Practical dry goods: How to interpret the "C-peptide release test results"?

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Introduction: In the process of diagnosis and treatment of diabetes, it is very important to understand the patient's "islet function".
The most common laboratory tests are blood glucose and glucose tolerance tests.
The most commonly used is the islet function test.

The "C-peptide release test" is a unique and important method for testing pancreatic islet function, but "why should this test be done? How should it be done? What are the precautions during the test? The core-how should the test results be interpreted?" Many people agree Not very understanding.

The author's reference below focuses on these issues, specifically talking about the "C peptide release test".

 1.
Do you really understand the C-peptide release test? To understand the C-peptide release test, you must first know what C-peptide is.

 C-peptide, also known as connecting peptide, is a peptide substance secreted by pancreatic β cells.
It shares a common precursor with insulin-proinsulin.

A molecule of proinsulin is cleaved into a molecule of insulin and a molecule of C-peptide under the action of pancreatic protease and carboxypeptidase.
C-peptide and insulin are finally secreted with equal molecular weight and released into the blood.
Therefore, in theory, C-peptide and insulin are Equivalent to secretion, measuring serum C-peptide is equivalent to measuring serum insulin, which can be used to understand the secretory function of pancreatic β-cells.

Studies have found that free C-peptide in serum is not easily degraded by the liver, and its half-life is significantly longer than that of insulin.
Therefore, clinically measuring C-peptide levels can more accurately reflect the functional state of β-cell synthesis and release of insulin.For patients who have been treated with insulin, the insulin antibodies produced in the body can interfere with the measurement of insulin.
At the same time, the current radioimmunoassay for measuring insulin cannot distinguish whether it is endogenous or exogenous insulin, which brings difficulties to understanding the function of cells.
There is a fairly stable proportional relationship between C-peptide and insulin, and there is no cross-immune reaction between C-peptide and insulin, which can overcome the interference of insulin receptor.
The injected exogenous insulin does not contain C-peptide, and the blood C is determined.
The peptide level can reflect the level of endogenous insulin and understand the function of β-cells.
It is especially suitable for patients on insulin therapy to measure islet function.

The principle of the C-peptide release test allows the patient to take glucose (or steamed bread meal) on an empty stomach to increase blood sugar and stimulate insulin β cells to release C-peptide and insulin.
The test is to measure the plasma C-peptide at different time points after fasting and after taking sugar.
To understand the reserve function of pancreatic β-cells.

The main purpose of C-peptide release test 1.
Judging the clinical type of diabetes: By measuring the concentration of C-peptide to understand the patient's islet function, it is helpful to correctly judge the clinical type of diabetes.

2.
Evaluation of clinical insulin treatment effect: Because C-peptide is not interfered by insulin antibodies, for patients receiving insulin treatment, the concentration of C-peptide can be directly measured to determine the patient's insulin treatment effect.

 Who needs to do C-peptide release test 1.
Diabetic patients who are diagnosed with diabetes; 2.
Diabetic patients who are clinically treated with insulin.

2.
What should the standard operating process be? The C-peptide release test is the same as the glucose tolerance test.
When the patient is fasted (8-10 hours fasting), give the patient a quantitative oral glucose (standard glucose 75g) or steamed bread (100g standard flour made steamed bread) to increase blood sugar and stimulate insulin β cells release C-peptide and insulin, and the plasma C-peptide levels were measured at 5 time points at 30 minutes, 60 minutes, 120 minutes, and 180 minutes after fasting and after taking sugar (or steamed bread).

 Clinically, the C-peptide release test is used to evaluate the function of pancreatic β-cells and is often combined with the glucose tolerance test.
Therefore, we do the C-peptide release test to measure the blood glucose value at each time point simultaneously.  3.
Perform C-peptide release test.
These details cannot be ignored.
1.
Discontinue drugs that may affect glucose metabolism.
Discontinue drugs that may affect glucose metabolism, such as glucocorticoids, contraceptives, and thiazide diuretics, 3 to 7 days before the test.
, Sulfa drugs, propranolol, etc.

