The World Health Organization (WHO) and many other organizations around the world such as National Diabetes Data Group of United States have issued diagnostic criteria for diabetes mellitus based on certain facts such as (1) the spectrum of fasting plasma glucose (FPG) and the response to an oral glucose load (OGTT or oral glucose tolerance test) varies among different normal individuals, and (2) diabetes mellitus should be diagnosed if the level of blood glucose is at or higher, at which diabetes-specific complications occur.

For example the criteria for diagnosis of diabetes mellitus given below may not be applicable to Native Americans (Pima Indian population) because the prevalence of retinopathy (a complication of diabetes) begins to increase at fasting plasma glucose of more than 6.4 mmol/Liter (116 mg/100 ml of blood).

Diabetes can be called an iceberg disease, as there are many individuals with diabetes who do not know they have diabetes, because they do not have any diabetes symptoms. By the time diabetes becomes symptomatic it may take several years. So screening is an important tool for diagnosis of diabetes among asymptomatic individuals and for prevention of complications and morbidity due to diabetes. It is to note that approximately 50% of patients have some diabetes related complication at the time of diagnosis, especially in type-2 diabetes, hence screening is very important.

Testing of Fasting Plasma Glucose (FPG) for screening of diabetes is recommended, especially for type-2 diabetes, because (1) there are large number of people who meet the criteria for diagnosis of diabetes, but unaware that they have diabetes, as they are asymptomatic, (2) approximately 50% of individuals with type-2 diabetes have one or more diabetes complications at the time of diagnosis (3) epidemiologic studies have proved that type-2 diabetes may be present asymptomatically for up to a decade before diagnosis made and (4) appropriate management of type-2 diabetes can favorably alter the natural history of diabetes and become beneficial for the patient.