People with these risk factors should be screened for diabetes at least once every three years. Diabetes risk can be estimated using online risk calculators. Doctors may measure fasting blood glucose levels and hemoglobin A1C level, or do an oral glucose tolerance test. If the test results are on the border between normal and abnormal, doctors do the screening tests more often, at least once a year.
Rates of type 2 diabetes have increased markedly since 1960 in parallel with obesity. As of 2015 there were approximately 392 million people diagnosed with the disease compared to around 30 million in 1985. Typically it begins in middle or older age, although rates of type 2 diabetes are increasing in young people. Type 2 diabetes is associated with a ten-year-shorter life expectancy. Diabetes was one of the first diseases described. The importance of insulin in the disease was determined in the 1920s.
Environmental factors are important, because even identical twins have only a 30-60% concordance for type 1 diabetes mellitus and because incidence rates vary in genetically similar populations under different living conditions.  No single factor has been identified, but infections and diet are considered the 2 most likely environmental candidates.
By simultaneously considering insulin secretion and insulin action in any given individual, it becomes possible to account for the natural history of diabetes in that person (e.g., remission in a patient with T1 diabetes or ketoacidosis in a person with T2DM). Thus, diabetes mellitus may be the result of absolute insulin deficiency, or of absolute insulin resistance, or a combination of milder defects in both insulin secretion and insulin action.1 Collectively, the syndromes of diabetes mellitus are the most common endocrine/metabolic disorders of childhood and adolescence. The application of molecular biologic tools continues to provide remarkable insights into the etiology, pathophysiology, and genetics of the various forms of diabetes mellitus that result from deficient secretion of insulin or its action at the cellular level.
Most cases of diabetes involve many genes, with each being a small contributor to an increased probability of becoming a type 2 diabetic. If one identical twin has diabetes, the chance of the other developing diabetes within his lifetime is greater than 90%, while the rate for nonidentical siblings is 25–50%. As of 2011, more than 36 genes had been found that contribute to the risk of type 2 diabetes. All of these genes together still only account for 10% of the total heritable component of the disease. The TCF7L2 allele, for example, increases the risk of developing diabetes by 1.5 times and is the greatest risk of the common genetic variants. Most of the genes linked to diabetes are involved in beta cell functions.
Aspirin should be used as secondary prophylaxis in all diabetic people with evidence of macrovascular disease, and it should be strongly considered as primary prevention in diabetic subjects with other risk factors for macrovascular disease, such as hypertension, cigarette smoking, dyslipidemia, obesity, and albuminuria (macro or micro).228 Because of the platelet defects associated with diabetes, it is recommended that the dose of aspirin should be 300 mg per day,228–230 although the American Diabetes Association’s position statement (http://www.diabetes.org/DiabetesCare/supplement198/s45.htm) advocates a dose of 81 to 325 mg enteric-coated aspirin per day. If the patient cannot tolerate aspirin, then clopidogrel231 can be used.
Diabetes can also be diagnosed if a blood glucose level taken any time of the day without regards to meals is 11.1 mmol/L or higher, plus you have symptoms characteristic of diabetes (e.g., increase thirst, increase urination, unexplained weight loss). A doctor may also examine the eyes for signs of damage to the blood vessels of the retina (back of the eye). Finally, diabetes mellitus is diagnosed if the 3-month cumulative blood sugar average test, known as hemoglobin A1C or glycated hemoglobin, is 6.5% or higher.
Diabetes that's triggered by pregnancy is called gestational diabetes (pregnancy, to some degree, leads to insulin resistance). It is often diagnosed in middle or late pregnancy. Because high blood sugar levels in a mother are circulated through the placenta to the baby, gestational diabetes must be controlled to protect the baby's growth and development.
According to the Mayo Clinic, doctors may use other tests to diagnose diabetes. For example, they may conduct a fasting blood glucose test, which is a blood glucose test done after a night of fasting. While a fasting blood sugar level of less than 100 milligrams per deciliter (mg/dL) is normal, one that is between 100 to 125 mg/dL signals prediabetes, and a reading that reaches 126 mg/dL on two separate occasions means you have diabetes.