1. Blunted insulin secretion,
2. A resistance to the action of the same, or
3. A mixture of both.

The vast majority of patients are diagnosed with one of the following categories: diabetes mellitus type 1, produced by an absolute deficiency in insulin secretion, diabetes mellitus type 2, caused by a combination of resistance to insulin action, and inadequate compensatory response of the pancreas.

Diagnostic Criteria
The criteria for diagnosis of diabetes mellitus have recently been reviewed by a panel appointed by the American Diabetes Association (ADA) and the World Health Organization (WHO). There are three different criteria to diagnose diabetes:

1.The presence of classic symptoms (polydipsia, polyphagia, polyuria and weight loss), with the incidental finding without considering the time elapsed since the last meal, a blood glucose (blood sugar) levels above 200 mg / dl ( 11.1 mmol / l).
2. A fasting glucose 126 mg / dl (7 mmol / l).
3. The presence of glucose levels above 200 mg / dl (11.1 mmol / l) in an analysis of two hours after an oral glucose load of 75 grams (test performed according to WHO criteria).

The isolated finding of either of these criteria is not sufficient to establish the diagnosis. Should be confirmed in the following days the same, or any of the other two.