C-Peptide Test: How It Helps Diagnose Insulin Problems

What Is C-Peptide?

C-peptide is a small protein made in the pancreas. It connects two parts of a bigger substance called proinsulin. When proinsulin breaks down, it turns into two things: insulin and C-peptide.

Insulin helps control your blood sugar. C-peptide doesn’t do this, but it still has an important job. It shows how much natural insulin your body is making. This is useful if you are taking insulin as a treatment because insulin shots don’t show how much your body makes on its own, but C-peptide does.

Doctors use C-peptide tests to check how well the pancreas works, to find out what type of diabetes someone has, or to look for problems like tumors in the pancreas.

When Is C-Peptide Testing Recommended?

Doctors may order this test when they need to check your insulin levels or find out what’s causing blood sugar problems.

The test is usually done to:

• Tell the difference between Type 1 and Type 2 diabetes
• Check how much insulin your body still makes
• Look into low blood sugar (hypoglycemia)
• See if diabetes treatment is working
• Monitor your pancreas after surgery or illness
• Check for insulin-producing tumors

How Is C-Peptide Testing Conducted?

This test is usually done with a blood test. A nurse or technician takes blood from your arm. Sometimes, a urine sample is also used, collected over 24 hours.

The test is not part of basic blood panels like a CBC or CMP. It is usually ordered by itself or with insulin testing.

Before the test:

• Don’t eat or drink for 8-12 hours before the test
• Avoid alcohol for 24 hours before
• Don’t smoke for at least 3 hours before
• Follow your doctor’s instructions if you take regular medications

What Are the Normal C-Peptide Levels?

C-peptide levels show how much insulin your body is making. Normal levels can vary depending on the lab.

Normal blood test range:

• 0.5 to 2.0 ng/mL

Normal urine test range (24-hour collection):

• 15 to 63 micrograms per day

Note: The normal range and measurement units can change depending on the lab and testing method.

What Do Abnormal C-Peptide Levels Mean?

C-Peptide High Levels

High levels mean your body is making too much insulin, or it’s not clearing out C-peptide properly.

Possible causes of high C-peptide:

• Type 2 diabetes - The body makes more insulin than needed at first.
• Insulin-producing tumor - A rare growth in the pancreas that creates extra insulin.
• Kidney problems - The kidneys remove C-peptide, so levels rise if they aren’t working well.
• Adrenal gland tumors - These may affect hormone balance and increase insulin production.
• Certain hormone conditions - Some rare problems like Cushing’s syndrome (too much cortisol) or acromegaly (too much growth hormone) can cause the body to make extra insulin.

• Talk to your doctor about the cause
• Exercise regularly to improve insulin use
• Eat fewer processed carbs and sugar
• Focus on whole foods, healthy fats, and proteins
• Add more fiber-rich foods to your meals
• Maintain a healthy weight
• Avoid sugary drinks and snacks
• Monitor your blood sugar if needed

C-Peptide Low Levels

Low levels mean your body is making very little or no insulin. This may happen if the pancreas is damaged or if insulin is being used up quickly.

Possible causes of low C-peptide:

• Type 1 diabetes - The body stops making insulin
• Pancreas removal or injury - Less tissue means less insulin production
• Severe stress or illness - Can lower insulin temporarily
• Alcohol-related low blood sugar - May cause a sudden drop in insulin
• Normal reaction to insulin therapy - The body reduces its own insulin when treated from outside

• Follow up with your doctor to find the exact cause
• Stick to your treatment plan carefully
• Focus on a balanced diet with omega-3s and vitamin D
• Avoid skipping meals
• Keep regular check-ups to monitor your condition
• Stay active to support overall health

Which Specialist Should You Consult?

If your C-peptide levels are abnormal, the right specialist can help:

• General Practitioner (GP) - Can guide your first steps and explain your test results
• Endocrinologist - A hormone and diabetes specialist who can provide expert care for insulin-related issues