What would be considered a low number for C peptide to indicate that the body is not producing insulin? Does the C peptide need to be drawn from a fasting blood sample?
The medical standard for insulin deficiency is probably the medicare standard for pumps. Namely that produce less than 110% of the lower end of the reference range for fasting c-peptide (with a fasting blood sugar < 200 mg/dl). But actually this test is not all that useful the c-peptide can be greatly affected by what your actual blood sugar reading is and whether you have been using exogenous insulin.
And yes a fasting c-peptide is supposed to be done when you are fasting. There is a version called the stimulated c-peptide where you consumer carbs to force a release of insulin, but this test is rarely done.
Unfortunately the c-peptide test results as an absolute number is not useful. Some doctors will use the test to see if your c-peptide is falling which may indicated a loss of beta cells, but the actual reading itself isn't all that useful. Clearly if it is nearly zero then you don't make insulin, but many T1 patients in honeymoon will still make what appears to be normal levels of insulin in a fasted state.
I looked through my records for the C peptide numberber:
2010 1.9
2008 1.5
I don't recall whether they were or not fasting; most likely they were. Where can I find some reliable information on the significance, if any of C peptide numbers?
If your body is not producing insulin then your C-peptide test will be something like
mine…fasting BG 90mg/dL and C-peptide less than <.05. Although in the Joslin study’s
they found many T1’s that still make some insulin, like around <.8 but still not enough to
fast without some added external insulin.
You may read more about the c-peptide test at lab tests online.
I just had my c-peptide drawn this week. Here are the numbers:
Component Standard Range Your Value FlagC Peptide 0.80 - 3.10 <0.10 L
The table wouldn't format well. The standard range of c-peptide for this lab is 0.80 to 3.1 and the measured value for me was less than 0.10, T1D > 30 years.
My most recent blood test was not from a fasting sample and still awaiting the results of the C peptide.
Will it make a difference that it was NOT fasting?
From Lab Tests Online:
Test Preparation Needed?
Fasting for 8 to 10 hours before blood testing is usually required.
I don't know if or how not fasting skews the results. The protocol would not usually require fasting if it wasn't needed.
This is what makes the c-peptide a bit inconclusive. My last fasting c-peptide test result as a T2 was 0.4 ng/dl (ref 1-3 ng/dl). So apparently I make less insulin than many long-term T1s. But as I mentioned earlier, the absolute number itself makes little sense. It may actually not be related to remaining beta cell function and it may just be that some people are inherently more or less insulin sensitive (which would mean they need more or less insulin to maintain fastings). Or it could be that your body has established a higher setpoint for blood sugars (which I think is my problem).