 2.
The intake of carbohydrates should be controlled, but should not be excessively restricted.
Since 3 days before the test, the daily carbohydrate intake should not be less than 150g, but should be controlled within the range of 250～300g, and normal activities should be maintained .

 3.
Sugar water preparation, taking and timing details instruct patients to fast for 8-10 hours, draw venous blood on an empty stomach before 8 o'clock in the morning, and then orally take 75g of anhydrous glucose powder dissolved in 300mL warm water, such as 1 molecule of water glucose 82.
5g (for children 1.
75g ​​per kilogram of body weight) syrup should be drunk within 3 to 5 minutes, counting from the first sip, taking venous blood at 30 minutes, 60 minutes, 120 minutes and 180 minutes, respectively, and measure the above Blood glucose and C-peptide values ​​at 5 time points.

Blood samples should be submitted for examination as soon as possible.

 If the subjects cannot tolerate glucose water, they can also choose steamed buns (about 3 taels) made of 100 grams of flour instead of glucose water.

Although the measurement results of C-peptide are not much different between steamed bread meal and oral glucose water, the latter is more accurate in terms of quantification.

 4.
The mentality should be kept calm and stable.
During the test, the subjects should not drink tea, coffee, smoke, or do strenuous exercise, but they do not need to stay in bed absolutely.

Keep your mood calm and avoid mental stimulation.

 5.
The test that requires fasting fingertip blood glucose <10mmol/L will generally require subjects to measure fasting fingertip blood glucose less than 10 mmol/L.
If the fasting blood glucose exceeds 10 mmol/L, this test is not recommended.

Otherwise, due to the toxic effects of hyperglycemia, the secretion of insulin and C-peptide is obviously inhibited, and the measured results cannot reflect the true condition of the subject's islet function.

Moreover, giving patients oral sugar water when the blood sugar is high will cause the blood sugar to rise further, and even induce diabetic ketoacidosis, which will cause unnecessary damage to the subjects.

Therefore, if the patient's blood sugar is high, it is best to perform intensive treatment first.
After the fasting blood sugar drops below 8mmol/L (maximum 10mmol/L), check again after 3 to 5 days.

 6.
The C-peptide release test for newly diagnosed diabetic patients is usually done by drinking standard 75g glucose; for hospitalized patients with confirmed diabetes, clinically, they can also eat 3 taels of steamed bread in the morning for C-peptide release test.

 4.
How to analyze the results of C-peptide release test? To understand the patient’s islet secretion function through the C-peptide release test, it is necessary to combine the blood glucose values ​​measured at each time point simultaneously (that is, the glucose tolerance test).
It is meaningless to look at the islet function from the blood glucose.

For the sake of intuition, we often draw the blood glucose value and C-peptide value of the above-mentioned time points (the coordinate point is 0 when fasting) as a graph, namely "glucose tolerance curve and C-peptide release curve".

Through analysis, to understand the patient's pancreatic islet β cell reserve function and the degree of failure.

Interpretation principle 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.
No peak delay; 3.
By observing the relationship between the value of each time point of the C-peptide release test and the simultaneously measured blood glucose level, determine whether there is relative hyperinsulinemia and insulin resistance; 4.
By observing the time when the peak of the C-peptide release test appears , To determine whether there is a peak delay (characteristics of type 2 diabetes); 5.
Determine the type of diabetes (type 1 or type 2) by observing whether the C-peptide release curve is low and flat, and combining the medical history and the results of diabetes autoantibody examinations to determine the type of diabetes (type 1 or type 2): if Considering it is type 2 diabetes, and considering the clinical characteristics of the patient (such as the length of the disease, the presence or absence of stress factors, the efficacy of hypoglycemic drugs, the daily blood sugar control status, etc.
), further analysis is "hyperglycemia toxicity inhibits pancreatic β-cell function.
" It is still caused by the long course of the disease leading to pancreatic β-cell failure.
” If it is the former, after active treatment, the pancreatic islet function is expected to improve.

