C Peptide

As C-peptide is released in the circulation along with insulin, it has widely been used as a measure of insulin secretion to assess the pancreatic beta-cell function.[10] It also has the advantage of bypassing clearance by the liver, unlike insulin, and hence, has a much longer half-life of around 30 minutes. The plasma concentration of C-peptide in a fasted state is 0.9 to 1.8 ng/ml, and the postprandial levels are 3 to 9 ng/m in healthy individuals.[1] The higher levels are observed in overweight individuals. C-peptide is catabolized in the kidneys, and only a small fraction is excreted in the urine. Modern ultrasensitive C-peptide immunoassays can detect plasma levels as low as 0.0045 to 0.0075 ng/ml. The presence of increased titers of anti-insulin antibodies that bind to both proinsulin and C-peptide could give false-positive results. Urinary C-peptide (UCP) measurement is a non-invasive test, and the urine can be collected in boric acid, where UCP is stable at room temperature for up to 3 days. With normal urinary function, the UCP excretion is reflective of 5% to 10% of total C-peptide secreted by the pancreas.[11]

Since C-peptide is a polypeptide and less stable due to its susceptibility to enzymatic proteolytic cleavage, the serum C-peptide measurements should be done immediately after the sample collection. Hence, gel tubes are required to collect samples on ice to transport the sample to the laboratory. The samples should be immediately centrifuged and stored under frozen conditions until the estimation is done. However, EDTA-prepared tubes can be used for plasma C-peptide determination, which increases its stability at room temperature for up to 24 hours, and hence, plasma is the preferred sample.[12]

The plasma C-peptide levels can be measured in random, fasting (8 to 10 hours) or stimulated state. Random non-fasting sampling (rCP) is the easiest method to test C-peptide (fCP) levels. The rCP has been shown to correlate with 90-minute mixed meal tolerance test (MMTT) C-peptide responses. The stimulation can be done using glucagon, intravenous/oral glucose, tolbutamide, sulfonylurea, and glucagon-like peptide 1, amino acids, or a mixed meal. Glucagon stimulation test (GST) is the most widely used test due to its high sensitivity in detecting residual insulin secretion using a dose of 1.0 mg. C-peptide can also be measured using the oral glucose tolerance test (75 g, OGTT) where the samples are collected at 0, 30, 60, 90, and 120 minutes.[13] This test significantly correlates with insulin secretion in type 2 diabetes patients. Both MMTT and GST provide sensitive and reproducible results for residual beta-cell function in type 1 diabetes with peak responses seen at 90 and 6 minutes, respectively.[14] Although the C-peptide levels are useful to classify diabetes, it always must be interpreted in the clinical context of disease duration, comorbidities, and family history. However, it is still a very valid measure in clinical research studies.

The 2 major indications for measuring C-peptide levels include fasting hypoglycemia and assessment of insulin secretory reserve in patients with diabetes. In patients with fasting hypoglycemia with concomitant hyperinsulinism, one needs to entertain a differential diagnosis comprising insulinoma, exogenous insulin administration (factitious), sulfonylurea therapy (factitious), insulin autoimmune syndrome due to endogenous anti-insulin antibodies (Hirata disease).[15] All of these conditions can result in hypoglycemia with elevated insulin levels. A C-peptide level is very useful in the differential diagnosis since it is only elevated with a beta cell tumor, insulinoma, and sulfonylurea therapy. A sulfonylurea drug screen can exclude the latter. C-peptide is not elevated with Hirata disease, which is confirmed by positive anti-insulin antibodies and is decreased with factious, exogenous insulin therapy. Hence, it is a very important test in the workup of fasting hypoglycemia with hyperinsulinism.

The other major indication is the assessment of insulin secretory reserve in patients with diabetes. Diabetes mellitus is characterized by hyperglycemia due to the lack of insulin secretion and/or insulin action. Insulin deficiency is associated with C-peptide-deficiency in type 1 diabetes due to beta-cell demise. A fasting C-peptide level of less than 0.6 ng/ml is consistent with beta-cell failure and predicts the requirement for insulin therapy. Although the origins of type 2 diabetes are insulin resistance, it only manifests clinically when there is beta-cell failure resulting in impaired insulin and C-peptide secretion culminating in fasting and postprandial hyperglycemia. Much more research is needed to define the biology of C-peptide and potential role in the pathogenesis of diabetic microvascular complications or as a novel therapeutic agent. 

Also, Medicare uses a C-peptide assessment of insulin reserve as a criterion for continuous subcutaneous insulin infusion therapy (insulin pump therapy).

Pearls

C-peptide is secreted in equimolar concentrations with insulin from the beta cells.

It is a valid measure of insulin secretion especially following challenges with glucagon or a mixed meal.

C-peptide is extremely useful in the differential diagnosis of hyperinsulinemic-hypoglycemia.