The term diabetes mellitus describes several diseases of abnormal carbohydrate metabolism that are characterized by hyperglycemia. It is associated with a relative or absolute impairment in insulin secretion, along with varying degrees of peripheral resistance to the action of insulin. Every few years, the diabetes community reevaluates the current recommendations for the classification, diagnosis, and screening of diabetes, reflecting new information from research and clinical practice.
The American Diabetes Association (ADA) issued diagnostic criteria for diabetes mellitus in 1997, with follow-up in 2003 and 2010 [1-3]. The diagnosis is based on one of four abnormalities: glycated hemoglobin (A1C), fasting plasma glucose (FPG), random elevated glucose with symptoms, or abnormal oral glucose tolerance test (OGTT) (table 1). Patients with impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT) are referred to as having increased risk for diabetes or prediabetes. (See 'Diagnostic criteria' below.)
Screening for and prevention of diabetes is reviewed elsewhere. The etiologic classification of diabetes mellitus is also discussed separately. (See "Screening for type 2 diabetes mellitus" and "Prevention of type 2 diabetes mellitus" and "Prevention of type 1 diabetes mellitus" and "Classification of diabetes mellitus and genetic diabetic syndromes".)
Type 2 diabetes is by far the most common type of diabetes in adults (>90 percent) and is characterized by hyperglycemia and variable degrees of insulin deficiency and resistance. The majority of patients are asymptomatic and hyperglycemia is noted on routine laboratory evaluation, prompting further testing. The frequency of symptomatic diabetes has been decreasing in parallel with improved efforts to diagnose diabetes earlier through screening (see "Screening for type 2 diabetes mellitus"). Classic symptoms of hyperglycemia include polyuria, polydipsia, nocturia, blurred vision and, infrequently, weight loss. These symptoms are often noted only in retrospect, after a blood glucose value has been shown to be elevated. Polyuria occurs when the serum glucose concentration rises significantly above 180 mg/dL (10 mmol/L), exceeding the renal threshold for glucose, which leads to increased urinary glucose excretion. Glycosuria causes osmotic diuresis (ie, polyuria) and hypovolemia, which in turn can lead to polydipsia. Patients who replete their volume losses with concentrated sugar drinks, such as non-diet sodas, exacerbate their hyperglycemia and osmotic diuresis.
Rarely adults with type 2 diabetes can present with a hyperosmolar hyperglycemic state, characterized by marked hyperglycemia without ketoacidosis, severe dehydration, and obtundation. Diabetic ketoacidosis (DKA) as the presenting symptom of type 2 diabetes is also uncommon in adults but may occur under certain circumstances (usually severe infection or other illness) and in non-Caucasian ethnic groups. (See "Diabetic ketoacidosis and hyperosmolar hyperglycemic state in adults: Clinical features, evaluation, and diagnosis" and "Syndromes of ketosis-prone diabetes mellitus".)