Why C-Peptide Serum test for Type 1 of 30 years?

What purpose is there for performing a C-Peptide test on someone definitively dependent on insulin for over 30 years? Total daily amount of insulin only changes when I change insulin type.
I assume that I am TYPE 1 as originally diagnosed and not not some sub-type. Could this test prove something different?
Is Low C-Peptide dangerous as is Low Vitamin D, B-12, etc and capable of pharmaceutical corrections?

The purpose of the C-peptide test is to determine how much insulin your body is actually making. There is a one for one correspondence between home-grown (endogenous) insulin and c-peptide in the blood.

I know that Medicare (US) will require a C-peptide test to determine eligibility for insulin pump coverage. They still discriminate against T2D people in this way, although some T2Ds are able to qualify.

If this test comes out above a certain threshold then it indicates your body is still making a significant, yet maybe still relatively low, amount of insulin. Use of external insulin, by itself, is not determinant of diabetes type. Many people with T2D take insulin and almost all (with the exception of early honeymoon T1Ds) T1Ds use insulin.

Low C-peptide is not dangerous, it is simply an indicator of the quantity of home-grown insulin.

so even if you’ve been Type1 for 30 or 50 years, Medicare would still require this test done? Does anyone know what the actual result that would be disqualifying for Medicare pump coverage?

@MarieB: If you have a positive antibody test, then the C-Peptide is not a requirement.

The maximum C-Peptide allowed is “less than or equal to 110 percent of the lower limit of normal of the laboratory’s measurement method.”

Here’s a link to the very detailed Decision Memo for Medicare coverage for pumps:

CMS has determined that the evidence is adequate to conclude that continuous subcutaneous insulin infusion (CSII) is reasonable and necessary for treatment of diabetic patients: 1) who either meet the updated fasting C-peptide testing requirement or are beta cell autoantibody positive; and 2) who satisfy the remaining criteria for insulin pump therapy detailed in the Medicare National Coverage Determinations Manual (Medicare NCD Manual 280.14, Section A.5).

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I’ve had T1 for over 50 years & I had my 1st C-Peptide test done last week as part of getting Medicare coverage for my Dexcom supplies. Guess what? It confirmed I have T1. The test probably didn’t exist when I was 1st diagnosed in 1961.


I asked here and emailed Endo to find out IF my large CASH price for the test would provide any beneficial info.
IF my body is producing ANY insulin would show that beta cells still exist and hope for regeneration. However, is there anything mainstream OR off the books that might get a T1D off insulin before man lands on Mars?

They used to only need evidence of one qualifying C-peptide test taken with blood glucose under some limit (220?).
One on file from any time should suffice… That hasn’t changed, has it?

I was diagnosed a year ago. Just for kicks I would love to have another one!

@Thas, the Summary in the Decision memo states, “Levels only need to be documented once in the medical records.”