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. The most common tests are the A1C, fasting blood sugar, and oral glucose tolerance test. Each uses different thresholds to sort results into normal, prediabetes, or diabetes ranges, and most positive results need to be confirmed with a second test before a diagnosis is made.
The Three Main Blood Tests
Any of these three tests can be used to diagnose Type 2 diabetes or prediabetes. Your doctor will choose based on your situation, but all three are considered equally valid.
A1C test. This 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. No fasting is required. A result below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes. An A1C result of 6.5% or higher on two separate tests confirms a diabetes diagnosis.
Fasting blood sugar test. You fast for 8 to 12 hours beforehand, then have your blood drawn. A level of 99 mg/dL or below is normal, 100 to 125 mg/dL falls in the prediabetes range, and 126 mg/dL or higher points to diabetes. Like the A1C, the result needs to hit that threshold on two separate tests to confirm the diagnosis.
Oral glucose tolerance test (OGTT). This one takes longer. After fasting overnight, you have your blood drawn, then drink a syrupy solution containing 75 grams of sugar. Your blood is drawn again at the one-hour and two-hour marks. At two hours, a reading of 140 mg/dL or below is normal, 140 to 199 mg/dL signals prediabetes, and 200 mg/dL or above indicates diabetes.
The Random Blood Sugar Test
If you walk into a doctor’s office with classic diabetes symptoms like excessive thirst, frequent urination, or unexplained weight loss, your doctor may not want to wait for a fasting test. A random plasma glucose 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 to diagnose diabetes on the spot.
How to Prepare for Fasting Tests
For the fasting blood sugar test and the OGTT, you’ll need to avoid eating or drinking anything other than plain water for 8 to 12 hours before your appointment. That means no coffee, juice, soda, or flavored water. You should also skip chewing gum, smoking, and exercise during the fast, since all of these can affect your blood sugar readings. Most people schedule these tests first thing in the morning so the fasting period falls overnight.
Why a Second Test Is Usually Needed
A single elevated result doesn’t automatically mean you have diabetes. Blood sugar can spike temporarily from stress, illness, or medication, so doctors typically require a second test to confirm. This can be a repeat of the same test or a different one. The exception is when a random blood sugar test comes back at 200 mg/dL or higher and you already have symptoms. In that case, a single result is considered sufficient.
Factors That Can Affect A1C Accuracy
The A1C test relies on red blood cells, so anything that changes how those cells behave can throw off the results. Anemia and other blood disorders can make A1C readings unreliable, as can kidney failure and liver disease. Pregnancy also affects red blood cell turnover and can skew results. If any of these apply to you, your doctor will likely use a fasting blood sugar test or an OGTT instead.
How Type 1 and Type 2 Are Told Apart
The blood sugar tests above can tell your doctor that you have diabetes, but they don’t reveal which type. That distinction matters because Type 1 and Type 2 have different causes and need different treatment.
Type 1 diabetes is an autoimmune condition where the body attacks the cells that make insulin. To check for this, doctors test your blood for specific antibodies that signal an immune attack on the pancreas. The American Diabetes Association now recommends antibody testing for people without symptoms who have a family history of Type 1 diabetes or other known risk factors. If the first antibody test is positive, a repeat test within three months confirms the result. The more types of antibodies detected, the higher the risk of developing the disease.
A C-peptide test can also help. C-peptide is a byproduct of insulin production, so low levels suggest your pancreas isn’t making enough insulin on its own, which points toward Type 1. Normal C-peptide levels fall between 0.5 and 2.0 ng/mL, though ranges vary slightly between labs. Higher or normal levels in someone with diabetes generally suggest Type 2.
Gestational Diabetes Screening
Pregnant women are typically screened for gestational diabetes between 24 and 28 weeks of pregnancy. In the U.S., most providers use a two-step approach.
The first step is a glucose challenge test. You drink a solution containing 50 grams of sugar (no fasting required), and your blood is drawn one hour later. If your blood sugar hits 140 mg/dL or higher, you move on to the second step: a longer glucose tolerance test. For this one, you fast overnight, then drink a solution with 100 grams of sugar. Your blood is tested at the fasting point, then again at one, two, and three hours after drinking. A diagnosis is made if two or more of those readings come in at or above the specified thresholds.
Some providers use a one-step approach instead, which involves fasting, drinking a 75-gram glucose solution, and having blood drawn at fasting, one hour, and two hours. With this method, just one elevated reading is enough for a diagnosis.
Screening for Children and Teens
Type 2 diabetes used to be rare in young people, but screening is now recommended for children who are at higher risk. The American Diabetes Association recommends testing starting at age 10 or the onset of puberty, whichever comes first, for kids who are overweight (BMI at the 85th percentile or above) or obese (95th percentile or above) and have at least one additional risk factor. Those risk factors include a family history of Type 2 diabetes, belonging to a higher-risk racial or ethnic group, or signs of insulin resistance. If the initial screening comes back normal, repeat testing is recommended every three years at minimum, or more often if the child’s BMI is climbing.
What Happens After a Diagnosis
If your tests confirm diabetes, your doctor will use the results to determine which type you have and how far along it is. For prediabetes, the focus is usually on lifestyle changes like diet and exercise, since blood sugar levels at this stage can often be brought back to normal. For Type 2 diabetes, treatment may involve lifestyle changes, medication, or both. Type 1 diabetes requires insulin from the start because the body can no longer produce its own. In all cases, the same blood tests used for diagnosis, particularly the A1C, become ongoing tools for tracking how well your blood sugar is being managed over time.