Diabetes is diagnosed through blood tests that measure how much sugar is in your blood, either at a single moment or averaged over several months. There are four main tests used, each with specific numerical cutoffs that separate normal blood sugar from prediabetes and diabetes. In most cases, you’ll need two abnormal results before receiving a formal diagnosis.
The Four Blood Tests Used for Diagnosis
Any of the following tests can lead to a diabetes diagnosis. Your doctor will choose based on your symptoms, risk factors, and whether you’ve eaten recently.
A1C Test
The A1C test measures your average blood sugar over the past two to three months. It works by looking at how much sugar has attached to your red blood cells. Because it reflects a longer window of time rather than a single snapshot, it’s one of the most commonly used diagnostic tools. No fasting is required.
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
The A1C test can give misleading results in people with certain types of anemia or other conditions that affect red blood cells. If your doctor suspects your results may be inaccurate, they’ll use one of the other tests instead.
Fasting Plasma Glucose Test
This test measures your blood sugar after you haven’t eaten for 8 to 12 hours, typically done first thing in the morning. It gives a clear picture of how well your body manages blood sugar without any food in the mix. A fasting level of 126 mg/dL or higher points to diabetes, while 100 to 125 mg/dL falls in the prediabetes range.
Oral Glucose Tolerance Test (OGTT)
The OGTT is a two-step process. You fast overnight, then drink a sugary solution containing 75 grams of glucose. Two hours later, your blood is drawn to see how effectively your body processed that sugar. A reading of 200 mg/dL or higher at the two-hour mark indicates diabetes. This test is especially useful for catching diabetes that a fasting test might miss, because it reveals how your body handles a sugar load in real time.
Random Blood Sugar Test
This test can be done at any time, regardless of when you last ate. A result of 200 mg/dL or higher suggests diabetes, particularly when combined with classic symptoms like excessive thirst, frequent urination, and unexplained weight loss. It’s often the first test performed when someone shows up with noticeable symptoms.
Why You Usually Need Two Abnormal Results
A single high reading isn’t enough for a diagnosis in most situations. Guidelines require two abnormal test results, either from the same blood sample or from two separate samples. Your doctor might repeat the same test or run a different one. If two different tests give conflicting results, the test that came back above the diabetes threshold gets repeated, and the diagnosis is based on that confirmed result.
The one exception is when someone has a random blood sugar of 200 mg/dL or higher along with obvious symptoms of diabetes. In that scenario, a single test can be sufficient.
How They Tell Type 1 From Type 2
The blood sugar tests above can confirm that diabetes is present, but they don’t reveal which type you have. That distinction matters because the two types have different causes and require different treatment approaches.
Type 1 diabetes is an autoimmune condition where the immune system attacks the cells in your pancreas that produce insulin. To check for this, doctors test your blood for specific autoantibodies, proteins that signal an immune attack against your own body. If autoantibodies are present, that points strongly toward Type 1.
A C-peptide test also helps. C-peptide is a molecule your pancreas releases alongside insulin, so measuring it tells doctors how much insulin your body is actually producing. Normal levels fall between 0.5 and 2.0 ng/mL. People with Type 1 diabetes typically have very low C-peptide levels because their insulin-producing cells have been destroyed, while people with Type 2 often have normal or even elevated levels early on because their body is overproducing insulin to compensate for resistance.
Most adults who develop diabetes have Type 2, so autoantibody testing isn’t routine for everyone. It becomes important when the clinical picture is unclear, such as a normal-weight adult diagnosed in their 20s or 30s, or someone who doesn’t respond to standard Type 2 treatments.
Screening During Pregnancy
Gestational diabetes develops during pregnancy in people who didn’t have diabetes before. Screening typically happens between 24 and 28 weeks of pregnancy using a two-stage approach.
The first step is a one-hour glucose screening. You drink a glucose solution, and your blood is drawn an hour later. A result of 140 mg/dL or below is considered normal, and no further testing is needed.
If your result comes back above that threshold, you’ll move on to a three-hour glucose tolerance test. This time, you fast overnight and then drink a solution with 100 grams of glucose. Blood is drawn at four points: before drinking, and at one, two, and three hours after. The cutoffs are:
- Fasting: 95 mg/dL or higher
- 1 hour: 180 mg/dL or higher
- 2 hours: 155 mg/dL or higher
- 3 hours: 140 mg/dL or higher
Meeting or exceeding the threshold at two or more of these time points confirms gestational diabetes. Women at high risk due to obesity, a family history of diabetes, or a prior gestational diabetes diagnosis may be screened earlier in pregnancy.
How to Prepare for Your Test
Preparation depends on which test you’re getting. The A1C and random blood sugar tests require no preparation at all. You can eat and drink normally beforehand.
For a fasting plasma glucose test or an oral glucose tolerance test, you’ll need to fast for 8 to 12 hours. Your doctor’s office will tell you the exact window, but in practice this usually means skipping breakfast and having your blood drawn in the morning. Water is fine during the fasting period.
Certain medications can affect blood sugar readings. If you take steroids, certain blood pressure medications, or hormonal treatments, let your doctor know before testing so they can account for potential interference.
What Continuous Glucose Monitors Can and Can’t Do
Continuous glucose monitors (CGMs), the small sensors worn on the arm or abdomen that track blood sugar around the clock, are widely used by people already living with diabetes. But they are not currently approved for diagnosing diabetes or prediabetes. The three standard methods remain the A1C, fasting glucose, and oral glucose tolerance test.
There is growing interest in using CGM data as a supplementary tool, particularly for helping people in the prediabetes range understand their blood sugar patterns. The FDA is currently evaluating an application to approve CGM use for prediabetes monitoring. For now, though, insurance doesn’t cover CGM for people without a diabetes diagnosis, so any use in that space requires paying out of pocket.