How to correctly interpret the results of the "C peptide release test", an important means of assessing islet function?

Introduction: Diagnosis and treatment of diabetes, it is very important to understand the patient's "islet function", except for blood glucose measurement and glucose tolerance test, the most commonly used islet function test

The "C peptide release test" is a unique and important means of examining the function of the islets, but "Why should this test be done?" How? What are the precautions during the testing process? How should the test results be interpreted? "A lot of people don't know very well

The following references focus on these issues, specifically about the "C peptide release test"

What is a C Peptide Release Test?

To understand C peptide release testing, you first need to know what C peptide

For patients who have been treated with insulin, the insulin antibodies produced in the body can interfere with insulin measurement, and the release method now used to determine insulin, can not distinguish whether it is endogenous or exogenous insulin, bringing difficulties to understanding the function of cells, and C peptide and insulin have a fairly stable proportional relationship, and there is no cross-immune response between insulin, can overcome the interference of insulin receptors, injected exogenous insulin does not contain C peptide, the determination of C-peptide levels in the blood, can reflect the level of endogenous insulin, Understand the function of β cells, especially suitable for determining islet function

Principles of C peptide release assay

Let the patient quantify oral glucose (or steamed meal) on an empty stomach, raise blood sugar, stimulate insulin β cells to release C peptide and insulin, and determine the level and change of plasma C-peptide at different time points after fasting and sugar to understand the reserve function

The main use of C peptide release testing

1.

2.

Who needs to have a CPTP?

1.
Diabetic patients who have been diagnosed with diabetes; 2.
Diabetic patients
who have been clinically treated with insulin.

How is the C PE TEST conducted?

The C peptide release test is the same as the glucose tolerance test, when the patient needs to take glucose (fasting for 8 to 10 hours) quantitative oral glucose (standard glucose 75g) or steamed bun (100g standard flour made of steamed bun), so that blood sugar rises, stimulates insulin β cells to release Cpeptide and insulin, and measures the plasma C-peptide level
at 30 minutes, 60 minutes, 120 minutes, 180 minutes after fasting and taking sugar (or steamed bun) for 5 time points.

Clinically, C peptide release tests are used to evaluate the function of islet β cells often combined with glucose tolerance tests, so we often do Cpeptide release tests to determine blood glucose values at various time points
.

Easy to overlook details

1.
Discontinuation of drugs that may affect glucose metabolism

Drugs that may affect glucose metabolism are discontinued 3 to 7 days before the test, such as glucocorticoids, birth control pills, thiazide diuretics, sulfonamides, propranolol, etc
.

2.
The intake of carbohydrates should be controlled, but should not be over-restricted

Patients should have a daily carbohydrate intake of not less than 150 g from 3 days before receiving the test, but should be controlled in the range of 250 to 300 g and maintain normal activities
.

3.
Sugar water preparation, dosage and timing details

Patients are instructed to draw venous blood on an empty stomach for 8 to 10 hours on an empty stomach before 8 o'clock in the morning, and then take oral anhydrous glucose powder dissolved in 300mL of warm boiled water 75g, if 1 molecule of water glucose is 82.
5g (children are given 1.
75g per kilogram of body weight) sugar water is drunk within 3 to 5 minutes, starting from the first sip, venous blood is drawn at 30 minutes, 60 minutes, 120 minutes and 180 minutes respectively, and the blood glucose and C peptide values
of the above 5 time points are measured respectively.

Blood samples should be sent for testing
as soon as possible.

If the subject cannot tolerate glucose water, a steamed bun made of 100 grams of flour (about 3 pairs) can also be selected instead of glucose water
.

Although the results of the determination of C peptide between the steamed bun meal and oral glucose water are not much different, the latter is more quantitative
.

4.
The state of mind should be kept peaceful and stable

During the trial, subjects should not drink tea, coffee, smoke, or exercise strenuously, but they do not need to be absolutely bedridden
.

Keep a calm mood and avoid mental stimulation
.

5.
Require fasting fingertip blood glucose < 10mmol/L

The test generally requires subjects to measure fasting fingertip blood glucose less than 10 mmol/L, and is not recommended if fasting blood glucose exceeds 10 mmol/L
.

Otherwise, due to the toxic effect of hyperglycemia, the secretion of insulin and C peptide is significantly inhibited, and the measured results do not reflect the true situation
of the subject's islet function.

Moreover, giving oral sugar water to patients in the case of high blood sugar will lead to further increased blood sugar, and even induce diabetic ketoacidosis, causing unnecessary damage
to subjects.

Therefore, if the patient's blood glucose is high, it is best to strengthen the treatment first, and then the fasting blood glucose drops below 8mmol/L (up to 10mmol/L), and then check
after 3 to 5 days.

6.
C peptide release test for newly diagnosed diabetic patients, generally drinking standard 75g of glucose; Patients who are hospitalized to confirm diabetes can also eat 3 or 2 steamed bun meals in the morning clinically and undergo A peptide release tests
.

How to analyze the C peptide release test results?

To understand the secretion function of the patient through the C peptide release test, it is necessary to combine the blood glucose value of each time point measured synchronously (that is, the glucose tolerance test), and it is of no practical significance
to see the islet function without blood glucose.

For the sake of intuition, we often plot the blood glucose values and C peptide values of the above time points (the coordinate point of 0 on fasting) into a curve graph, that is, "glucose tolerance curve and C peptide release curve"
.

