C-Peptide Information: where can I find it?

Hi there!
I’m looking for information on the c-peptide test in the context of diabetes T1. Any good overview articles out there (more than what’s on Wikipedia) which cover the medical/diagnostic side?
Cheers, Timm

I have been quite dissappointed with information on the use of c-peptide as a diagnostic tool. When my c-peptide was tested and it came in low, I thought it would be a useful piece of information. But no, apparently that piece of information alone was not meaningful. What I did learn was that

If you have constantly elevated blood sugars and you have a low c-peptide, that indicates an insulin deficiency

If you have constantly elevated blood sugars and you have a high c-peptide, that indicates insulin resistance

In my case I had elevated blood sugars (130 mg/dl fasting), but not “highly” elevated fasting, so according to my endo, my c-peptide did not indicate anything.

All I have is bits and pieces I have gleaned from multiple sources. I’d be interested in what others have found.

From what I’ve understood from a talk with an endocrinologist, c-peptide is a molecule which the body produces in the course of creating insuline, but is not included in injectable insuline. So the amount of c-peptide is an indicator of how much insuline the body produces by itself, which in turn gives you an indication about the state of your pancreas and the question if you are a T1 (low c-peptide) or T2 (normal/high c-peptide) - especially if you are already injecting insuline.

Any endos or biochemists around who can confirm this?

Here are some references about C-Peptide levels and diabetes Dx:

Here is another link


Thanks for the references. While these are helpful, the c-peptide itself may or may not be useful in helping us determine what type of diabetes we have. As I have noted, unless you have a marked elevated fasting blood sugar, a low c-peptide reading in itself is apparently not considered a sign of insulin deficiency (at least by my endo). Clearly if you present with a seriously low c-peptide (i.e. 0.01 ng/mL) and a high blood sugar (i.e. 300 mg/dl), there won’t be any question, you have a nearly absolute insulin deficiency. But what about a diabetic who is like me, testing at 1.8 ng/mL (reference range 1.3-5.0 ng/mL) and a fasting of 130 mg/dl?

I think it is important that if diabetics are insulin deficient, we need diagnostic tests that focus on properly characterizing that. If I am insulin deficient, I want my doctors to stop prescribing me inneffective medications to increase my insulin sensitivity, to stop harrassing me about losing weight and help me manage my insulin deficiency. This is why we have lots of patients walking around with a T2 diagnosis, when in fact they are LADA or autoimmune and have serious insulin deficiency. These patients are not getting appropriate care at least partly because of the poor diagnostic tests.

I totally agree. Although, your c-peptide is on the low side even for that fasting BG.

My endo told me to eat a candy bar (and not bolus for it) before my c-peptide blood draw. I had a super-high BG, but an almost undetectable c-peptide.

So that is actually an ad hoc “stimulated” c-peptide. Performing an OGTT and taking blood sugar, insulin and c-peptide measurements gives a pretty darn accurate picture of your insulin response and enables you to clearly determine whether there is insulin deficiency or insulin resistance in the patient. But it is a three hour intrusive test and I think doctors are hesitant to order it and many patients would not be happy having it performed. Personally, I’d do the test (although the candy bar version sounds better).

Hi bsc: On www.diabeteshealth.com, look up “An Old Test Teaches Doctors New Tricks: C-Peptide Exam Becoming an Accepted Tool for Diabetes Treatment” (sorry, I tried to use the link feature, but it didn’t work).

It’s an article from the old “Diabetes Interview” which is now Diabetes Health. The article relies heavily on information from Richard Bernstein.

That is a good article, you can either search on the title at diabeteshealth.com or use this link here. I am not sure I agree with everything it says, for instance it suggests that T2s have “normal” insulin levels, late-state T2s may have “normal,” but that is usually because of declining beta cells. T2s would generally register as high. Other than that it presents much of the conflict that I have seen, particularly from people like my endo who don’t think the c-peptide is worth much.

When I test c-peptide on my patients I always tell them to do it after the meal. I want their BS to be elevated at the time of c-peptide draw. (I get a call from the lab all the time, telling me that my patient is NOT fasting, because by the lab protocol it is “a fasting” test, and I have to confirm that I do indeed want after meal value, and tell my patients as well to insist on blood draw, or they can be turned away). As you all noted, normal c-peptide is not helpful if BS normal, but normal (or low) c-peptide if BS is high is an evidence of insulin deficiency (I use term “inappropriately normal”, because after the meal your c-peptide should be elevated. You do not need to do OGTT, just have a meal - if you insulin deficient, your BS will be high enough.
Low c-peptide is most often seen in type 1 diabetes, including LADA, but it is also seen on diabetes caused by pancreatitis, in patients with cystic fibrosis, etc., or in patients with years and years of type 2. It is a sign of insulin deficiency.

Thank you Doctor for the detailed response on this.

That is really helpful. I hope you don’t mind some questions.

What would you consider a high BS?

Do you (can you) order a stimulated c-peptide and is the glucagon-stimulated c-peptide the only version available?


High BS is any BS above the normal range, that is >140 for post-meal (or >100 for fasting).
Since I am ordering c-peptide on someone with diabetes - their post-meal BS usually will be >170. There is no reference range for post-meal c-peptide (at least I am not aware of). The reference rage usually given is for fastinf value. So, if you just ate and your BS is elevated (at any level above fasting) - c-peptide should exceed fasting range, as more insulin being secreted in response to carb load. I never tried to order stimulated c-peptide.I do not think I needed it.

This is really valueable information for the diabetic community. I thank you for this. Many people ask about the c-peptide as a way of trying to clarify their diagnosis and it has been confusing. You’ve certainly helped clear up some things in my mind.

Would a high fasting glucose (above 200) and C-peptide below 0.1 NG/ML indicate for that particular morning exogenous insulin wasn’t sufficient and most likely no endogenous insulin production? What effect does the dawn phenomenon have on expected C-peptide readings?

Yes, any high BS and low c-peptide is an evidence of inadequate or absent (in case of c-peptide < 0.1) ENDOGENOUS insulin production. The reason I recommeded post-meal measure just in case a person has normal fasting BS, and test will not be helpful, but post meal c-peptide level should be always above fasting reference range.
Down phenomenon is a phenomenon in diabetes only, not in a healthy state. Down phenomenon has NO effect on c-peptide/insulin secretion. It is an opposite cause-effect relationship: lack of c-peptide/insulin causes down phenomenon, when endogeneous insulin can not be secreted in the amounts adequate to counteract peak of cortisol, growth hormone, etc and maintain euglycemia (normal BS).
Hope, it helps.

Thanks Marina R. for the information. These results were not surprising to me (I was diagnosed with diabetes in 1958), but there have been studies that show that some can have residual beta cell function even after 50 or more years (Apparently I don’t)