Diabetes is diagnosed through blood tests that measure how your body handles sugar. The most common are the A1C test, the fasting blood sugar test, and the oral glucose tolerance test. Each one measures something slightly different, and your doctor may use one or more 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 two to three months. It works by looking at the percentage of your red blood cells that have sugar attached to them. The higher your blood sugar has been, the higher that percentage climbs.
The results break down into three categories:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
The A1C is convenient because you don’t need to fast beforehand. You can eat normally and have blood drawn at any time of day. That makes it a popular choice for routine screening. However, certain conditions like anemia or sickle cell disease can affect the results, in which case your doctor will 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, typically overnight. It gives a snapshot of your baseline glucose level, the amount of sugar circulating in your blood when your body isn’t actively processing a meal.
The thresholds are straightforward:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or above
If your result comes back in the diabetes range, your doctor will typically repeat the test on a different day to confirm. A single elevated reading isn’t enough for a diagnosis on its own unless you also have clear symptoms like excessive thirst, frequent urination, or unexplained weight loss.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how well your body processes sugar in real time. You fast for eight hours, then have your blood drawn. After that, you drink a solution containing 75 grams of glucose (essentially a very sweet drink), and your blood is drawn again two hours later.
For the two-hour reading:
- Normal: 139 mg/dL or below
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or above
This test is more time-consuming than the others because you need to stay at the lab for the full two hours. It’s particularly useful for catching cases where fasting blood sugar looks borderline but your body actually struggles to bring sugar levels back down after eating.
Random Blood Sugar Test
When someone walks into a clinic with obvious diabetes symptoms (extreme thirst, frequent urination, blurred vision, unexplained weight loss), doctors don’t always want to wait for a fasting test. A random blood sugar test can be done at any time regardless of when you last ate. A result of 200 mg/dL or above, combined with symptoms, is enough for a diabetes diagnosis.
This test isn’t used for routine screening. It’s reserved for situations where a quick answer is needed because symptoms are already present.
Gestational Diabetes Screening
Pregnant women are screened for gestational diabetes between 24 and 28 weeks of pregnancy using a slightly different process. The most common approach in the U.S. has two steps. First, you drink a smaller sugar solution (50 grams) and have your blood drawn one hour later. If your blood sugar exceeds a certain threshold, you come back for a longer follow-up test: fasting overnight, drinking a 100-gram glucose solution, then having blood drawn at one, two, and three hours. Two or more elevated readings on that second test confirm gestational diabetes.
Some providers use a one-step approach instead, which skips the initial screening drink and goes straight to a 75-gram, two-hour glucose tolerance test after fasting. Only one elevated value is needed for a diagnosis with this method. Your provider will tell you which approach they use.
Tests That Distinguish Type 1 From Type 2
The tests above confirm that you have diabetes, but they don’t tell you which type. In most cases, doctors can figure this out from context: your age, weight, how quickly symptoms appeared, and whether you have a family history. But when the picture isn’t clear, two additional tests help sort it out.
Autoantibody Tests
Type 1 diabetes is an autoimmune condition where the immune system attacks the insulin-producing cells in the pancreas. Autoantibody tests look for immune markers that signal this attack. Five antibodies are commonly tested, with GAD being the most widely used. If you test positive for two or more of these antibodies, the likelihood of type 1 diabetes is above 90%. Doctors typically test for at least two antibodies at the same time rather than checking just one.
C-Peptide Test
C-peptide is a substance your pancreas releases alongside insulin in equal amounts. Because it stays in the blood longer than insulin and isn’t affected by insulin injections, it gives an accurate picture of how much insulin your body is actually producing on its own. A low C-peptide level points toward type 1 diabetes, where the pancreas can barely make insulin. A high or normal level is more consistent with type 2, where the body produces insulin but can’t use it effectively.
Urine Ketone Tests
Urine tests don’t diagnose diabetes, but they play a role once you have it. Ketone testing checks for a dangerous buildup of acids in your blood called ketoacidosis. This happens when your body doesn’t have enough insulin and starts burning fat for energy instead of sugar. The byproducts of that fat-burning, called ketones, accumulate and make your blood dangerously acidic.
Ketoacidosis is most common in type 1 diabetes but can also occur in type 2. Early warning signs include extreme thirst, frequent urination, and dehydration. As it progresses, symptoms can escalate to nausea, abdominal pain, confusion, difficulty breathing, and fruity-smelling breath. Home urine test strips are available for people whose doctors recommend frequent monitoring. Results typically show as small, moderate, or large amounts of ketones.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends screening for prediabetes and type 2 diabetes in adults aged 35 to 70 who are overweight or obese. But screening should start earlier if you belong to a group with higher diabetes rates, including American Indian/Alaska Native, Black, Hispanic/Latino, Native Hawaiian/Pacific Islander, or Asian American populations. For Asian Americans, screening is also recommended at a lower BMI (23 or above instead of 25). A family history of diabetes, a history of gestational diabetes, or polycystic ovary syndrome are also reasons to start screening sooner.
If your results come back in the prediabetes range, repeat testing every one to three years makes sense, since prediabetes progresses to type 2 diabetes in a meaningful percentage of people but can also be reversed with lifestyle changes.