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Permalink Reply by Sam on May 17, 2012 at 7:34pm

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

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Permalink Reply by Ruth on May 17, 2012 at 7:52pm

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.

Ruth

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Permalink Reply by Super_sally on May 17, 2012 at 8:14pm

Hmmm, thanks.

Hi blood sugar would trigger the pancreas to release insulin, hence the c-peptide should be high under that scenario.

But if one is taking injected insulin then wouldn't that suppress your body's natural insulin production (assuing partially working pancreas)? But then one needs that inject insulin to keep blood sugars where they should be....

I am confusing myself here.

I guess the standard is to take the c-peptide fasting (and with as normal as possible blod sugars, regardless of how they're achieved)??

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Permalink Reply by Jonah on May 18, 2012 at 4:13pm

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.

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Permalink Reply by Brian (bsc) on May 18, 2012 at 5:21am

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.

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Permalink Reply by Super_sally on May 18, 2012 at 4:53pm

Thank you everyone!

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