthy man only within a narrow range.

Plasma C-peptide in uraemic patients

The kidney has been suggested as the main organ for the degradation of C-peptide. This hypothesis was tested in subjects with normal fasting blood glucose concentration and varying degrees of renal failure. Forty-nine subjects with endogenous creatinine clearance ranging from 0--25 ml/min were studied. The basal steady state concentrations of C-peptide (CP) and the immunoreactivity of insulin (IRI) were determined in plasma from fasting patients. The average IRI was similar to that found in normal subjects while a higher CP was found in all patients but two. The average CP in the nephrectomized patients was six times higher than the mean CP in normal subjects (0.35 pmol/ml). There was a significant inverse correlation between clearance and CP (r = 0.51, P less than 0.001) with the highest CP in nephrectomized patients. It is concluded that the increased CP in renal failure, and especially the markedly increased CP in the nephrectomized group supports the hypothesis of the kidney being the organ mainly responsible for the degradation of C-peptide also in man.