The right classification for diabetes mellitus (DM) allows a more adequate treatment and comprises four categories: type 1 DM, type 2 DM, other types, and gestational diabetes. In some cases, there might be a superposition of situations, especially with regard to the DM that initiates in the young adult or is initially presented with diabetic ketoacidosis intermediately to type 1 and 2 DM. Thus, additions to the classic classification system have been proposed as assessing the presence of autoimmunity (antibody) and b cell function (C-peptide) to precisely define the subtypes. The aim of this literature review was to analyze these diagnostic indexes’ performance in the DM classification and to describe subtypes with details. The antibodies against pancreas confirm autoimmunity, and the antibody against insulin is more accurate before 5 years old, while the anti-glutamic acid decarboxylase is more accurate after 20 years old, a test which remains positive for a longer period. The measurement of C-peptide evaluates the pancreatic insulin reserve, and the most largely used methods of stimulation are the measurement after meals or after intravenous glucagon. C-peptide values < 1.5 ng/ml define a patient with absent pancreatic function and, above this value, patients with preserved function. When the presence of antibodies (A+) directed to the pancreas is combined to its insulin secretion capability (β+), it is possible to subdivide DM’s classification in type 1A (A+β-) and 1B (A+β-); and type 2A (A+β+) and 2B (A-β+), which allows a more precise classification and treatment besides opening horizons for the understanding of DM pathogenesis.
Diabetes mellitus; glutamic acid; carboxy-lyases; C-peptide
