"Type 1.5 diabetes" is a term I don't really use, because I somehow feel it gives people a shorthand without really explaining what underlies the condition. People use this term to mean diabetes in which there is significant, but not complete, deficiency of insulin production. Thus someone with "type 1.5 diabetes" doesn't have complete deficiency of insulin production, as does someone with true type 1 diabetes, but has lost so much insulin production that they cannot be effectively treated without insulin.
The reason I tend not to use this term is that this situation can be true of people who have both type 1 or type 2 diabetes, and I don't really think "type 1.5" reflects a specific additional form of diabetes. Some people with type 1 diabetes have a slow progression in the loss of insulin or retain enough insulin production that their own insulin can really help with glucose control, so they might end up being called "type 1.5." At the same time, many people with type 2 diabetes lose enough insulin production over time that they require treatment with multiple daily doses of insulin or an insulin pump just as does a type 1 individual, so they too might get the moniker "type 1.5." Although I don't object to someone using the term "type 1.5 diabetes," I think the important point is that the physician and the patient understand how the diabetes is likely to progress over time.
More Answers from Jack Merendino, MD