Diabetes is diagnosed through 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 works differently, and the one your doctor orders depends on your symptoms, risk factors, and whether you’re pregnant. Here’s what each test involves and what the results mean.
The A1C Test
The A1C test is one of the most widely used screening tools because it doesn’t require fasting or drinking anything special. It measures your average blood sugar over the past two to three months by looking at how much sugar has attached to your red blood cells. A simple blood draw is all it takes.
Results fall into three ranges:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
One important caveat: the A1C test can give misleading results if you have certain blood conditions that affect your red blood cells, such as sickle cell trait or some forms of anemia. If you have one of these conditions, your doctor will likely rely on a different test instead.
Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for at least eight hours. Fasting means nothing to eat or drink except sips of water. For the most reliable results, the test is typically done first thing in the morning.
A fasting blood sugar below 100 mg/dL is normal. Between 100 and 125 mg/dL indicates prediabetes. A result of 126 mg/dL or higher, confirmed on a second test, means diabetes. The fasting test is straightforward and inexpensive, which is why it remains a go-to option in many clinical settings.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how efficiently your body processes sugar in real time. You fast for at least eight hours, then drink a syrupy solution containing 75 grams of sugar. Your blood is drawn before the drink, then again at one hour and two hours afterward.
A two-hour blood sugar below 140 mg/dL is normal. Between 140 and 199 mg/dL points to prediabetes. A reading of 200 mg/dL or higher indicates diabetes. This test is more time-consuming than the others, so it’s used less often for routine screening. But it can catch problems that the fasting test misses, particularly in the early stages of insulin resistance.
Random Blood Sugar Test
If you’re already showing symptoms of diabetes, like excessive thirst, frequent urination, unexplained weight loss, or blurry vision, your doctor may skip the fasting requirement entirely. A random blood sugar test can be done at any time of day regardless of when you last ate. A result of 200 mg/dL or above, combined with symptoms, is enough for a diabetes diagnosis.
Testing During Pregnancy
Gestational diabetes develops during pregnancy and is typically screened between 24 and 28 weeks. The most common approach in the U.S. is a two-step process.
In step one, you drink a smaller sugar solution (50 grams) without needing to fast. Your blood is drawn one hour later. If your blood sugar exceeds a certain threshold, you move to step two: a longer test. For this, you fast overnight, drink a solution with 100 grams of sugar, and have your blood drawn at one, two, and three hours. A diagnosis of gestational diabetes is made when two or more of those blood sugar readings come back above the cutoff values.
Some providers use a one-step approach instead, which involves fasting and drinking a 75-gram solution, then testing at one and two hours. Only one elevated reading is needed for diagnosis with this method.
How Type 1 and Type 2 Are Told Apart
The blood sugar tests above can diagnose 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 strategies.
Type 1 diabetes is an autoimmune condition where the immune system attacks the cells in the pancreas that produce insulin. To confirm it, doctors test for specific autoantibodies in the blood, particularly anti-GAD and anti-IA2 antibodies. They may also measure C-peptide, a molecule released alongside insulin. A very low C-peptide level (below 0.2 nmol/L) strongly correlates with Type 1 diabetes, because it signals the pancreas is producing little to no insulin on its own.
These tests are also useful for diagnosing LADA, sometimes called “Type 1.5 diabetes,” which develops more slowly in adults and can initially look like Type 2. People with LADA have lower C-peptide levels than those with Type 2 but may still produce some insulin early on. Autoantibody testing confirms the diagnosis.
Who Should Get Tested and When
The American Diabetes Association recommends that all adults begin screening at age 35, even without symptoms. If you’re younger than 35 but overweight or obese with additional risk factors (a family history of diabetes, a history of gestational diabetes, polycystic ovary syndrome, or belonging to a higher-risk ethnic group), screening should start earlier.
If your results come back normal, repeat testing every three years is generally sufficient. If you gain significant weight or develop new risk factors in the meantime, earlier retesting makes sense. A prediabetes result (A1C between 5.7% and 6.4%) typically prompts more frequent monitoring, often annually.
What About At-Home Tests?
You can buy over-the-counter A1C kits at most pharmacies. These use a finger-prick blood sample and deliver results in minutes. However, their accuracy is noticeably lower than lab tests. A 2022 study comparing three commercial home A1C kits found that none met the national standardization goal of at least 90% of readings falling within 5% of the true lab value. The best-performing kit hit 82% accuracy, while the worst managed only 29%.
A home kit can give you a rough sense of where you stand, but it’s not reliable enough to confirm or rule out a diagnosis. If a home test shows an elevated result, follow up with a lab test. And if your result looks normal but you have symptoms or risk factors, don’t assume you’re in the clear.
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. For a fasting blood sugar test or oral glucose tolerance test, you need to fast for at least eight hours beforehand. Water is fine, but skip food, coffee, juice, and anything else with calories. Schedule these tests for first thing in the morning so you can sleep through most of the fasting window.
If you’re on medications, ask your provider ahead of time whether any of them could affect your results. Some medications can temporarily raise blood sugar, which could lead to a falsely elevated reading.