The Tests Done Before Genetic Testing

Genetic testing is not the first step. Two much cheaper blood tests usually come first, and they are what justify going further.

Islet autoantibodies. These are the markers left behind when the immune system attacks the insulin-producing cells. If they are present, the diabetes is autoimmune — Type 1 or LADA — and monogenic diabetes becomes unlikely. If they are absent in someone labelled Type 1, that is a genuine finding worth pursuing.

C-peptide. This measures how much insulin your own body is still producing. It is the most useful single test here. Type 1 values fall to near zero over time. Type 2 values are normal or high. Monogenic values sit in between — in one Asian Indian study, fasting C-peptide averaged around 1.5 ng/ml and stimulated C-peptide around 3.9 ng/ml in confirmed MODY, between the Type 1 and Type 2 groups.

That study found the stimulated C-peptide discriminated better than clinical features alone, with suggested referral ranges of roughly 1.2 to 2.1 ng/ml fasting and 2.1 to 4.5 ng/ml stimulated.

C-peptide should be measured with a glucose sample taken at the same time, because the result only means something in the context of what your glucose was doing.

If antibodies are negative and C-peptide shows preserved insulin production years into diabetes, that combination is the standard basis for genetic testing.