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 stops producing insulin, a hormone that allows cells in the body to take in glucose for energy. Glucose is a simple sugar that's produced when food is digested. Without insulin, glucose is unable to enter the cells and instead builds up in the bloodstream, eventually causing symptoms including increased thirst, extreme hunger, and frequent urination. 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 measuring the amount of glucose in a sample of blood.

At-Home Testing

Although there are devices you can buy over the counter that measure the levels of glucose in blood, they are meant to be used only by people who have already been diagnosed to monitor their blood sugar levels as part of managing the disease.

Blood glucose monitors are not viable instruments for diagnosing type 1 diabetes, warns the National Institute of Diabetes and Digestive and Kidney Diseases (NIKKD). Only by having at least one of the blood tests discussed below can a doctor determine if you have diabetes.

Attempting to self-diagnose type 1 diabetes can delay starting treatment, which could put you at risk of developing serious and even life-threatening complications of the disease: By the time you begin to experience symptoms, it's likely your pancreas has already begun to shut down.

Labs and Tests

According to the NIKKD, only when a person experiences symptoms of type 1 diabetes will they undergo any of the blood tests that follow. However, because diabetes runs in families, it can be useful to be screened for the disease if someone in your family already has been diagnosed.

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. It usually is 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 your 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. Sometimes the combination of having fasted and having blood drawn causes 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.

Understanding the Numbers

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 is very much like a fasting blood glucose test. The main difference is that a blood sample is taken without fasting or consideration of when a person last ate or drank a beverage other than water. It's typically used in emergency situations when someone (usually a child) 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 manages 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. The person will next need to consume a very sweet drink—an 8-ounce solution consisting of 75 grams of sugar.

Over the next two hours, his or her blood glucose levels will be taken every 30 minutes. this will provide a picture of how they change 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 stick (glycate) 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 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 necessary to determine which type of diabetes a person has. This 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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