Through analysis, the reserve function and degree of
failure of the patient's islet β cells are understood.

Interpret the principles

1.
It depends on the level of the curve, that is, how the patient's C peptide secretion level (indirectly represents the level of insulin secretion) - high, low or normal;

2.
It depends on the shape of the curve, that is, whether there is a peak in secretion after taking sugar, and whether there is a peak delay;

3.
By observing the relationship between the value of each time point of the C peptide release test and the blood glucose level measured synchronously, it is determined whether there is relative hyperinsulinemia and insulin resistance;

4.
By observing the time of the peak occurrence of C peptide release test, it is determined whether there is a peak delay (characteristic of type 2 diabetes mellitus);

5.
By observing whether the C peptide release curve is low, and at the same time combining the medical history and the results of diabetic autoantibody examination, etc.
, the type of diabetes mellitus (type 1 or type 2) is judged: if it is considered to be type 2 diabetes, it is also necessary to combine the clinical characteristics of the patient (such as the length of the disease, whether there are stress factors, the efficacy of hypoglycemic drugs, the status of blood glucose control on weekdays, etc.
), and further analyze whether it is "high blood glucose toxicity inhibits the cell function of the islet β, or the long course of the disease leads to the failure of islet β cells", if it is the former, after active treatment, Islet function is expected to improve
.

Experience Sharing: 6 Clinically Common C Peptide Release Curve Types

Theoretically, the pathogenesis of diabetes is mainly insulin resistance and insulin secretion relative or absolute insufficiency, which will be manifested in different types
of C peptide release curves.

Common type one

Interpretation: Fasting plasma C peptide is 0.
3~1.
3nmol/L, 0.
5~1.
0h secretion reaches a peak after oral glucose, the peak is about 5 to 6 times of the fasting value, 2~3h gradually returns to the fasting level, and the glycotenance test points are in the normal range, which is the normal C peptide release curve
of people with normal glucose metabolism.

Common type two

Interpretation: C-peptide levels are elevated, especially in fasting C-peptide, the peak (still 5 to 6 times the fasting value) and the peak time are normal, and the glycotenance test is within the normal range, indicating the existence of fasting insulin resistance, which is more common in overweight and obese patients
.

Common type three

Interpretation: The level of fasting C-peptide is elevated, the peak after taking sugar is more than 5 times the fasting value, but the peak is delayed, such patients are prone to preprandial hypoglycemia, although the sugar tolerance test points are in the normal range, but it is a high-risk group of diabetes, which is the early manifestation of
type 2 diabetes.

Common type four

Interpretation: The level of fasting C peptide is elevated, the peak appears at 0.
5 to 1.
0h, the peak is more than 5 times the fasting value, but the 3h is still a high level, does not fall back to the fasting level, and the glucose tolerance test suggests impaired glucose regulation (IFG, IGT or IFG +IGT), which is prediabetes, and if there is no active intervention, it may gradually progress to clinical diabetes
.

Common type five

Interpretation: The level of fasting C-peptide can be normal, high or low, the release curve rises slowly after taking sugar, the peak is delayed, and the release curve at 3h still does not fall back to the fasting level, and as the course of diabetes is prolonged, the function of islet β cells gradually decreases, its fasting C-peptide level gradually decreases, the postprandial release curve rises more slowly, the peak shift is more obvious, and the glucose tolerance test suggests that clinical diabetes patients are characteristics of
typical type 2 diabetes.

 

Common type six

Interpretation: The level of fasting C-peptide is low, the release curve rises slowly after taking sugar, almost straight, there is no peak characteristics, the glucose tolerance test suggests that the typical clinical diabetes patients, this situation indicates that the islet β cell function is very exhausted, excluding the toxic effect of hyperglycemia so that insulin secretion is significantly inhibited:

➤ If it is a juvenile patient, accompanied by ketosis, mostly type 1 diabetes;

➤ If the middle-aged and elderly patient suddenly becomes ill, isting, and the disease progresses rapidly, accompanied by recurrent ketosis, it is necessary to consider adult occult autoimmune diabetes mellitus (LADA), combined with islet-related antibodies to confirm the diagnosis;

➤ If a diabetic patient with a long course of disease has been treated with adequate amounts of hypoglycemic drugs, islet β cell failure
should be considered for patients with type 2 diabetes.

Conclusion

Clinical glucose tolerance test and C peptide release test combined determination, analysis of the characteristics of C peptide release curve, to assess the patient's islet β cell function, judgment of diabetes typing and treatment has important guiding significance, as an endocrinology clinician must be proficient
.

Resources:

[1] Han Guangyu, Xu Zhan, Shi Li, Li Quanshuang, Liang Jun, Zhang Wei.
Analysis of serum insulin C peptide release test results in patients with type 2 diabetes mellitus and impaired glucose tolerance[J].
Chinese School Doctor,2016,30(07):541-543.

Clinical analysis of C peptide release response curve type of type 2 diabetes mellitus[J].
Journal of Mathematical Medicine,2018,31(12):1743-1745.

Jia Yanfang.
Clinical significance of serum C-peptide detection in patients with LADA with NAFLD[J].
Clinical Research,2021,29(02):128-130.

LI Feng.
Relationship between C-peptide and glycemic control, complications and metabolic indices in patients with type 2 diabetes[J].
Diabetes New World,2020,23(23):47-48+51.