Doctors test for diabetes using blood tests that measure how your body handles sugar. The most common are the A1C test, fasting blood sugar test, and oral glucose tolerance test. Each one looks at blood sugar from a slightly different angle, and your doctor may use one or a combination depending on your symptoms, risk factors, and whether you’re pregnant.
The A1C Test
The A1C test is one of the most widely used diabetes screening tools because it doesn’t require fasting and gives a longer-term picture of your blood sugar. It measures the percentage of red blood cells that have glucose stuck to their hemoglobin, a protein that carries oxygen through your blood. Since red blood cells live about three months, the A1C reflects your average blood sugar over the past two to three months rather than a single moment in time.
The results break down into three categories:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The A1C is convenient, but it isn’t accurate for everyone. Conditions that affect red blood cells or hemoglobin can throw off results. These include severe anemia, sickle cell anemia, thalassemia, kidney failure, liver disease, recent blood transfusions, and certain medications like opioids or some HIV drugs. Pregnancy, particularly early or late stages, can also skew results. If any of these apply to you, your doctor will likely rely on a different test instead.
Fasting Blood Sugar Test
The fasting plasma glucose test measures your blood sugar after you haven’t eaten for at least 8 hours. Most people schedule it first thing in the morning so the fasting period happens overnight. It’s a simple blood draw, typically from a vein in your arm.
Here’s how the numbers line up:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
This test captures how well your body manages blood sugar at rest, without the influence of a recent meal. It’s straightforward and inexpensive, which makes it a common first step in screening. One important note: if your result comes back in the diabetes range but you don’t have obvious symptoms, your doctor will typically repeat the test on a separate day before making a formal diagnosis.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) shows how your body responds to a large dose of sugar in real time. You fast overnight, get a baseline blood draw, then drink a syrupy solution containing 75 grams of glucose. Your blood is drawn again at the one-hour and two-hour marks.
The two-hour result determines the diagnosis:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
The OGTT is more involved than the other tests, since you need to stay at the lab or clinic for at least two hours. It’s particularly useful for catching prediabetes and early diabetes that fasting tests might miss, because some people have normal fasting levels but struggle to bring their blood sugar back down after eating. For that reason, it’s considered the most sensitive screening option, though it’s used less often in routine checkups simply because of the time commitment.
Random Blood Sugar Test
If you walk into a doctor’s office with classic diabetes symptoms, including frequent urination, extreme thirst, and unexplained weight loss, your doctor can order a random plasma glucose test. This is a blood draw taken at any time of day regardless of when you last ate. A result of 200 mg/dL or higher, combined with those symptoms, is enough for a diabetes diagnosis on its own without repeat testing.
This test isn’t used for routine screening. It’s reserved for situations where symptoms strongly suggest diabetes and the doctor needs a quick answer.
Do You Need More Than One Test?
In most cases, yes. If a single test comes back in the diabetes range and you don’t have obvious symptoms, the standard practice is to confirm with a second test. That can be a repeat of the same test on a different day or a different type of test altogether. For example, your doctor might follow up an elevated A1C with a fasting blood sugar test, or vice versa. If two different tests both point to diabetes from the same blood sample, that’s also considered sufficient for diagnosis.
The exception is the random blood sugar test. When someone has a reading of 200 mg/dL or above and clear symptoms of high blood sugar, no confirmation is needed.
Gestational Diabetes Screening
Pregnant women are screened for gestational diabetes between 24 and 28 weeks of pregnancy. The most common approach in the U.S. is a two-step process. First, you drink a smaller glucose solution (50 grams) without fasting, and your blood sugar is checked one hour later. If that result is elevated, you return for a full three-hour test using a 100-gram glucose solution, with blood draws at fasting, one hour, two hours, and three hours.
A diagnosis is made when two or more of the blood draws fall at or above specific thresholds. The commonly used thresholds (Carpenter and Coustan criteria) are 95 mg/dL for fasting, 180 at one hour, 155 at two hours, and 140 at three hours.
Some providers use a one-step approach instead, which skips the initial screening drink and goes straight to a 75-gram glucose tolerance test with blood drawn at fasting, one hour, and two hours. With this method, just one elevated reading is enough for diagnosis.
Tests That Distinguish Type 1 From Type 2
The tests above tell your doctor whether you have diabetes, but not which type. That distinction matters because Type 1 diabetes is an autoimmune condition where the immune system attacks the cells in the pancreas that produce insulin, while Type 2 involves insulin resistance, where the body still makes insulin but can’t use it effectively.
To tell them apart, doctors can order an autoantibody panel. This blood test looks for specific immune proteins that attack pancreatic cells. The panel typically checks for four types of autoantibodies: islet cell antibodies, glutamic acid decarboxylase antibodies, insulinoma-associated-2 antibodies, and insulin antibodies. If one or more come back positive, it strongly suggests Type 1 diabetes or a slow-onset form of it sometimes seen in adults.
A C-peptide test is another useful tool. C-peptide is a byproduct released when the pancreas makes insulin, so measuring it reveals how much insulin your body is still producing on its own. In Type 1 diabetes, C-peptide levels are very low or undetectable because the insulin-producing cells have been destroyed. In Type 2, levels are often normal or even elevated because the pancreas is working overtime to compensate for insulin resistance. Together with the autoantibody panel, this gives doctors a clear picture of which type of diabetes you’re dealing with and, more importantly, what treatment approach will work best.
How to Prepare
Preparation depends entirely on which test you’re getting. The A1C test requires no preparation at all. You can eat and drink normally beforehand. The fasting blood sugar test and oral glucose tolerance test both require an overnight fast of 8 to 12 hours, so water only (no food, coffee, or juice) starting the night before. Your provider will give you specific timing instructions when they order the test.
If you’re unsure whether your test requires fasting, ask when you schedule the appointment. Eating before a fasting test will produce inaccurate results and mean you’ll need to come back and repeat it.