Immunoassay Measures Serum C-Peptide Levels

The conventional understanding of the pathophysiology of type 1 diabetes is that pancreatic ß-cell function drops off dramatically after diagnosis and this well-described process can be tracked through declines in C-peptide values over the same period.

Scientists at Massachusetts General Hospital (Boston, MA, USA) studied 182 type 1 diabetics, whose median age was 39 years, and had had the disease for a median of 15 years, and were a median of age 13 at disease onset. C-peptide values for all these subjects were measured first with an ultrasensitive C-peptide assay and only those with values higher than the detection range of the ultrasensitive assay also were measured using a standard assay.

The second part of the study involved four diabetics who provided serum samples weekly for up to 20 consecutive weeks. C-peptide values in these samples were measured using a different standard C-peptide assay but with the same ultrasensitive assay that had been used to measure samples from the 182 patients. The standard C-peptide assays have lower limits of detection of 15 pmol/L and 33.1 pmol/L, respectively, whereas the ultrasensitive assay has a lower limit of detection of 1.5 pmol/L with inter- and intra-assay coefficients of variation of 5.5 and 3.8% at 37 pmol/L.

The 182 samples were categorized into six groups based on the patients’ years of disease duration and found declining levels of C-peptide detection across the groups; however, the decline was gradual over decades, rather than a few years. Nearly 80% of samples in the 0–5 years’ duration of disease group had C-peptide levels above the ultrasensitive assay’s limit of detection, versus 10% from patients who had had diabetes for 31–40 years. The ultrasensitive assay detected C-peptide in 10% of individuals 31–40 years after disease onset and with percentages higher at shorter duration. Levels as low as 2.8 ± 1.1 pmol/L responded to hyperglycemia with increased C-peptide production, indicating residual β-cell functioning.

The scientists also assessed subjects’ ß-cell functional capacity by evaluating fasting and/or nonfasting glucose levels in both the group of four patients who provided serial samples and in samples from the 182 patients. In both groups, samples from hyperglycemic subjects with glucose values greater than150 mg/dL had significantly higher C-peptide levels than those from normoglycemic subjects. The C-peptide enzyme-linked immunosorbent assay (ELISA) kits were both products of Mercodia (Uppsala, Sweden).

Denise Faustman, MD, PhD, the senior author of the study, said, "For an endocrinologist evaluating a patient with a suspected tumor that’s making too much insulin from the tumor, the standard assays with range of detection from 40 to 660 pmol/L are totally appropriate. However, I would think clinicians following people with type 1 diabetes would want to pair results from this ultrasensitive assay with HbA1c measurements to discern whether a patient is struggling with compliance or if their pancreatic function is shutting down." The study was published in the March 2012 issue in the journal Diabetes Care.

Related Links:
Massachusetts General Hospital
Mercodia