Tests for Type 1 Diabetes
If your doctor suspects type 1 diabetes, one or more of the following tests may be ordered.
Fasting Plasma-Glucose Test
A fasting plasma-glucose test measures blood sugar (glucose) levels after a fast. A fast means that you should have no foods or liquids other than water for at least eight hours before the test. During a fast, your blood glucose is supplied by your liver, not by your diet. The liver makes glucose and keeps it in storage. By the end of a fast your glucose levels are fairly constant. You won't have the "bounces" that can occur after meals.
At the end of the fasting period, a small sample of blood is drawn. The blood is tested to measure its level of sugar.
Normally after a fast, glucose is in a range of 70 to 100 milligrams per deciliter (mg/dl). People with type 1 diabetes do not make enough insulin to move glucose efficiently from the blood into cells. Therefore, the test will show a high blood sugar level. A fasting plasma glucose level of 126 milligrams per deciliter (mg/dl) or higher can confirm a diagnosis of diabetes.
A single plasma glucose test is not enough. Your doctor cannot make a firm diagnosis using this test until you have blood tests on two separate days. Both results must be consistent with diabetes.
A blood test called hemoglobin A1C can diagnose diabetes. Glucose molecules attach to hemoglobin, a protein in the blood. Only a fraction of your hemoglobin gets attached to glucose molecules. Hemoglobin proteins attach more readily when there is more glucose in the blood. The attachment lasts for several weeks. In some cases, it lasts for as long as the lifespan of the cell, two to three months.
Hemoglobin proteins that have bound glucose in this way are called A1C proteins. They can be identified by a blood test. This test tells you the percentage of your hemoglobin proteins that are in the A1C form. It can give an overall picture of your blood glucose control in recent weeks. The higher your A1C percentage, the higher your average blood sugar level has been.
A normal A1C result is less than 5.7%. If your result is at least 5.7% but no higher than 6.4%, you are in a range called "pre-diabetes." If you have an A1C that is 6.5% or higher, this diagnoses diabetes.
Oral Glucose Tolerance Test
This test measures your body's response after you have a sugary drink. The oral glucose tolerance test also is performed after a fast of at least eight hours. Blood is drawn. Then you drink a liquid containing a measured amount of glucose (usually 75 grams). Blood is drawn again one or more times after you consume the drink.
In a person without diabetes, blood-sugar levels typically rise within the first hour. Then they quickly return to normal levels. In a person with diabetes, the blood sugar reaches a higher level. It also takes longer to drop back to normal. A diagnosis of diabetes is made if your blood sugar level is still at least 200 mg/dl two hours after you have the sugary drink.
Random Plasma-Glucose Test
This test measures blood glucose, but there is no need to fast ahead of time. A reading of 200 mg/dl or higher suggests diabetes. However, this test is less reliable than the fasting tests.
If you have symptoms that suggest diabetes, then a random glucose test with a result of 200 mg/dl or higher is enough to make a diagnosis of diabetes. Examples of typical symptoms are:
- Frequent urination
- Unusually frequent thirst
- Weight loss
- New blurring of vision
Usually, if you don't have obvious symptoms, you will need one of the fasting tests to confirm a diagnosis of type 1 diabetes.
Urine can be tested for glucose. Normally, urine does not contain any glucose. However, when blood sugar is high, some may pass into the urine as blood is filtered by the kidneys. Glucose in the urine can be a sign of diabetes. However, this test is not as accurate as blood glucose tests for diagnosing diabetes.
Urine also can be tested for the presence of ketones. Your body produces ketones when it uses fat to get energy. This occurs when you don't have enough insulin to help you use glucose for energy. A test for ketones is not used to diagnose diabetes. It can be important for other reasons, however. It can signal that diabetes may be getting out of control.
Type 1 diabetes results from damage to your pancreas, where insulin is made. This damage usually occurs when your own immune system attacks the pancreas. If you have type 1 diabetes, sometimes a doctor can find antibodies in your blood that are a part of this process.
Antibody tests are not usually needed for diagnosis of type 1 diabetes. Antibody tests can be used to confirm the diagnosis of type 1 diabetes, if the type is not clear. People who make antibodies that damage the pancreas also can make antibodies that damage the thyroid gland. For this reason, it is helpful to check for thyroid disease in people with type 1 diabetes.