How is type 2 diabetes diagnosed?

William Lee Dubois
Endocrinology Diabetes & Metabolism
The current standard of care guidelines from the American Diabetes Association advocate the use of the hemoglobin A1C test to diagnose diabetes. In plain English the A1C test gives us a picture of the blood sugar environment in your body over the last three months.
The A1C was chosen to replace the older methods of diagnosis mainly for expedience. Using the fingersticks method required getting the fingerstickee to return for a second test. You can imagine the compliance rate when you tell someone, “Come back tomorrow and we’ll see if you have diabetes or not.” The other traditional method of screening, the test where someone has to come in fasting, drink nasty super-sweet high-tech Kool-Aid and hang around for a few hours has a whole host of problems as you can imagine (we still use this method to test for gestational diabetes).
By comparison, and A1C test can be done with a single drop of blood. It doesn’t matter if you are fasting or not. Many clinics and doctor’s offices can do the test onsite in about six minutes.
The magic number for diagnosis is 6.5%, which roughly translates to an average blood sugar of 140 mg/dL over the last three months. Some experts, myself included, think this is too high a number and the diagnosis threshold should have been lower. That said, at least the choice of 6.5 is unequivocal. There can be no argument. If you are at 6.5 you soooooooooooooo have diabetes, and you’ve had it for a while.
One bad thing about using A1Cs, at least as it’s set up right now, is that people who are at 6.4%, for an average blood sugar of 137mg/dL, are not considered diabetic—or anything else for that matter. The old fingerstick system also had criteria for “impaired glucose” or “prediabetes.” I’m not really a fan of the concept of prediabetes, as you can’t be a little bit pregnant; but I’d love to see some sort of criteria that let people know they were on their way to trouble if their A1C is higher than normal blood sugar but lower than diabetes.
Diabetes is typically diagnosed by measuring the blood sugar level. This is typically done in a medical laboratory or doctor's office on a small sample of blood, or at home or other location with a glucometer or automatic measuring device. Since blood sugar can fluctuate moment to moment, there are several methods used by the medical laboratory to standardize the measurement. These include testing the blood sugar after fasting for 10 hours or more (FBS), testing the blood sugar 2 hours after a meal (2-hour postprandial sugar), and, finally, the glucose tolerance test (GTT).

In addition, the hemoglobin A1c is typically checked, as it shows what the average blood sugar has been over a longer interval of time. Sometimes, diabetes specialists will also use additional tests to better characterize a person's diabetes. These include serum C-peptide, C-reactive protein (CRP), and insulin levels. These are used only in special cases and usually require special expertise to interpret.

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, indicating 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, which is conducted two hours after drinking 75 grams of glucose dissolved in water, that results in a blood glucose level of 200 mg/dL or higher is an indication of diabetes.

Medical experts now recommend a newer test, the glycated hemoglobin (A1C) test, to diagnose type 2 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.

Some people with diabetes develop symptoms that include frequent urination, excessive thirst, pervasive fatigue, an increase in infections, blurred vision, tingling in hands or feet, and absence of menstrual periods. Many people have no symptoms at all and find out they have diabetes only through testing. One-third of adults with diabetes have type 2 diabetes but don't know it.

Diabetes is diagnosed by measuring the blood sugar level. If it is higher than normal but not yet high enough to indicate full-blown diabetes, you may have what is called pre diabetes or glucose intolerance. This means you are more likely to develop diabetes and heart attack or stroke.

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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.