V.
Practical exercises-6 common types of C-peptide release curves.
Theoretically, the pathogenesis of diabetes is mainly insulin resistance and relative or absolute insufficiency of insulin secretion.
There are different types of C-peptide release curves. Interpretation: Fasting plasma C-peptide is 0.
3-1.
3nmol/L, and the secretion peaks at 0.
5-1.
0h after oral glucose, the peak is about 5-6 times the fasting value, and it gradually returns to the fasting level after 2-3h.
Glucose tolerance test at each point All in the normal range, this is the normal C-peptide release curve of a person with normal glucose metabolism.

Interpretation: Elevated C-peptide levels, especially fasting C-peptide, peak (still 5-6 times the fasting value) and peak time are normal, and the points of the glucose tolerance test are within the normal range, indicating that there is fasting insulin resistance.
Seen in overweight and obese people.

Interpretation: Fasting C-peptide levels increase, and the peak after taking sugar is more than 5 times the fasting value, but the peak is delayed.
Such patients are prone to pre-meal hypoglycemia.
Although the glucose tolerance test points are within the normal range, it is People at high risk of diabetes are early manifestations of type 2 diabetes.

Interpretation: The fasting C-peptide level rises, the peak appears in 0.
5-1.
0h, the peak is more than 5 times the fasting value, but the level is still high at 3h, and it does not fall back to the fasting level, and the glucose tolerance test indicates impaired glucose regulation ( IFG, IGT or IFG+IGT), this is the pre-diabetes stage, if not actively intervened, it may gradually progress to clinical diabetes.

Interpretation: fasting C-peptide levels can be normal, high or low.
After taking sugar, the release curve rises slowly, and the peak is delayed.
The release curve still does not fall to the fasting level after 3 hours, and as the course of diabetes prolongs, pancreatic β-cell function gradually declines.
The fasting C-peptide level gradually decreases, the postprandial release curve rises more slowly, and the peak shift is more obvious.
The glucose tolerance test indicates that it is a clinical diabetic patient, which is a characteristic of typical type 2 diabetes.

 The fasting C-peptide level is low, and the release curve after taking sugar rises gently, almost into a straight line, without peak characteristics.
The glucose tolerance test indicates that it is a typical clinical diabetic patient.
This situation indicates that the pancreatic β-cell function is very exhausted and the toxicity of hyperglycemia is excluded.
After the effect makes insulin secretion significantly inhibited: ➤If it is a teenager with ketosis, it is mostly type 1 diabetes; ➤If it is a middle-aged and elderly patient with sudden onset, weight loss, rapid progress of the disease, and repeated ketosis, consider adults Latent autoimmune diabetes (LADA) can be diagnosed with pancreatic islet-related antibodies; ➤If patients with diabetes who have been treated with a sufficient amount of hypoglycemic drugs for a long course of time, they should consider pancreatic β-cell function in patients with type 2 diabetes.
Exhaustion.  Conclusion Clinically, the combined measurement of glucose tolerance test and C-peptide release test is used to analyze the characteristics of C-peptide release curve, which has important guiding significance for assessing the patient’s pancreatic β-cell function, judging the type and treatment of diabetes, as an endocrinologist clinician must Be proficient.

 Reference materials: [1] Han Guangyu, Xu Zhan, Shi Li, Li Quanshuang, Liang Jun, Zhang Wei.
Analysis of the results of serum insulin C peptide release test in patients with type 2 diabetes and impaired glucose tolerance[J].
Chinese School Doctor,2016,30 (07):541-543.
[2]Li Suyan.
Clinical analysis of type 2 diabetes C peptide release response curve type[J].
Journal of Mathematical Medicine,2018,31(12):1743-1745.
[3]Jia Yanfang .
Clinical significance of serum C-peptide detection in LADA patients with NAFLD[J].
Clinical Research,2021,29(02):128-130.
[4].
C-peptide and blood glucose control and complications in patients with type 2 diabetes And the relationship between metabolic indicators[J].
New World of Diabetes, 2020,23(23):47-48+51.
Contribution email: tougao@medlive.
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