Diabetes is a condition where people don't produce enough insulin to meet their body's needs or their cells don't respond properly to insulin. Insulin is a protein that is made in the pancreas, which is a small organ situated close to your stomach. Insulin is important because it moves glucose, a simple sugar, into the body's cells from the blood. It also has a number of other effects on metabolism.
The food people eat provides the body with glucose, which is used by the cells as a source of energy. If insulin isn't available or doesn't work correctly to move glucose from the blood into cells, glucose will stay in the blood. High blood glucose levels are toxic and cells that don't get glucose are lacking the fuel they need. These two problems cause the symptoms of diabetes.
Diabetes is diagnosed by taking a blood glucose level. When screening for diabetes, the doctor might take what is called a "random" blood sugar level. This is a blood test that measures what your current sugar level is without preparation for the test. Normal levels are usually between 72 mg/dL and 144 mg/dL (4-8 mmol/L). If you are outside of those levels or if your doctor wants to test you for diabetes, he may order a fasting plasma glucose test, where you must prepare by not eating for at least 8 hours. Current standards suggest that if your fasting plasma glucose levels are 126 mg/dL (7 mmol/L) or higher, you have diabetes.
Sometimes your doctor may diagnose diabetes based on the random blood sugar level and whether you have any symptoms of diabetes, such as frequent urination, excessive thirst, and unexplained weight loss.
Doctors may also diagnose diabetes based on the results of an oral glucose tolerance test (OGTT). With this test, a person fasts and then is given some glucose, after which their blood glucose is tested at various times to determine how quickly the sugar travels out of the bloodstream and into cells. In an OGTT, if the blood glucose level taken 2 hours after 75 g of glucose is 200 mg/dL (11.1 mmol/L) or higher, you have diabetes.