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To be able to diagnose diabetes mellitus, a physician must take a complete patient history and perform a physical examination. A positive diagnosis is made when fasting blood tests show high glucose levels of 126 mg/dL or greater.
When test results come back ambiguous or inconclusive, a glucose tolerance test may be administered to get a more accurate reading. The glucose tolerance test is a simple non-invasive test where the patient is asked to swallow a drink containing 75 grams of glucose. Blood glucose levels are then measured every 30 minutes over a two-hour time span.
There are several ways to diagnose diabetes. Each way usually needs to be repeated on a second day to diagnose diabetes. Testing should be carried out in a health care setting (such as your doctor’s office or a lab). If your doctor determines that your blood glucose level is very high, or if you have classic symptoms of high blood glucose in addition to one positive test, your doctor may not require a second test to diagnose diabetes.
A1C. The A1C test measures your average blood glucose for the past 2 to 3 months. The advantages of being diagnosed this way are that you don’t have to fast or drink anything.
- Diabetes is diagnosed at:
A1C ≥ 6.5%
Fasting Plasma Glucose (FPG). This test is usually done first thing in the morning, before breakfast and checks your fasting blood glucose levels. Fasting means after not having anything to eat or drink (except water) for at least 8 hours before the test.
- Diabetes is diagnosed at:
Fasting blood glucose ≥ 126 mg/dl
Oral Glucose Tolerance Test (also called the OGTT). The OGTT is a two-hour test that checks your blood glucose levels before and 2 hours after you drink a special sweet drink. It tells the doctor how your body processes glucose.
- Diabetes is diagnosed at:
2-h blood glucose ≥ 200 mg/dl
Random (also called Casual) Plasma Glucose Test. This test is a blood check at any time of the day when you have severe diabetes symptoms.
- Diabetes is diagnosed at:
Blood glucose ≥ 200 mg/dl
How is gestational diabetes (GDM) diagnosed?
If you have a high risk of having diabetes, you will be checked during your first visit with a standard test for diabetes. If you don’t have diabetes, you will be checked for GDM at 24-28 weeks of pregnancy with an oral glucose tolerance test.
The oral glucose tolerance test for pregnant women measures blood glucose at the beginning of the test and at 1 and 2 hours after consuming a special sweet drink.
- Above normal results are shown below. If your levels are above normal during the test, you have gestational diabetes.
Fasting: ≥ 92 mg/dl
1-h: ≥ 180 mg/dl
2-h: ≥ 153 mg/dl
The most common way to diagnose diabetes in children and nonpregnant adults is through blood tests. These tests measure milligrams of blood sugar per deciliter, which is expressed by mg/dL. One test, the fasting blood glucose test, is usually performed after an eight-hour fast. If this test reveals a blood glucose level of 126 mg/dL or higher, diabetes is present. A random blood glucose test, taken at any time of the day, that indicates a blood glucose level of 200 mg/dL or higher also confirms diabetes, especially when accompanied by symptoms. Another test, called the oral glucose tolerance test, is conducted two hours after drinking 75 grams of glucose dissolved in water. If this test results in a blood glucose level of 200 mg/dL or higher, it is an indication of diabetes.
Medical experts now recommend a newer test, the glycated hemoglobin (A1C) test, to diagnose diabetes. The A1C test will measure average blood sugar level over a longer period of time. Certain conditions such as pregnancy can make these tests unreliable, so it is important to ask your doctor about which test is the right one for you.
In order to have a diagnosis of diabetes, your fasting blood sugar needs to be greater than or equal to 126 mg/dL on two separate occasions.
Fasting blood glucose test: Fasting blood glucose testing checks blood glucose levels after fasting for between 12-14 hours. The individual can drink water during this time, but should strictly avoid any other beverage. Individuals with diabetes may be asked to delay their diabetes medication or insulin dose until the test is completed. This test can be used to diagnose diabetes or prediabetes. The fasting plasma glucose (FPG) is the preferred test for diagnosing diabetes due to convenience and is most reliable when done on an empty stomach in the morning, so the presence of food and natural biorhythms do not cause fluctuations in blood sugar levels. If the fasting glucose level is 100-125 milligrams/deciliter, the individual has a form of prediabetes called impaired fasting glucose (IFG), meaning that the individual is more likely to develop type 2 diabetes but does not have the condition yet. A level of 126 milligrams/deciliter or above, confirmed by repeating the test on another day, means that the individual has diabetes.
