How Type 1 Diabetes Is Diagnosed

Doctor showing young patient how to use diabetes pen

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Type 1 diabetes (also known as diabetes mellitus or insulin-dependent diabetes) is an autoimmune disease in which the pancreas does not produce enough insulin, a hormone that allows cells in the body to take in glucose for energy.

Glucose is a sugar that's produced as the breakdown product of digested food, especially carbohydrates. Without insulin, glucose is unable to enter the cells and builds up in the bloodstream, eventually causing symptoms including increased thirst, frequent urination, and fatigue. If left untreated, type 1 diabetes can have serious and even life-threatening complications.

The disease most often affects children and teenagers (and so at one time was known as juvenile diabetes), but it can develop in adults. Diagnosing type 1 diabetes involves several steps, including measuring the amount of glucose in a sample of blood.

type 1 diabetes diagnosis
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At-Home Testing

Although there are devices you can buy over the counter to measure the level of glucose in your blood, they are meant to be used to monitor blood sugar levels as part of managing diabetes for those who have already been diagnosed.

Blood glucose monitors are not viable instruments for diagnosing type 1 diabetes at home, warns the American Diabetes Association.

Attempting to self-diagnose type 1 diabetes can interfere with proper treatment. By the time you begin to experience symptoms, it's likely that you have begun to develop potentially permanent complications.

Labs and Tests

The blood tests that you have at your annual physical include a fasting blood glucose, which is a screening test that may be abnormal if you have diabetes. If your blood sugar is high or if you have risk factors of diabetes (such as a family history of the condition), you may need to have another test as well.

To that end, there is an international program for asymptomatic family members of people with type 1 diabetes called TrialNet that provides free risk screening for relatives of people with type 1 diabetes.

Fasting Blood Glucose Test (FBG)

This is the most commonly used test for diagnosing diabetes. A fasting blood glucose test (FBG) involves taking a blood sample after a period of fasting for at least eight hours. You can have it done first thing in the morning so that the fasting part of the test takes place overnight. Most people advised to not eat or drink anything (except sips of water as necessary to quell thirst) after midnight of the day before the test.

The test itself is simple and straightforward. It will take place at a lab, hospital, or doctor's office. A lab technician, nurse, or phlebotomist (a medical professional trained to draw blood) will insert a needle into a vein in your arm and extract a small vial of blood. This part of the test will take about five minutes. Fasting and having your blood drawn may cause weakness or light-headedness, so it's advisable to have a snack before leaving the testing center.

The blood sample then will be sent to a lab to be analyzed. The results may come back as soon as that day or it may take a week to get them.

The results of an FBG will be expressed as milligrams of glucose per deciliter of blood (mg/dL). A reading of 126 mg/dl or higher indicates diabetes.

To confirm the diagnosis, it usually is necessary to repeat the test a second time on a different day.

Random Blood Glucose

A random blood glucose test has some similarities to a fasting blood glucose test, except that the blood sample is taken without fasting or consideration of when a person last ate or drank a beverage other than water. It might be needed in emergency situations such as prior o surgery or when someone has such high glucose levels they may be drifting into a diabetes-induced coma. The results of this test can be available within minutes. A glucose level of more than 200 mg/dl indicates diabetes.

Oral Glucose Tolerance Test (OGTT)

The oral glucose tolerance test evaluates the body's management of glucose after a meal. To do this, the person being tested will be required to fast for eight to 12 hours prior to the test, which likely will be scheduled for first thing in the morning.

Upon arrival at the testing site, a small vial of blood will be drawn; it will serve as a baseline reading of blood glucose. You will next need to consume a very sweet drink—an 8-ounce solution consisting of 75 grams of sugar.

Over the next two hours, your blood glucose levels will be taken every 30 minutes. This will provide a picture of how your glucose level changes over time in response to the flood of sugar in the bloodstream. A sharp rise in glucose that is sustained over the two-hour test period is an indication the pancreas isn't delivering the insulin needed to normalize blood glucose.

Diagnosis: Diabetes?

A final blood glucose level of 200 mg/dL or more after an OGGT indicates diabetes. Typically the test will be repeated on a different day to confirm the diagnosis.

A1C Test

The A1C test is a measure of a person's average blood glucose levels over a two-to-three month period. Other names for this test are hemoglobin A1C, HbA1C, glycated hemoglobin, and glycosylated hemoglobin test. It's used most often to diagnose type 2 diabetes but sometimes is performed to diagnose type 1 diabetes.

Rather than providing a straightforward measurement of the amount of glucose in the blood, the A1C test looks at a protein called hemoglobin A that's found in red blood cells. Glucose can glycate (stick to) to this protein. The more glucose present in the blood, the higher the percentage of glycated hemoglobin proteins in blood as well.

Once glucose sticks to a hemoglobin protein, it typically remains there for the lifespan of the hemoglobin A protein (as long as 120 days). This means that, at any moment, the glucose attached to the hemoglobin A protein reflects the level of your blood sugar over the last two to three months.

An A1C result of 6.5 percent or higher is an indication of diabetes. The higher the percentage, the higher the glucose levels in the blood.

Certain people aren't likely to get accurate results from an A1C test:

  • Those who have anemia or other problem with their blood.
  • Those of African, Mediterranean, or Southeast Asian descent; their test results may be falsely high or low. 

Autoantibody Test

Once diabetes is diagnosed it's not always clear if it is type 1 or type 2. Since the former is believed to be an autoimmune disease, in which antibodies attack the beta cells of the pancreas, a follow-up test to detect the presence of antibodies may be helpful in determining which type of diabetes a person has. This test involves taking a small sample of blood that will be examined in a lab to look for one or more of several types of antibodies specific to diabetes.

A Word From Verywell

While a diagnosis of type 1 diabetes can be scary, it can be made fairly easily, swiftly, and accurately. This means that treatment, which will be focused on taking supplemental insulin to replace the insulin the pancreas is no longer able to make, can begin right away. This, along with diet and straightforward lifestyle changes, will allow you to begin managing your diabetes and feeling better as soon as possible.

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Article Sources
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  7. Ismail HM, Xu P, Libman IM, et al. The shape of the glucose concentration curve during an oral glucose tolerance test predicts risk for type 1 diabetes. Diabetologia. 2018;61(1):84-92. doi:10.1007/s00125-017-4453-6

  8. NIH National Institute of Diabetes and Digestive and Kidney Diseases. Diabetes blood tests for people of African, Mediterranean, or Southeast Asian descent. Updated October 2011.

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