Three main blood tests are used to diagnose diabetes: the A1C test, the fasting plasma glucose (FPG) test, and the oral glucose tolerance test (OGTT). Each measures blood sugar differently, 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 measures your average blood sugar over the past three months. It doesn’t require fasting, which makes it one of the most convenient options. Instead of capturing a single moment, it reflects how much sugar has been attaching to your red blood cells over their roughly 90-day lifespan.
The results break down like this:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The A1C test isn’t reliable for everyone. Certain conditions can skew results higher or lower, including severe anemia, kidney failure, liver disease, blood disorders like sickle cell anemia or thalassemia, recent blood transfusions, and early or late pregnancy. Some medications, including opioids and certain HIV drugs, can also affect it. If any of these apply to you, your doctor will likely rely on a different test instead.
The Fasting Plasma Glucose Test
This test measures your blood sugar at a single point in time after you’ve fasted for at least eight hours. You can drink water, but nothing else. Most people schedule it first thing in the morning to make the fasting period easier.
The diagnostic ranges are:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
The FPG test is straightforward and widely available, but because it only captures one moment, things like stress, illness, or a poor night’s sleep can temporarily push the number up. That’s one reason a single abnormal result usually needs to be confirmed with a second test.
The Oral Glucose Tolerance Test
The OGTT is a two-part test that shows how well your body handles a large dose of sugar. You fast for eight hours, then have your blood drawn. After that, you drink a liquid containing 75 grams of sugar, and your blood is drawn again two hours later.
The two-hour results fall into these categories:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
The OGTT is more time-consuming than the other tests, and the sugary drink can cause nausea in some people. It’s used less often for routine screening, but it’s particularly useful for catching cases where fasting levels look normal yet the body still struggles to process sugar after a meal. It’s also the standard test for diagnosing gestational diabetes.
The Random Plasma Glucose Test
If you’re already experiencing obvious symptoms of diabetes, like frequent urination, excessive thirst, or unexplained weight loss, your doctor may not want to wait for you to fast. A random plasma glucose test can be done at any time of day, regardless of when you last ate. A result of 200 mg/dL or higher, combined with those classic symptoms, is enough to diagnose diabetes on the spot without a follow-up test.
Why You Might Need a Second Test
Unless you have clear symptoms and a random glucose reading of 200 mg/dL or above, a single abnormal result doesn’t confirm a diagnosis. You’ll need a second test to be sure. This can happen in two ways: your doctor can run two different tests at the same visit (for example, an A1C and a fasting glucose), or repeat the same test on a different day.
If two different tests give conflicting results, the one that came back in the diabetes range gets repeated. The diagnosis is based on whichever confirmatory test comes back abnormal. This built-in check prevents a bad night’s sleep or a stressful morning from triggering a false diagnosis.
Testing for Gestational Diabetes
Pregnant women go through a slightly different process. Most are screened between 24 and 28 weeks, though women at higher risk may be tested earlier. The initial screening is a glucose challenge test: you drink a syrup containing 50 grams of sugar, and your blood is drawn one hour later. No fasting is required for this step.
If that result comes back high, you move on to a longer follow-up. The three-hour version requires an overnight fast and involves drinking a more concentrated sugar solution, with blood drawn at multiple intervals. A two-hour version that uses a 75-gram solution is also used, depending on the approach your provider follows.
Tests That Distinguish Type 1 From Type 2
The tests above tell you whether you have diabetes, but they don’t tell you which type. That distinction matters because Type 1 and Type 2 have different causes and different treatment paths. If there’s any uncertainty, your doctor may order a C-peptide test. C-peptide is a byproduct of insulin production, so measuring it reveals how much insulin your body is making on its own. People with Type 1 diabetes produce very little or no insulin, so their C-peptide levels are low. People with Type 2 typically have normal or even elevated levels, at least early on.
The C-peptide test is also helpful for people already taking insulin, since it can distinguish between insulin your body makes and insulin from medication. In some cases, doctors also test for specific antibodies that attack insulin-producing cells, which is a hallmark of Type 1 diabetes and a related condition called LADA (latent autoimmune diabetes in adults), sometimes described as slow-onset Type 1.
How to Prepare
Preparation depends entirely on which test you’re getting. The A1C and random glucose tests require no preparation at all. The fasting glucose test and the OGTT both require an eight-hour fast, with water being the only thing you can consume during that window. If you’re unsure which test has been ordered, ask your doctor’s office before your appointment so you don’t show up having eaten breakfast only to need a fasting draw.
For any of these tests, results typically come back within a day or two. If your numbers fall in the prediabetes range, that’s not a diabetes diagnosis, but it is a signal that your blood sugar is trending in the wrong direction and that lifestyle changes can make a measurable difference.