Table 3 Criteria for the diagnosis of prediabetes and diabetes mellitus

Prediabetes Impaired fasting glucose: plasma glucose (after 8 h of fasting) between 100 (5.6 mmol/l) and 125 mg/dl (6.9 mmol/l)

Impaired glucose tolerance: plasma glucose after 2 h of the OGTT between 140 and 199 mg/dl (7.8 mmol/l)

HbA1c between 5.7–6.4% (39–47 mmol/mol)

Type 2 diabetes Random glycemia ≥200 mg/dl (11.1 mmol/l) and symptoms suggestive of diabetes (glycosuria without ketonuria, polydipsia, weight loss). Confirmation with a second test is not necessary. If symptoms are lacking, diagnosis is made whether one of the following criteria is fullfilled:

 1. Fasting glycemia ≥126 mg/dl after 8 h of fasting.

 2. Glycemia ≥200 mg/dl after 2 h of the OGTT.

 3. HbA1c ≥6.5% or ≥ 48 mmol/l (IFCC reference method using high-performance liquid chromatography (caution in pediatric age).

If one test is positive, the diagnosis must be confirmed by a second test. Whenever the two tests are discordant, the patient should be strictly monitored and the positive test repeated within 3–6 months. If the diagnosis of diabetes is made, the assessment of the autoimmune markers (ICA, GAD, IA2, IAA o ZnT8) is needed to exclude type 1 diabetes.

Genetic screening for monogenic diabetes is recommended in the rare cases presenting with obesity, diabetes, negative autoimmunity tests and family history for T2D.