C-Peptide (Insulin C-Peptide, Proinsulin C-Peptide)

Sample

1. Venous blood is collected to prepare the serum.
2. A fasting sample is needed.
3. Glucose samples should also be taken at the same time.
4. Keep the blood at 4 °C.
5. Stable for 30 days when it is freezed.
6. A urine 24 hours sample is needed.
   1. Neutralize the urine to pH 7.0 to 7.5.
   2. Can store at -15 °C.

Precaution

1. As the majority of the C-peptide is degraded in the kidneys, renal failure will increase the level.
2. Take H/O drugs that may increase the level of hypoglycemic agents (sulfonylureas).

Purpose of the test (Indications)

1. This test is done to evaluate diabetic patients.
2. This test is the best tool for the diagnosis of hypoglycemia.
3. It provides a reliable indication of the pancreatic secretory function and insulin secretion.
4. It is helpful to diagnose Insulinoma (Tumor of islets of Langerhans).
   1. It is advised for the follow-up of a patient treated for insulinoma.
5. To find patient injecting exogenous insulin.
6. The C-peptide level can be advised to diagnose insulin resistance syndrome.
7. Helpful in the case of patients with pancreatectomy where it will be undetectable.

Precaution

1. Patients with renal failure may have a high level of C-peptide because mostly it is degraded in the kidney.
2. Oral hypoglycemic agents may increase the C-peptide level.

Pathophysiology

1. C-peptide is a connecting peptide for the β and α chain of proinsulin.
2. C-peptide is formed during the conversion of proinsulin to Insulin.
   1. C-peptide is released into a portal vein in an equal amount.
   2. It has a longer half-life than insulin. So more C-peptide is present in the circulation.
3. Proinsulin is cleaved into Insulin + C-peptide (inactive biologically).
   

   Insulin production and function

 

4. The C-peptide assay provides the difference between the endogenous and exogenous insulin.
5. In general C-peptide level and insulin level correlates with each other (except obese patient and patient with insulinoma).
6. The capacity of beta cells of the pancreas to produce insulin can be measured either by measuring C-peptide or insulin directly.
7. C-peptide level estimation is helpful in the following conditions:
   1. Differentiate type 1 and type 2 diabetes mellitus. In type 1 DM there will low level of C-peptide and insulin, while in type 2 DM will have a normal or high level of C-peptide.
   2. In patients who are taking exogenous insulin, C-peptide is a more accurate test of islet cell function. This will also help to see the endogenous production of insulin.
   3. DM patients treated with insulin and have anti-insulin antibodies. These antibodies falsely increase the level of insulin.
   4. In people who produce hypoglycemia by giving them insulin. Where there will be raised levels of insulin but the C-peptide level will be normal. Exogenous given insulin suppresses endogenous insulin and C-peptide production.

8. A rise in the C-peptide level in a treated patient of insulinoma indicates recurrence.
9. Advantages of C-peptide over Insulin are:
   1. C-peptide is a better indicator of β- cells functions due to its good level of concentration in the blood than is peripheral insulin concentration.
   2. It does not cross-react with the insulin antibody, which interferes with insulin immunoassay.
   3. The C-peptide assay doesn’t measure the exogenous insulin.
   4. C-peptide is not found in the commercial preparation of insulin.
   5.  C-peptide has a longer half-life than insulin.
   6. C-peptide is a better indicator of the fasting hypoglycemia.

Normal C-peptide

Source 2

Fasting level	0.78 to 1.89 ng/mL
One hour after glucose load	5 to 12 ng/mL
Urinary C-peptide	74 t± 26 µg/L

• Fasting level = 0.78 to 1.89 ng/mL (0.26 to 0.62 nmol/L
• One hour after glucose  load = 5 to 12 ng/mL.
• Urinary C-peptide = 74 ± 26 µg/L.
  • (values varies with lab to lab)

Source 1

Normal C-Peptide 

Sample	ng/mL	nmol/L
Serum (fasting)	0.78 to 1.89	0.26 to 0.63
Urine	µg/dL
24 hours	64 ± 20.5	21.5 ± 6.8

The raised level of C-peptide seen in:

1. Insulinoma.
2. Oral hypoglycemic drugs.
3. Islet cell tumor producing insulin (Insulinomas).
4. Type 2 DM (non-insulin dependant).
5. Renal failure.
6. Hyperthyroidism.
7. Cirrhosis.

The decreased C-peptide level is seen in:

1. Factitious hypoglycemia.
2. Type 1 diabetes mellitus.
3. Pancreatectomy.

 

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