Can one validly test c-peptide if one is taking insulin?

If so, how? I am assuing the injected insulin would suppress at least some natural insulin.

Should one then test with known high blood sugars?

Am back on the path of wanting to know what type I am, in a few months it will affect if my Dr tries to put me on oral meds instead of insulin (though I may just refuse to change) in a few months when I stop breastfeeding.

I have tested Anti-GAD and it was negative. There are no other antibody tests available here as far as I know.

C-peptide labs should not be affected by exogenous insulin. They should reflect your bodies natural ability to produce its own insulin-- to the best of my limited understanding. Other antibody tests include insulin antibody and islet cell antibody-- which are both often referred to by their technical laboratory names, which are far beyond my grasp. I see you're in PI-- don't know if they are common practice there or not

Someone on another board explained why you need to have a fairly low blood sugar when you test for C-peptide as if your blood sugar is high, your C-peptide will likely be high. Wish I could remember why, but unfortunately it was more techincal information than I have the ability to remember.

At any rate, he told me this right before I was having to prove to Medicare that I qualified for the pump. I was high right before the test and my C-peptide consequently was also high. We retested it on a day when my fasting BG was in the 80's, and my C-peptide came out significantly below normal. I'm a T2, but I've been on insulin for over 25 years, and I've already been on a pump for 10 and yes, Medicare did approve me staying on the pump. Don't know what I would have done if they hadn't, as there's no way I could achieve this great control on shots.

Long-winded answer, I guess, to explain that it's not the insulin that affects C-peptide, but rather the blood sugar levels that may affect it.


Exogenous insulin will suppress your natural insulin production, it just isn't needed. This will result in a lower reading, both for fasting c-peptide and for stimulated c-peptide. The reasons for the c-peptide test will determine which test is best. If your doctor is interested in evaluation your remaining beta cell function, they may wish to do a stimulated c-peptide, making you ingest carbs and seeing what the maximum c-peptide you can generate. In this case, you want a high blood sugar, at least 100 mg/dl, but not too high (above 250 mg/dl) as that might result in poor results. The results of such a c-peptide test have no normal reference range and are simply used to track your individual progression of beta cell function.

The other reason is for Medicare and comparable diagnosis. This is usually done with a fasting c-peptide and Medicare requires your blood sugar blood sugar to be less than 250 mg/dl I believe. There is a c-peptide threshold used by Medicare and many insurance companies for insulin pump qualification. Some doctors use the fasting c-peptide to try to diagnose insulin resistance. If you have a high blood sugar and a high fasting c-peptide, that suggests insulin resistance. I high blood sugar and a low fasting c-peptide suggest insulin deficiency. If you have a normal blood sugar (80 mg/dl), then a moderately low or high fasting c-peptide means basically nothing.

The gold test c-peptide exam is a mixed meal tolerance test in which you do not take your mealtime insulin if you usually take mealtime insulin. If you are taking a basal insulin, such as Lantus, that is not thought to interfere with the validity of the c-peptide test. It should not be suppressing your own ability to make insulin.
A fasting c-peptide is not considered a meaningful indicator of whether or not you are making insulin or what your ability to make insulin might be.