Type 2 diabetes is diagnosed through blood tests that measure how your body handles sugar. The most common are the A1C test, the fasting blood sugar test, and the oral glucose tolerance test. Any of these can confirm a diagnosis, though your doctor will typically need two abnormal results, either from the same blood sample or from separate visits, before making it official.
The A1C Test
The A1C test measures your average blood sugar over the past two to three months. It works by checking how much sugar has attached to your red blood cells, which gives a longer-term picture than a single snapshot of your blood sugar on one particular morning. No fasting is required, so you can have blood drawn at any time of day.
The results come back as a percentage:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The A1C is the most convenient option since it doesn’t require any preparation, but it isn’t reliable for everyone. Conditions that affect red blood cells can throw off the results. Iron deficiency anemia tends to push A1C readings artificially high, while conditions that shorten the lifespan of red blood cells (like sickle cell trait or recovery from significant blood loss) can make the number falsely low. Kidney disease and pregnancy can also interfere. If any of these apply to you, your doctor will likely use one of the other tests instead.
The Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for at least 8 hours, typically overnight. You go to the lab first thing in the morning, have blood drawn, and the result tells your doctor how well your body manages glucose without any food in the system. Water is fine during the fast.
The cutoffs from the CDC are straightforward:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
The fasting test is widely available and inexpensive, which makes it one of the most commonly ordered diabetes tests. The main drawback is that it captures only a single moment. Stress, illness, or a bad night’s sleep can nudge results in either direction, which is one reason a single high reading isn’t enough for a diagnosis on its own.
The Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how efficiently your body clears sugar from the bloodstream. You fast overnight, have a baseline blood draw, then drink a sugary solution containing 75 grams of glucose. Two hours later, your blood is drawn again.
At the two-hour mark, a blood sugar of 200 mg/dL (11.1 mmol/L) or higher indicates diabetes. Results between 140 and 199 mg/dL fall in the prediabetes range.
This test is more sensitive at catching early insulin resistance than the fasting test alone, because it shows how your body responds to an actual sugar load. The tradeoff is time: you’ll be sitting in the lab for over two hours. It’s used less frequently for routine screening but can be valuable when other test results are borderline or conflicting.
The Random Blood Sugar Test
Unlike the other tests, a random blood sugar test doesn’t require fasting or scheduling. Blood is drawn at whatever time you happen to be at the doctor’s office. A result of 200 mg/dL or higher suggests diabetes, but only when you’re also showing classic symptoms: excessive thirst, frequent urination, unexplained weight loss, or blurred vision. Without those symptoms, this test alone isn’t used to make a diagnosis.
Why Two Abnormal Results Are Required
A single high reading doesn’t mean you have diabetes. Blood sugar fluctuates throughout the day and can spike temporarily from stress, medication, or illness. To confirm a diagnosis, guidelines from the NIDDK require two abnormal results. These can come from the same blood sample (for example, both your A1C and fasting glucose come back high) or from two separate tests on different days. If two different tests give conflicting results, the one that came back above the diagnostic threshold is repeated, and the diagnosis is based on that confirmed result.
The one exception is the random blood sugar test in someone with obvious symptoms of high blood sugar. In that scenario, a single reading of 200 mg/dL or higher is enough.
Can Home Glucose Monitors Diagnose Diabetes?
Home finger-prick monitors and continuous glucose monitors (CGMs) are designed to help people who already have diabetes track their levels day to day. They are not approved for diagnosing diabetes. The readings from home devices have a wider margin of error than lab-grade equipment, so a high number on your home meter is worth mentioning to your doctor, but it won’t replace a proper lab test. The FDA classifies these devices as monitoring tools for adjusting treatment, diet, and exercise, not as diagnostic instruments.
Who Should Get Tested
Current guidelines recommend that all adults begin screening at age 35, even without symptoms or risk factors. If you’re younger than 35 but overweight or obese and have at least one additional risk factor, earlier testing is appropriate. Those risk factors include a family history of diabetes, a history of gestational diabetes, polycystic ovarian syndrome, and being part of a population with higher diabetes rates (including Black, Hispanic/Latino, Asian American, American Indian/Alaska Native, and Native Hawaiian/Pacific Islander communities).
For Asian American adults, a lower BMI threshold applies. Screening is recommended at a BMI of 23 or higher rather than the standard 25, because type 2 diabetes develops at lower body weights in this population. Overweight and obesity remain the single strongest risk factors for developing type 2 diabetes, but they aren’t the only ones, so people at a healthy weight with other risk factors should still discuss testing with their doctor.
What to Expect on Test Day
If you’re scheduled for a fasting blood sugar test or an oral glucose tolerance test, you’ll need to avoid eating or drinking anything other than water for 8 to 14 hours beforehand. Most people schedule a morning appointment and skip breakfast. Some medications can affect blood sugar readings, so let your doctor know what you take so they can advise whether to adjust anything before the test.
The A1C test and random blood sugar test require no preparation at all. Results for the A1C typically come back within a day or two. Fasting and glucose tolerance results are often available the same day or the next morning. If your results fall in the prediabetes range, your doctor will likely recommend retesting in one to three years, along with lifestyle changes that can slow or prevent progression to diabetes.