Hiya, I have had both c peptide and GAD tests done.
I have an appointment next week with my consultant to discuss results but.....
I understand that c peptide is produced at same time as insulin but lasts longer in the body. So it gives a good indicator of how much insulin you are producing naturally. If it’s low, it’s likely you are not producing enough insulin so could suggest type 1. If you are producing lots of c peptide it would suggest you could be type 2 and are ‘insulin resistant’.
The GAD test looks for antibodies associated with ‘killing’ beta cells in the pancreas and therefore if you have these it’s likely type 1. Saying that, I think up to 25% of type 1 do not have these antibodies.
I think most of the time both tests are done together to get better results.