I'm confused as usual. There have been a number of threads about testing for insulin production and insulin resistance, many of which say that the tests are not available on the NHS and you have to arrange for blood to be drawn at your surgery then send it off to a laboratory. For example https://www.medichecks.com/diabetes-tests/insulin-resistance-test which claims to run 3 tests (but doesn't seem to say what they are in detail). However there is also much mention of the c-peptide test. http://www.diabetes.co.uk/c-peptide-test.html. The description says: " What is C-peptide? C-peptide is released at the same time as insulin. For each molecule of insulin produced there is a molecule of c-peptide. C-peptide does not itself influence blood sugar. C-peptide is a useful marker of insulin production because c-peptide tends to remain in the blood longer than insulin. " As I understand it, a c-peptide test (along with GAD) is the standard method of distinguishing between T1 and T2 on or after diagnosis. As such it must surely be a standard test available under the NHS. The description looks pretty good - an accurate and longer term marker for the level on insulin production. So what are the tests for insulin production and insulin resistance which are not a c-peptide test and are not available under the NHS? How are they better than c-peptide? Supplementary: I am on LCHF. I read on this forum that if you are on long term LCHF and in nutritional ketosis your insulin production drops, so much so that if you need an OGTT you should increase your carbohydrates for a few days before hand, to kick start your beta cells to produce more insulin so that you don't get a false reading when you take the OGTT. Given that, would a c-peptide test for someone on LCHF show a misleadingly low reading? Should you increase the carbohydrates for a couple of days before the test in the same way you do for the OGTT? Or doesn't it matter? My concern is to establish if I am insulin resistant. Would someone insulin resistant automatically be running high c-peptide levels?