I was diagnosed in 1999 with type 2 diabetes. My blood sugar was over 400 mg/dl [22.2 mmol/L]. Later, I was referred to an endocrinologist. My general practitioner (GP) says I have type 2 diabetes. The endocrinologist says I am type 1.5, but he has to say type 1 because of insurance. My fasting C-peptide is 0.4. What type am I? Do I really need insulin? Should I have a two hour postprandial C-peptide?

Your C-peptide is low, similar to a patient with type 1 diabetes. However, the C-peptide is best interpreted in light of the simultaneous glucose level. That being said, some that are diagnosed with a latent form of type 1 diabetes that may actually look like type 2 diabetes at the time of diagnosis. It is over time that beta cell function and insulin secretion is lost to the point that insulin replacement therapy is required. Additional studies that might be helpful include antibody tests, such as anti-GAD AB, anti-insulin AB. The anti-GAD stays elevated the longest and, if positive, would be a marker of type 1 diabetes. If you really have type 1 diabetes, and your C-peptide is low, you need to be on insulin therapy.

JTL