Diabetes is diagnosed through blood tests that measure how your body handles sugar. There are four main tests used, and each one looks at blood sugar in a slightly different way. Most people will start with either an A1C test or a fasting blood sugar test, both of which require only a simple blood draw.
The A1C Test
The A1C test is one of the most common ways to screen for diabetes because it requires no fasting and no special preparation. It measures the percentage of your red blood cells that have sugar attached to them, which 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
Because the A1C captures a long-term average, it’s less affected by what you ate the night before or how stressed you felt that morning. That makes it convenient and reliable for most people. However, certain conditions like sickle cell disease or recent blood transfusions can skew the results, so your doctor may choose a different test in those situations.
Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for at least eight hours. Most people schedule it first thing in the morning so the fasting period overlaps with sleep. You can drink water during the fast, but nothing else.
The thresholds are straightforward:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
A fasting blood sugar test gives a snapshot of how well your body manages glucose overnight, when no food is coming in. If your body can’t keep blood sugar in a normal range even without food, that’s a sign insulin isn’t working effectively. The test is quick and widely available at any lab or clinic.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) is more involved but gives a detailed picture of how your body responds to a sugar load. You fast overnight, then drink a sweet liquid containing 75 grams of glucose. Your blood is drawn two hours later to see how efficiently your body cleared that sugar from your bloodstream.
At the two-hour mark:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
This test is particularly useful for catching prediabetes and early diabetes that other tests might miss. It’s also the standard method for screening during pregnancy, typically performed between 24 and 28 weeks. Women with higher risk factors may be screened earlier.
Random Blood Sugar Test
A random blood sugar test can be done at any time of day, regardless of when you last ate. It’s not typically used for routine screening. Instead, it’s ordered when someone is already showing classic symptoms of diabetes, like excessive thirst, frequent urination, unexplained weight loss, or blurred vision.
A result of 200 mg/dL or higher, combined with those symptoms, is enough for a diagnosis without further confirmation. This is the one scenario where a single test result can lead directly to a diabetes diagnosis.
How Diagnosis Gets Confirmed
Outside of that clear-cut situation with obvious symptoms and a high random blood sugar, a single abnormal result isn’t enough to diagnose diabetes. The American Diabetes Association’s current standards require two abnormal results for confirmation. Those can come from two different tests done at the same visit (for example, an A1C and a fasting blood sugar drawn on the same day) or from the same test repeated on a separate occasion.
If two different tests give conflicting results, where one is in the diabetes range and the other isn’t, the abnormal test gets repeated. The diagnosis is based on whichever confirmatory result comes back above the threshold. This two-test rule exists because blood sugar levels naturally fluctuate, and a single reading can be misleading.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends screening for adults aged 35 to 70 who are overweight or obese (a BMI of 25 or higher). Earlier screening is recommended for people from populations with higher diabetes rates, including American Indian/Alaska Native, Black, Hispanic/Latino, and Native Hawaiian/Pacific Islander communities. Asian Americans are advised to screen at a lower BMI threshold of 23.
Outside of these guidelines, testing makes sense anytime you have risk factors like a family history of diabetes, a history of gestational diabetes, polycystic ovary syndrome, or physical inactivity. Many people with prediabetes have no symptoms at all, which is why routine screening matters. Catching prediabetes early gives you a window to reverse it through lifestyle changes before it progresses.
Testing for Type 1 vs. Type 2
The blood sugar tests above can tell you whether you have diabetes, but they don’t distinguish between Type 1 and Type 2. That distinction matters because the two conditions have different causes and require different treatment approaches. Type 1 is an autoimmune condition where the immune system attacks the cells that produce insulin. Type 2 involves the body becoming resistant to insulin over time.
To tell them apart, doctors use autoantibody blood tests. These look for immune proteins that target insulin-producing cells. A panel typically checks for four different autoantibodies, and when all four are tested together, the detection rate for Type 1 diabetes reaches about 98%. Fewer than 3% of people with Type 2 diabetes test positive for any of these antibodies, making the distinction quite reliable. The more antibodies that come back positive, the stronger the evidence for a Type 1 diagnosis.
This testing is most commonly ordered when the diagnosis is unclear, for example, in adults who develop diabetes suddenly or in people who don’t fit the typical profile for Type 2.
At-Home A1C Test Kits
Over-the-counter A1C kits are available at pharmacies and online, and they can give you a rough sense of where your blood sugar stands. However, their accuracy varies significantly. The accepted benchmark is that a home test should produce results within 5% of a lab-drawn blood test at least 90% of the time.
A University of Florida study of 219 people with diabetes tested three commercial home kits against lab results. The best performer, called Home Access, met that accuracy standard 82% of the time, which still falls short of the 90% benchmark. The other two products tested hit the mark only 46% and 29% of the time. That level of inaccuracy could easily place someone in the wrong diagnostic category, making them think they’re in the normal range when they actually have prediabetes, or vice versa.
Home kits can be useful for general monitoring between doctor visits, but they aren’t reliable enough to diagnose or rule out diabetes on their own. A lab-based test remains the standard for any clinical decision.