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New Simpler C-Peptide Test Available

A single blood test measuring C-peptide at 90 minutes after a mixed-meal challenge was a sensitive and specific measure of endogenous insulin secretion among children with new-onset type 1 diabetes…. 

According to Rachel E. J. Besser, PhD, from the Peninsula National Institute for Health Research Clinical Research Facility, Peninsula Medical School, the findings, suggest that the 90-minute mixed-meal tolerance test (MMTT)-stimulated C-peptide of 0.2 nmol/L or more (90CP) may offer a more practical alternative to the gold-standard full MMTT. The full MMTT requires sampling every 30 minutes for 2 hours to allow for measurement of area under the curve (AUC).

The full MMTT is mainly used in research and is rarely performed in routine clinical practice because of the intensity of sampling. Although the fasting CP (FCP) is often used in clinical practice, studies have shown it to be inferior to the AUC after an MMTT in assessing residual insulin secretion, the authors note.

This study compared both the 90CP and the FCP with the full 150-minute MMTT in 421 children with type 1 diabetes, with assessments done at 3, 9, 18, 48, and 72 months’ diabetes duration. The children were 55% boys, with an average age at diagnosis of 11 years. A total of 1334 MMTTs were included in the primary analysis.

Previous studies have demonstrated that CP usually peaks at approximately 90 minutes during an MMTT and that a 90CP of 0.2 nmol/L or more is related to improved clinical outcomes with fewer complications and less severe hypoglycemia. In the current study, the full MMTT established that 23 nmol/L/150 minute or more was the equivalent AUC CP to detect a peak CP of 0.2 nmol/L or higher with 98% sensitivity and 97% specificity.

The mean AUC CP was highly correlated to the mean 90CP. If CP was measured at just 90 minutes, rather than performing a full MMTT, it would have correctly classified 96% of patients for detecting both peak CP of 0.2 nmol/L or higher and AUC CP of 23 nmol/L/150 minute or more.

The mean FCP was strongly correlated with AUC CP, but this association was weaker than that seen with the 90CP. A previously derived cutoff of an FCP of 0.1 nmol/L or higher still correctly classified 83% of patients according to peak CP of 0.2 nmol/L or higher and 85% by AUC CP of 23 nmol/L/150 minute or higher, but was less sensitive and specific compared with a 90CP of 0.2 nmol/L or higher.

The mean time to reach peak CP peak occurred earlier in patients with longer diabetes duration and in those diagnosed at a young age," the authors write. The time to CP peak decreased by 6.1 minutes for every year increase in diabetes duration, and by 2.5 minutes for every year increase in age at diagnosis. Overall, both peak and fasting CP values were higher in patients with shorter diabetes duration and in those diagnosed at an older age.

The investigators saw only a small difference between peak and 90CP (median 0.01–0.05 nmol/L) for the six different diabetes durations.

These findings suggest that the 90CP could replace the FCP as a means of offering more patients entry into intervention studies, according to the authors, and could also be used in studies assessing beta-cell function.  "In practical terms, this would mean fewer blood samples (one compared with five in a standard MMTT), a shorter duration required for the patient to stay in the research facilities (90 compared with 120 min), and a reduced cost to run the study and analyze the samples," the authors conclude.

Diabetes Care. Published online October 30, 2012. Abstract