Getting tested for diabetes is straightforward: a doctor orders a blood test, and results typically come back within a couple of hours. The most common screening tests are the A1C blood test, the fasting blood sugar test, and the oral glucose tolerance test. Any of these can confirm or rule out diabetes, and most require little or no preparation beyond skipping breakfast.
The Three Main Diabetes Tests
Doctors use three primary blood tests to diagnose diabetes and prediabetes. Each measures blood sugar differently, and your doctor will choose one based on your situation, symptoms, and whether you’ve already eaten that day.
The A1C test measures your average blood sugar over the past three months. It doesn’t require fasting, so you can eat and drink normally beforehand. A result below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or above means diabetes. Because it reflects a three-month average rather than a single moment, it’s one of the most widely used screening tools.
The fasting blood sugar test measures your glucose level after you haven’t eaten for at least eight hours. Most people schedule it first thing in the morning so the fasting period happens overnight. Water is fine during the fast. A result of 99 mg/dL or below is normal, 100 to 125 mg/dL falls in the prediabetes range, and 126 mg/dL or above indicates diabetes.
The oral glucose tolerance test (OGTT) is more involved. You fast for eight hours, then give a blood sample. Next, you drink a syrupy glucose solution containing 75 grams of sugar. Your blood is drawn again one hour and two hours later to see how efficiently your body processes the sugar. This test is especially useful for catching prediabetes and gestational diabetes because it shows how your body responds to a sugar load in real time, not just a snapshot.
What to Expect During the Tests
For the A1C and fasting blood sugar tests, the entire visit is quick. A technician draws blood from a vein in your arm, and results are often ready in one to two hours. Some clinics use a finger prick for the A1C, with results available during your appointment.
The oral glucose tolerance test takes longer. Plan on being at the lab for about two and a half hours total. After your fasting blood draw, you’ll drink the glucose solution, which most people describe as flat, overly sweet soda. Then you wait. You can’t eat, and you’ll need to stay at the lab until the final blood draw at the two-hour mark. Some people feel nauseous or lightheaded from the sugar on an empty stomach, but the sensation usually passes quickly.
If your doctor suspects diabetes based on symptoms like extreme thirst, frequent urination, or unexplained weight loss, they may skip the fasting requirement entirely and order a random blood sugar test. This can be done at any time regardless of when you last ate.
How to Prepare
For fasting tests, don’t eat or drink anything except water for eight to twelve hours before your appointment. Your doctor will tell you the exact window. Most people find it easiest to schedule the blood draw early in the morning and stop eating after dinner the night before.
For the A1C test, no preparation is needed. You don’t need to fast, avoid certain foods, or change your routine. This makes it the most convenient option and a common choice for routine screenings during a regular checkup.
Confirming a Diagnosis
A single abnormal result doesn’t always mean you have diabetes. Unless your blood sugar is very high and you already have clear symptoms, doctors typically confirm the diagnosis by repeating the same test on a different day or by running a second type of test. For example, if your A1C comes back at 6.6%, your doctor might order a fasting blood sugar test to confirm the result.
If both tests point to diabetes, the diagnosis is confirmed. If the two tests disagree, the one with the abnormal result is usually repeated.
Distinguishing Type 1 From Type 2
The standard blood sugar tests can tell you whether you have diabetes, but they don’t reveal which type. This distinction matters because Type 1 and Type 2 diabetes have different causes and require different treatment approaches.
If there’s any question about the type, doctors can order autoantibody tests that look for immune markers called GAD, IA2, and ZnT8. These antibodies show up when the immune system is attacking the insulin-producing cells in the pancreas, which is the hallmark of Type 1 diabetes. These tests are most accurate within the first three years of diagnosis.
For people who’ve had diabetes for many years, a different test called C-peptide becomes more useful. It measures how much insulin your body still produces on its own. People with long-standing Type 1 diabetes produce very little or no insulin, while those with Type 2 typically still produce some.
Testing During Pregnancy
Pregnant women are routinely screened for gestational diabetes between 24 and 28 weeks of pregnancy. If you enter prenatal care after 28 weeks, your provider will screen you as soon as possible.
In the United States, most practices use a two-step process. First comes a glucose challenge test: you drink a solution with 50 grams of sugar, and your blood is drawn one hour later. No fasting is required for this step. If your blood sugar is above a certain threshold, you move on to the second step, a full three-hour oral glucose tolerance test. For this one, you fast overnight, drink a solution with 100 grams of sugar, and have your blood drawn at the one-hour, two-hour, and three-hour marks. A diagnosis of gestational diabetes is made when two or more of those readings come in above the target levels.
Some practices use a one-step approach instead, skipping the initial challenge and going straight to a two-hour glucose tolerance test with 75 grams of sugar. In this version, just one elevated reading is enough for a diagnosis.
At-Home A1C Test Kits
Over-the-counter A1C test kits are available at pharmacies and online, and two products, Home Access and A1cNow+, are FDA-cleared for home use. However, their accuracy varies significantly. A 2022 study involving 219 people with diabetes compared home kits against standard lab results. Home Access had 82% of its samples within an acceptable accuracy range. A1cNow+ hit that same benchmark only 46% of the time, and a third product, CoreMedica, managed just 29%.
A home kit can give you a rough idea of where your blood sugar stands, but the results aren’t reliable enough to diagnose diabetes on their own. If a home test shows an elevated number, follow up with a lab test through your doctor. And if it comes back normal but you’re having symptoms, don’t assume you’re in the clear.
Who Should Get Tested
The American Diabetes Association recommends that all adults get screened starting at age 35, with repeat testing every three years if results are normal. Testing should start earlier if you have risk factors like a family history of diabetes, a BMI of 25 or higher, a history of gestational diabetes, or if you belong to a racial or ethnic group with higher diabetes rates, including Black, Hispanic, Native American, Asian American, or Pacific Islander communities.
You can request a diabetes screening at a routine physical, an urgent care visit, or a community health clinic. Many pharmacies also offer basic blood sugar checks, though these finger-prick readings are less precise than a lab draw. If cost is a concern, diabetes screening is covered as a preventive service under most insurance plans with no out-of-pocket cost.