Getting diagnosed with type 2 diabetes typically starts with a simple blood test ordered by your primary care doctor. Three main tests are used, each with specific cutoff numbers that separate normal blood sugar from prediabetes and diabetes. In most cases, you’ll need two abnormal results to confirm the diagnosis, either from the same blood sample or from a follow-up test.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends screening for all adults aged 35 to 70 who are overweight or obese (a BMI of 25 or higher). If you’re Asian American, screening is recommended at a lower BMI of 23 or higher because type 2 diabetes develops at lower body weights in this population. Screening is also recommended earlier than age 35 for Black, Hispanic/Latino, American Indian/Alaska Native, and Native Hawaiian/Pacific Islander adults, who face higher rates of diabetes overall.
You don’t need symptoms to get screened. In fact, many people with type 2 diabetes have no noticeable symptoms for years. But if you’re experiencing frequent urination, unusual thirst, unexplained weight loss, constant fatigue, blurry vision, or recurring yeast or urinary tract infections, those are strong reasons to ask your doctor for testing right away rather than waiting for a routine screening.
The Three Main Diagnostic Tests
A1C Test
The A1C test measures your average blood sugar over the past two to three months. It doesn’t require fasting, so it can be done at any time of day. The results come back as a percentage:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
Because the A1C reflects a longer window of blood sugar control rather than a single moment, it’s one of the most commonly used tests for diagnosis. Certain conditions like anemia or recent blood transfusions can affect the accuracy of the result, 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 8 hours, which is why it’s usually done first thing in the morning. You can drink water, but nothing else. The thresholds are:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
Oral Glucose Tolerance Test
This test is more involved. You fast overnight, then have your blood drawn. After that, you drink a sugary solution containing 75 grams of glucose. Your blood is drawn again one hour later and two hours later. A blood sugar reading of 200 mg/dL or higher at the two-hour mark indicates diabetes. This test is less commonly used for routine screening because it takes longer and requires you to stay at the lab, but it can catch cases that a fasting test or A1C might miss.
What Happens If Your Numbers Are High
One abnormal result usually isn’t enough. Diagnosis requires two abnormal test results, either from the same blood draw or from two separate visits. Your doctor might repeat the same test or run a different one to confirm. If two different tests give conflicting results (say your A1C is in the diabetes range but your fasting glucose is not), the test that came back above the diagnostic threshold gets repeated. The diagnosis is based on whichever test is confirmed.
There is one exception to the two-test rule. If you show up with classic symptoms of high blood sugar, such as excessive thirst, frequent urination, and unexplained weight loss, and a random blood sugar test (taken at any time of day, regardless of meals) comes back at 200 mg/dL or higher, that single result is enough for a diagnosis. This scenario typically applies when someone is already feeling quite sick.
How Type 2 Is Distinguished From Type 1
The blood sugar tests themselves don’t tell your doctor which type of diabetes you have. They only confirm that your blood sugar is too high. In most adults, especially those who are overweight and over 35, type 2 is the presumed diagnosis based on the overall clinical picture: age, weight, family history, and how gradually the symptoms developed.
But the line isn’t always clear. Some adults develop type 1 diabetes later in life, and a slow-onset form called latent autoimmune diabetes in adults (LADA) can look exactly like type 2 in the early stages. Misdiagnosis between the two types is a recognized problem. If your doctor suspects type 1 or LADA, they can order antibody tests that detect an immune attack on the insulin-producing cells in your pancreas, along with a C-peptide test that measures how much insulin your body is still making on its own. These tests aren’t part of routine diagnosis but become important if your blood sugar is hard to control with standard type 2 treatments or if your clinical profile doesn’t quite fit.
Getting the Process Started
If you think you should be screened, the first step is straightforward: ask your doctor for a blood test at your next visit. You can request an A1C at any appointment since it doesn’t require fasting. If your doctor orders a fasting glucose test instead, you’ll need to schedule a morning blood draw and skip breakfast beforehand. Results typically come back within a day or two.
Many people find out they have prediabetes before they ever reach a diabetes diagnosis. If your numbers fall in the prediabetes range, that’s actually useful information. It means your blood sugar is elevated but hasn’t crossed the threshold yet, and lifestyle changes at this stage, particularly losing 5 to 7 percent of your body weight and increasing physical activity, can significantly delay or prevent progression to type 2 diabetes.
If your results do confirm type 2 diabetes, your doctor will likely order additional blood work to check kidney function, cholesterol levels, and other markers that help guide your treatment plan. You’ll also have your A1C rechecked periodically, usually every three to six months, to track how well your blood sugar is being managed over time.