Blood tests are used to diagnosis diabetes and prediabetes because early in the disease type 2 diabetes may have no symptoms. All diabetes blood tests involve drawing blood at a health care providers office or commercial facility and sending the sample to a lab for analysis. Lab analysis of blood is needed to ensure test results are accurate. Glucose measuring devices used in a health care providers office, such as finger-stick devices, are not accurate enough for diagnosis but may be used as a quick indicator of high blood glucose.
                
Testing enables health care providers to find and treat diabetes before complications occur and to find and treat prediabetes, which can delay or prevent type 2 diabetes from developing.
                
Any one of the following tests can be used for diagnosis:*
                
- an A1C test, also called the hemoglobin A1c, HbA1c, or glycohemoglobin test  - a fasting plasma glucose (FPG) test  - an oral glucose tolerance test (OGTT)
                
*Not all tests are recommended for diagnosing all types of diabetes. See the individual test descriptions for details.
                
Another blood test, the random plasma glucose (RPG) test, is sometimes used to diagnose diabetes during a regular health checkup. If the RPG measures 200 milligrams per deciliter or above, and the individual also shows symptoms of diabetes, then a health care provider may diagnose diabetes.
                
Symptoms of diabetes include
                
- increased urination  - increased thirst  - unexplained weight loss
                
Other symptoms can include fatigue, blurred vision, increased hunger, and sores that do not heal.
                
Any test used to diagnose diabetes requires confirmation with a second measurement unless clear symptoms of diabetes exist.
                
The following table provides the blood test levels for diagnosis of diabetes for nonpregnant adults and diagnosis of prediabetes.
                
A1C Test
                
The A1C test is used to detect type 2 diabetes and prediabetes but is not recommended for diagnosis of type 1 diabetes or gestational diabetes. The A1C test is a blood test that reflects the average of a persons blood glucose levels over the past 3 months and does not show daily fluctuations. The A1C test is more convenient for patients than the traditional glucose tests because it does not require fasting and can be performed at any time of the day.
                
The A1C test result is reported as a percentage. The higher the percentage, the higher a persons blood glucose levels have been. A normal A1C level is below 5.7 percent.
                
An A1C of 5.7 to 6.4 percent indicates prediabetes. People diagnosed with prediabetes may be retested in 1 year. People with an A1C below 5.7 percent maystill be at risk for diabetes, depending on the presence of other characteristics that put them at risk, also known as risk factors. People with an A1C above 6.0 percent should be considered at very high risk of developing diabetes. A level of 6.5 percent or above means a person has diabetes.
                
Laboratory analysis. When the A1C test is used for diagnosis, the blood sample must be sent to a laboratory using a method that is certified by the NGSP to ensure the results are standardized. Blood samples analyzed in a health care providers office, known as point-of-care tests, are not standardized for diagnosing diabetes.
                
Abnormal results. The A1C test can be unreliable for diagnosing or monitoring diabetes in people with certain conditions known to interfere with the results. Interference should be suspected when A1C results seem very different from the results of a blood glucose test. People of African, Mediterranean, or Southeast Asian descent or people with family members with sickle cell anemia or a thalassemia are particularly at risk of interference.
                
However, not all of the A1C tests are unreliable for people with these diseases. The NGSP provides information about which A1C tests are appropriate to use for specific types of interference and details on any problems with the A1C test at www.ngsp.org.
                
False A1C test results may also occur in people with other problems that affect their blood or hemoglobin such as chronic kidney disease, liver disease, or anemia.
                
More information about limitations of the A1C test and different forms of sickle cell anemia is provided in the NIDDK health topic, For People of African, Mediterranean, or Southeast Asian Heritage: Important Information about Diabetes Blood Tests, or by calling 18008608747.
                
Changes in Diagnostic Testing
                
In the past, the A1C test was used to monitor blood glucose levels but not for diagnosis. The A1C test has now been standardized, and in 2009, an international expert committee recommended it be used for diagnosis of type 2 diabetes and prediabetes.2
                
More information about the A1C test is provided in the NIDDK health topic, The A1C Test and Diabetes, or by calling 18008608747.
                
Fasting Plasma Glucose Test
                
The FPG test is used to detect diabetes and prediabetes. The FPG test has been the most common test used for diagnosing diabetes because it is more convenient than the OGTT and less expensive. The FPG test measures blood glucose in a person who has fasted for at least 8 hours and is most reliable when given in the morning.
                
People with a fasting glucose level of 100 to 125 mg/dL have impaired fasting glucose (IFG), or prediabetes. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.
                
Oral Glucose Tolerance Test
                
The OGTT can be used to diagnose diabetes, prediabetes, and gestational diabetes. Research has shown that the OGTT is more sensitive than the FPG test, but it is less convenient to administer. When used to test for diabetes or prediabetes, the OGTT measures blood glucose after a person fasts for at least 8 hours and 2 hours after the person drinks a liquid containing 75 grams of glucose dissolved in water.
                
If the 2-hour blood glucose level is between 140 and 199 mg/dL, the person has a type of prediabetes called impaired glucose tolerance (IGT). If confirmed by a second test, a 2-hour glucose level of 200 mg/dL or above means a person has diabetes.