Insulin and C-peptide determination

Insulin and C-peptide are secreted in parallel

which means that one molecule of insulin is produced along with one molecule of C-peptide.

The levels of insulin and C-peptide in the blood are the main indicators reflecting the functional status of islet β cells and can also reflect the severity of diabetes.

Insulin and C-peptide determination
Insulin determination

Due to the rapid degradation of insulin in the blood circulation, with a half-life of 4-5 minutes, and the longer half-life of C-peptide, as well as the fact that more than 50% of insulin in the liver is cleared, the clearance rate of C-peptide is very low. The ratio of C-peptide to insulin in peripheral blood is greater than 5. When diabetes patients are treated with insulin, the C-peptide determination is rarely affected, thus making it more valuable to determine the severity of diabetes through C-peptide measurement.

  1. Insulin and C-peptide determination methods. The main samples used are plasma and serum. Currently, more radioactive immunoassay techniques are used, which have the characteristics of high sensitivity, specificity, and small error. The best detection method is enzyme-linked immunosorbent assay (ELISA), which is currently being promoted by medical institutions. However, the equipment is relatively expensive. Regardless of the method used, it requires determination by specialized technical personnel, and patients only need to understand the differences between the methods.
  2. Normal levels of fasting plasma insulin and C-peptide. The fasting insulin concentration in normal individuals is 5-24 mU/L. The C-peptide concentration is approximately 400 pmol/L. The insulin level in type 1 diabetes patients is below 4 mU/L or even undetectable, while the level in type 2 diabetes patients is often slightly lower than normal. Obese individuals may have higher levels (hyperinsulinemia). The changes in C-peptide levels are similar to those of insulin.
  3. Insulin and C-peptide release tests. This is a synchronous test with OGTT, used to evaluate the functional level and potential of islet β cells. Generally speaking, patients who have not used insulin only need to perform the insulin release test, while those receiving insulin treatment should undergo C-peptide release testing.

During the OGTT test, blood samples are collected and fasting, 0.5, 1, 2, and 3-hour insulin or C-peptide levels are measured. The insulin or C-peptide release curve is drawn based on these five points. In normal individuals, the insulin or C-peptide secretion reaches its peak 1-2 hours after a meal, approximately 4-10 times the fasting insulin level. In type 1 diabetes, due to insufficient islet β cell function, insufficient insulin secretion after glucose loading results in a flat curve, separated from the hyperglycemic curve. In type 2 diabetes, the insulin (or C-peptide) curve shows delayed release. Although some obese patients sometimes have higher levels than normal, compared to their blood glucose levels, they still have relative insulin deficiency.

Insulin is a hormone produced by the pancreas that plays a critical role in regulating glucose metabolism in the body. When we consume food, especially carbohydrates, the pancreas releases insulin into the bloodstream in response to rising blood sugar levels. Insulin acts as a key that unlocks cells, allowing glucose to enter and be used for energy production. This process is essential for providing energy to cells throughout the body, including muscles, fat cells, and the brain.

In addition to facilitating glucose uptake, insulin also helps in the storage of excess glucose. It promotes the conversion of glucose into glycogen, a form of stored energy, which is primarily stored in the liver and muscles. This stored glycogen can be later broken down into glucose when energy is needed, ensuring a steady supply of fuel for the body.

Insulin also plays a crucial role in inhibiting the production of glucose by the liver. By suppressing the liver’s glucose production, insulin helps prevent

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