Is an insulin lab inferior to a c-peptide lab if the patient has never taken exogenous insulin? Such as at initial presentation? Why?
I’ve tried to interrogate my mother, a pediatrician, about this— who still says that the peds endos she refers new onset children diabetics to wants the insulin level measured right away instead of c-peptide… But she couldn’t give me a good explanation of why.
Basically, Insulin level is inaccurate. The level itself is dependent upon many factors including rapid breakdown. C-Peptide is a very accurate measurement that doesn’t breakdown like insulin.
John
When I was tested. my (now ex-) endo did both, C-peptide and insulin tests along with anti-GAD65 test. That first time, the Assay of insulin came out in the normal range for the lab, c-peptide came out at 1.2 at a lab where normal (fasting) was 0.7 to 2.2 – however, the test was not done fasting - rather it was around 90min after meal. The antibodies test was negative. Doc’s analysis was that it “clearly” indicated Type 2, though to me, those results are clear as mud.
Five months later, while trying to get a Dexcom approved, c-peptide alone was repeated - fasting this time. Different lab this time. Result was 0.7 and the normal range 1.1 to 4.4. A year later, c-pep (same as lab as 2nd time) was 0.6.
Overall, I’m not sure what to make of the results, other than to perhaps test again one day. My current endo’s attitude is that as I’m already taking insulin, there will be no substantive change in treatment, so why worry about it…
This is a common problem. Endos will do tests because you are in their office at the time and ignore the fasting.
John
Of course, that makes a big difference in interpretation of the results. At the time of the first c-peptide test, I was on a very low TDD of insulin. Shortly after a meal - one that included a sandwich, so not low-carb - I think a typical Type 2 might expect to see a somewhat high number both c-peptide and insulin (though that test is, as you said, @DrJohn, unreliable), rather than in the low-to-middle of the normal range. In my case, the numbers were quite normal; my BG at the time was 101. Other than that the results were “below-normal,” the second and third tests are statistically similar, so I don’t know if I have a trend to produce less and less insulin or not.
I’m a bit of a needle-phobic, so I hate blood tests – even more-so when they’re taken “wrong” and provide usless information.
This is pretty much my doctors position on it too-- though i do have a lab referral from him to retest gad65 and cpeptide next month… He considers it prett much just an exercise of intellectual curiousity— which kinda makes sense, after all at this point what difference does it really make what we call it?
Ps your case sounds eerily similar to mine, although my original gad65 lab showed very very small positive results… I questioned him as to if there are such things as false positives, is the level of gad65 lab meaningful or is it just “yes/no” etc. he agreed to retest it.
Blood draws don’t bother me… and never have. Shots still do, but I’ve realized for me it’s less about the pain and more about not being in control of it. I can give myself a shot no problem - but someone else does it, and all I want to do is freak out.
I’m finding this thread interesting. When I was misdiagnosed with T2 following a pregnancy where they were convinced I just had GD (despite being young, no family history of T2, not overweight, no other risk factors, and being diagnosed substantially earlier than is typical with MUCH higher BG’s than most women see with GD), and my c-peptide was only 0.7, which was only just above the lab threshold of 0.6. My fasting BG was well over 300 at the time (I was off insulin at the time, because this doctor told me to stop taking it), and the doctor I was seeng at the time was convinced I couldn’t possibly be a T1 because I was still making “some” insulin. She wasn’t my doctor for long - I fired her after she yelled at me in the office for treating high BG’s with insulin because I wasn’t giving the oral meds she prescribed “time to work”. My next doctor did confirm with antibody testing (which was very positive) that I was T1… at that point I was taking insulin (on my own since R and NPH were available without an RX) and my c-peptide came back at 0.3 and things got much better in terms of how my treatment was approached.
I’ve been curious for a while about what my c-peptide is now, 15 years later, and I might ask that it be repeated just to satisfy my curiosity (mostly I just want to see if it dropped off to nearly nothing). If my doctor won’t do it, it’s only about $30 from most directlabs type places… I’d be willing to spend that to find out 
Still having a lot of insulin production (and often not needing meal-time insulin) is what is lovingly referred to as “the honeymoon” phase. You know, everything is semi-hunky-dory and your diabetes doesn’t make you work so hard to get a decent A1C. Your diabetes can do no wrong and you still think everything it does is cute.
After eighteen months of marital bliss (only basal insulin), I woke up to a nasty, snarling beast who was not happy, because it had an A1C of 10.something%. My metformin was not enough and my pancreas could not be “persuaded” (flogged) to put out more insulin. Suddenly that big, red sharps container that should last me a year won’t last me a season. You’re now at the, “yes, dear” phase of diabeetus.