C-Peptide Test Question

I’m a LADA type 1 diagnosed May of 2018. At diagnoses I had a C-Peptide of 0.92

Last month (15 months later) I had a fasting C-Peptide of 0.35.

I’m planning to pay for another C-Peptide on my own but this time not fast.

My thoughts are because I was fasting and I also take basal/bolus insulin there was little need for my body to make much insulin at the time leading to the 0.35 rest result. At test time my fasting glucose was 80.

My theory is if I eat something prior to the blood draw causing my sugar to rise my body will respond making what insulin it can and leading to a higher C-Peptide result.

My Question:
How long before the blood draw do I need to eat for it to make a detechable change in C-Peptide? (I’m guessing an hour should be enough).

Thank you for your help.

1 Like

@ChrisP It might depend on what you eat. If you eat a spaghetti dinner, is your BG at its highest one, two, or three hours later? When you test the cPeptide, include the BG too.

Just enough to spike your blood sugar… I lived about 15 minutes from the lab so I just drank a bottle of apple juice on the way there…

Fasting c peptides are essentially meaningless unless they’re high, or near zero and you’re not taking insulin… that’s why they don’t generally keep doing them after the diagnosis stage…

A challenged c peptide is much more meaningful. I tried to get my bg up to 200+

1 Like

I’m curious, Chris…why are you trying to raise your C-peptide results?

It seems to me that it matters if you are still making any insulin, whether it can be done.

1 Like

There are two reasons.

  1. I’d like to monitor my rate of decline in insulin production
  2. My doctor said at some point insurance may stop covering Januvia/Jardiance which I currently take since those are only type 2 medications.

I suspect my actual C-Peptide is higher then 0.35 so I wanted to re-test to see if that is correct.

1 Like

I’m certain I am making insulin still. Since I started on insulin 15 months ago my daily insulin requirements have not changed much. So my expectation is I might be making less then my starting point 0.92 but I don’t think I’m down to 0.35 either.

Just for fun I’ll guess its more like 0.7 - 0.85. I will likely get the test in the next 10 days so we’ll see.

1 Like

Your c peptide level will never be possible to exactly pin down. The lab level will depend on not only your bodies ability, but also the amount of exogenous insulin you’re injecting, and also the demand for insulin production at that given moment… that’s why they don’t continue to track it indefinitely, as it becomes meaningless almost… the only remaining value it has at this point would be to jack your blood sugar up through the roof and see if it elevates accordingly… which we basically already know that it won’t… because you have diabetes

But yeah c peptide after your diabetes is under control and you’re on insulin is a meaningless measure unless it’s sky high or zero… and we basically know that yours won’t be either of those

Thanks for the explanation, Chris. As a T2, I had to prove my C-peptide was low in order to qualify with Medicare to keep getting insulin pump supplies.

How do you get a c-peptide test when you doctor refuse to run one?

  1. Offer to pay for the test
  2. Ask for a referral to another doctor or lab that will run test
  3. Find another doctor

Thanks CJ114. I haven’t had a c-peptide test ran since my diagnosis which was back in 2013 by another doctor and I’ve had two doctors since then. Whenever I would ask they’d say I don’t need to test again because they already knew I was a diabetic. :roll_eyes:

My readings are constantly all over the place, the endo before wanted me to have bariatric surgery. The PCP I have now says as long as I’m running under 200 I’m o.k. and to eat keto and fast which I don’t think are optimal good answers—had a T1 diabetic mom, (her parents) T1 grandfather, and T2 grandmother.

I thought I found the perfect internist but she doesn’t accept patients unless referred from other doctors and she doesn’t really like my insurance said my receptionist (BCBS—PPO, but with referrals). So frustrating. I had an internist when I was young, but it’s hard to find those types of doctors anymore.

I’m planning to use this lab service.


You pay them and they order the lab. At least for me it looks like they still do the blood draw at my local Quest office. You can order any test you want on your own.

1 Like

@Kate25 are you saying your doctor said as long as your sugar is under 200 you’re OK?

Reading your comment its not clear if your saying your diagnosed as Type 1 or Type 2.

Either way averaging around 200 is likely to cause long term health issues.

I was diagnosed at T2 in 2013; however, another endo diagnosed me as steroid diabetes due to the steroids and other meds I take for my RA, but would not rule out LADA since I have other autoimmune issues.

I used to have large swings from 80s to 260s. Then I leveled off to 90s - 140s for a while. Now I’m 120 - 160s on a regular basis. But no diabetic medications. Metformin & Met ER, Tulicity, Januvia–all made me extremely sick (extreme gastro problems and bad liver labs). The 3rd endo threw her arms in the air and said for me to have surgery.

The PCP so far hasn’t seemed to be concerned and just said to stay on a keto diet and fast. But I’m still have the BG creep. She said as long as I stay below 200 not to worry. And my RA is out of control too…it’s getting to be too much.

This past weekend I pretty much crashed in bed all weekend because I felt so bad…not hungry just so tired. I go from 89 to 140 from eating two boiled eggs and shredded cheese. That’s all I could get down.

Thanks so much ChrisP for the information. I will check into it definitely.

Have you ever been tested for diabetes related antibodies?

GAD65, IA-2 etc…

Not everyone has them but if you do along with low c-peptide that could confirm type 1

I don’t think so. I don’t think they tested me. Just made the decision I was type 2.based on my a1c and blood glucose.

My mom was a brittle diabetic who died from a hypoglycemic coma. :confused:

I’ve read stories on this forum many times of adults being diagnosed as type 2. Then over months/years their treatments are not working/helping and they finally find out they were slow developing type 1 the whole time.

Also some type 2’s could benefit from Basel insulin depending on their situation.

If you can I would suggest discussing this with your doctor.

Ask them to check your C-Peptide and GAD65,IA-2 Antibodies.

If you have the antibodies and a low C-Peptide that is a sign your a type one.

Some type 1’s don’t have the antibodies so if you have a low C-Peptide that alone could be enough to treat you as a type one.


Ok…thanks Chris I’m going to have to do the testing on my own. I know my current PCP will not do the testing and will not refer me to get it done.

Others here have suggested that I need to get a better doctor. I’ve been trying to get chanced over to an internist that’s on my insurance but no avail—either they aren’t taking new patients or they won’t take me on without a referral and become my primary.