Diagnosis of Type 1 Diabetes

The doctor will ask about your symptoms, medical and family history, and perform a physical exam. He or she will also do some lab tests. There are three main tests used to diagnose diabetes. A diagnosis is made based on symptoms combined with a positive result on any one or more of these three tests and a second positive test on a different day.

The tests are:

Random Plasma Glucose Test—This is taken any time of day, without regard to when you have last eaten. A sample of your blood is taken and the blood glucose level is measured. A measure of 200 milligrams per deciliter (mg/dl) [11.1 mmol/L] is considered a positive result.

Fasting Plasma Glucose Test—This is considered a more accurate test. You will need to fast for eight hours before this test is done. A sample of your blood is taken and the blood glucose level is measured. On this test, a measure of 126 mg/dl (7 mmol/L) is a positive result.

Two-hour Glucose Tolerance Test (OGTT)—After a 3 day intake of a diet consisting of at least 150 grams of carbohydrate, you’ll be asked to fast overnight (between 8-16 hours). The test is generally done in the morning, in your doctor's office.

A blood sample will be obtained from you to measure blood sugar. Then, you will consume a drink that contains 75 grams of glucose dissolved in water. Two hours later, another blood sample will be obtained to measure blood sugar. If you have diabetes, your blood glucose level will rise higher than normal and remain high for a much longer time than is normal. A measure of 200 mg/dl (11.1 mmol/L) or above at two hours is considered a positive test. Because this test is cumbersome, some experts recommend that it not be used routinely.

Other Commonly Ordered Tests—After the diagnosis of diabetes is confirmed, your doctor will most likely order the following tests:

  • Glycosylated hemoglobin (HbA1c)—a test which measures how well your blood sugar is controlled in the previous 2-3 months
  • Urine microalbumin—to see if there is any damage to your kidneys
  • Thyroid function tests
  • Blood lipids levels (total, LDL and HDL cholesterol, triglycerides)
  • Renal function tests including serum creatinine and potassium

Other Less Commonly Ordered Tests—These tests may include: plasma insulin levels, islet cell antibodies, and plasma C-peptide.


American Diabetes Association website. Available at: http://www.diabetes.org/home.jsp.

Diagnosis and classification of diabetes mellitus. Daibetes Care. 2005;28(Suppl 1):S37-42.

National Institute of Diabetes & Digestive & Kidney Diseases website. Available at: http://www.niddk.nih.gov/.

Last reviewed April 2007 by David Juan, MD

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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