People may first suspect diabetes if they experience warning signs, such as extreme thirst or unexplained weight loss. For some individuals, the condition may not be diagnosed until they experience severe symptoms. Diabetic ketoacidosis, a life-threatening condition that can lead to a diabetic coma, may be the first indication that an individual has type 1 diabetes. Because this is a dangerous condition, early symptoms of diabetes should not be ignored.

Children and adolescents who are considered at high risk for diabetes should be screened every two years after the age of 10 or at the time of puberty if puberty occurs at an earlier age. Physicians may complete more extensive tests on children and young adults if they are in a high-risk group or exhibit signs of diabetes.

There are several diagnostic tests that are administered to measure the glucose (sugar) level in the blood. Although the amount of glucose in a person’s blood varies, the range is relatively narrow. After fasting all night, most people have levels between 70 and 100 milligrams of glucose per deciliter of blood (mg/dL). If an individual consistently has a fasting glucose level greater than 126 mg/dL, it is likely that person needs to be evaluated for diabetes.

Common tests for diagnosing type 1 diabetes include:

• Fasting plasma glucose test (FPG).  A sample of blood is taken from a fingerstick after an individual has not had anything to eat or drink for eight to 10 hours. The blood is tested for the amount of glucose that is present in the sample. If the glucose level is elevated, the physician suspects diabetes. Physicians usually make a firm diagnosis of diabetes when two FPG tests, done on different days, are elevated.
  
  
• Random plasma glucose test (RPGT). This test may be performed when a person has a routine physical examination. A sample of blood is taken from a vein and tested for the amount of glucose present in the blood. This is not done after fasting, so the glucose in the blood may be high. However, the level should not be over 200 mg/dL. If the level is higher than 200 mg/dL, diabetes is a very likely diagnosis. An FPG test may be ordered to confirm the diagnosis.
  
  
• Oral glucose tolerance test (OGTT). This test requires an individual drink 8 ounces of a sweet sugar solution after fasting for six hours. The blood glucose is measured before drinking the liquid, then every hour for a three-hour period. If blood glucose rises more than 200 mg/dL and does not return to a normal level, diabetes is suspected. This test is also routinely performed for gestational diabetes.

Once the glucose tests confirm diabetes, the physician must determine if the individual has type 1, type 2 diabetes or another form of diabetes, such as maturity-onset diabetes of the young (MODY). The physician will check the medical history to determine if there are other members of the family with either type 1 or type 2 diabetes. A blood test called the C-peptide test can determine if the pancreas is producing insulin. C-peptide is a byproduct of insulin production.

If the patient’s pancreas is not producing insulin, c-peptide levels will be near zero. If the patient’s blood has higher-than-normal C-peptide levels, type 2 diabetes will be diagnosed (because the pancreas is producing insulin, but the body is unable to use it efficiently).

Autoantibody tests can also help distinguish type 1 from other forms of diabetes.

Patients who are diagnosed with type 1 diabetes, especially pregnant women, may wish to ask about having a thyroid blood test because they are at greater risk for some thyroid diseases.