C-peptide is a pancreatic protein formed during the conversion of proinsulin to insulin. Proinsulin is cleaved (holds α- and β-insulin chains together in the proinsulin molecule) into insulin and biologically inactive C-peptide. C-peptide assay provides distinction between exogenous and endogenous circulating insulin.

The main use of C-peptide is to evaluate hypoglycemia. C-peptide levels provide reliable indicators for pancreatic and secretory functions and insulin secretions. In a patient with type 1 T1D, C-peptide measurements can be an index of insulin production and mark endogenous β-cell activity. C-peptide levels can also be used to confirm suspected surreptitious insulin injections (i.e., factitious hypoglycemia). Findings in these patients reveal that insulin levels are usually high, insulin antibodies may be high, but C-peptide levels are low or undetectable. This test also monitors the patient’s recovery after excision of an insulinoma. Rising C-peptide levels suggest insulinoma tumor recurrence or metastases.

Fasting: 0.51–2.72 ng/mL or 0.17–0.90 mmol/L

Values vary with laboratory.

1. Draw a 1-mL venous blood sample from a fasting patient using a red-topped chilled tube. Serum is needed for test. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Date and time must be correct. Centrifuge blood for 30 minutes. Follow standard precautions.

2. Separate the blood at 4 °C and freeze if it will not be tested until later.

3. A sample for glucose testing is usually drawn at the same time.

4. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place specimen in a biohazard bag.

1. Increased C-peptide values occur in the following conditions:

   1. Endogenous hyperinsulinism (insulinemia)

   2. Oral hypoglycemic drug ingestion

   3. Pancreas or β-cell transplantation

   4. Insulin-secreting neoplasms (islet cell tumor)

   5. T2D

2. Decreased C-peptide values occur in the following conditions:

   1. Factitious hypoglycemia (surreptitious insulin administration)

   2. Radical pancreatectomy

   3. T1D

3. C-peptide stimulation test can determine the following:

   1. Distinguish between T1D and T2D

   2. Identify patients with diabetes whose C-peptide stimulation values are >1.8 ng/mL (>0.59 nmol/L) who can be managed without insulin treatment

Pretest Patient Care

1. Explain the test purpose and blood-drawing procedure. Obtain history of signs and symptoms of hypoglycemia.

2. Ensure that the patient fasts, except for water, for 8–12 hours before blood is drawn.

3. If a radioisotope test is necessary, it should take place after blood is drawn for C-peptide levels.

4. If the C-peptide stimulation test is done, give IV glucagon after a baseline value blood sample is drawn.

5. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Increased C-Peptide

1. Kidney disease

2. Ingestion of sulfonylurea