Oral glucose tolerance test: During an oral glucose tolerance test (OGTT), a high-glucose drink is given to the individual. Blood samples are checked at regular intervals for two hours. Glucose tolerance tests are used when the results of the fasting blood glucose are borderline. They are also used to diagnose diabetes in pregnancy (gestational diabetes). This test can be used to diagnose diabetes or prediabetes.
Gestational diabetes diagnosis: Gestational diabetes is diagnosed based on blood glucose levels measured during the oral glucose tolerance test (OGTT). Glucose levels are normally lower during pregnancy, so the cutoff levels for diagnosis of diabetes in pregnancy are lower. Blood glucose levels are measured before a woman drinks a beverage containing glucose. Then levels are checked one, two, and three hours afterward. If a woman has two blood glucose levels meeting or exceeding any of the following numbers, she has gestational diabetes: a fasting blood glucose level of 95 milligrams/deciliter, a one-hour level of 180 milligrams/deciliter, a two-hour level of 155 milligrams/deciliter, or a three-hour level of 140 milligrams/deciliter.
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You need to have two abnormal blood sugar levels to be diagnosed with diabetes. The first would be a fasting blood sugar over 126. The second is a two-hour post-meal sugar that is over 200. Any combination of two abnormal numbers meeting this criteria is a diagnosis of diabetes. Any two random blood sugar levels over 200 also make a diagnosis of diabetes.
Diabetes can be diagnosed in four ways. First, a fasting (no caloric intake for at least 8 hours) blood glucose can be checked. If the result is 126 mg/dl or higher, it's indicative of diabetes.
The second way to diagnose diabetes is by measuring blood glucose 2 hours after a 75 gram oral glucose tolerance test (OGTT). A result of 200 mg/dl or higher is indicative of diabetes.
The third way to diagnose diabetes is by the presence of classic symptoms of hyperglycemia (e.g., thirst, frequent urination) and a random blood glucose of 200 mg/dl or higher. And the fourth way to diagnose diabetes is by checking an A1C level. A level of 6.5% or higher is indicative of diabetes. In general, these tests should be repeated on a different day, unless the results are unequivocally high.
If diabetes is suspected, your doctor will test your blood glucose levels for diabetes and prediabetes -- a condition in which blood glucose levels are higher than normal but not elevated enough to be classified as diabetes. Your doctor should also check your blood pressure, body mass index (BMI) and cholesterol levels to provide youwith a fuller picture about your risk for developing diabetes.
Diabetes is diagnosed with a blood test. The most common type of diabetes is type 2 diabetes. It is diagnosed with one of several different blood tests.
The simplest test is a fasting blood sugar. This test simply measures the patient's blood sugar level when he is on an empty stomach. Another test that does not require a fast is called a Hemoglobin A1C test (also known as Glycohemoglobin). Less commonly, your doctor may decide to do a test called the glucose tolerance test. This test is done by having you drink a glucose (sugar) drink and then testing your blood sugar one or two hours later.
Type 1 diabetes is much less common and generally presents in childhood. It may be diagnosed with the same tests as type 2 diabetes, but most often it is diagnosed in the hospital when a patient becomes quite ill from the diabetes. It is very difficult to diagnose type 1 diabetes before significant symptoms develop.
The blood test most commonly used in the evaluation of diabetes is the fasting plasma glucose test. The test is performed after an overnight or eight-hour fast during the day. Your blood is taken and sent to the lab to measure your glucose levels. The test results indicate whether your blood glucose level is normal, whether you have diabetes or whether you have impaired glucose tolerance, which we now refer to as prediabetes. Prediabetes is now more commonly used to emphasize the fact that without some lifestyle and nutrition intervention, the majority of people will go on to develop diabetes.
- Normal: Normal blood sugar levels measure less than 100 milligrams per deciliter (mg/dl) after the fasting glucose test.
- Prediabetes: Blood glucose levels of 100-125 mg/dl after an overnight or eight-hour fast is diagnosed as prediabetes. People with these results are considered to have impaired fasting glucose.
- Diabetes: Diabetes is diagnosed when the blood glucose is 126 mg/dl or above.
Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.