Getting tested for diabetes is straightforward and usually requires nothing more than a simple blood draw. There are four main tests doctors use, and most can be done during a routine office visit. Which test you get depends on your symptoms, risk factors, and whether you’re pregnant.
The Four Main Diabetes Tests
Each test measures blood sugar in a slightly different way. Some require fasting, others don’t. Here’s what to expect from each one.
A1C Test
This is the most convenient option. It measures your average blood sugar over the past two to three months, so it gives a broader picture than a single-day snapshot. No fasting is required. A technician draws blood from your arm, and results typically come back within a day or two. An A1C below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
Fasting Blood Sugar Test
This test measures your blood sugar at a single point in time, after you haven’t eaten or drunk anything (besides water) for at least eight hours. It’s usually scheduled first thing in the morning, before breakfast. A result of 99 mg/dL or below is normal. Between 100 and 125 mg/dL falls in the prediabetes range, and 126 mg/dL or above indicates diabetes.
Oral Glucose Tolerance Test
This is the most involved test, but it’s also the most sensitive for catching prediabetes and gestational diabetes. You fast overnight, then have your blood drawn. Next, you drink a sugary liquid, and your blood is drawn again at intervals, typically at one hour, two hours, and sometimes three hours afterward. A two-hour reading of 140 mg/dL or below is normal. Between 140 and 199 mg/dL suggests prediabetes, and 200 mg/dL or above means diabetes. The whole process takes about two to three hours at the lab.
Random Blood Sugar Test
If you’re showing symptoms and your doctor doesn’t want to wait for you to fast, they can order this test at any time of day. No preparation is needed. A result of 200 mg/dL or higher, combined with symptoms like excessive thirst or frequent urination, points to diabetes. This test isn’t used to diagnose prediabetes since it only captures a single moment without a fasting baseline.
Do You Need a Second Test to Confirm?
In most cases, yes. If your first test comes back in the diabetes range and you don’t have obvious symptoms, your doctor will typically repeat the same test or run a different one to confirm the diagnosis. Two abnormal results are generally needed before a formal diagnosis is made. The exception is when someone already has clear symptoms like unexplained weight loss, constant thirst, and frequent urination, paired with a random blood sugar of 200 mg/dL or higher. In that situation, one test is often enough.
How Gestational Diabetes Screening Works
Pregnant women are usually screened between 24 and 28 weeks. The process typically starts with a one-hour glucose challenge: you drink a sugary solution, and your blood is drawn an hour later. A result of 190 mg/dL or higher at the one-hour mark indicates gestational diabetes. If your result falls below that threshold but is still elevated, your doctor may follow up with a longer two-hour or three-hour test. On the two-hour version, a reading of 153 mg/dL or higher points to gestational diabetes.
If you had gestational diabetes in a previous pregnancy or have other risk factors, your doctor may screen you earlier than 24 weeks.
How Doctors Tell Type 1 From Type 2
The standard blood sugar tests can tell you whether you have diabetes, but they don’t always distinguish between type 1 and type 2. In most cases, your age, weight, symptoms, and how quickly the condition developed make the answer obvious. Type 1 tends to appear suddenly, often in children or young adults, while type 2 develops gradually and is more common in adults with excess weight.
When the picture isn’t clear, doctors can order a C-peptide test. C-peptide is a substance your pancreas releases alongside insulin. A low level means your body isn’t producing much insulin, which is the hallmark of type 1 diabetes. A high level means your body is producing plenty of insulin but isn’t using it efficiently, which is typical of type 2. Doctors may also test for specific autoantibodies that attack the pancreas, since their presence strongly suggests type 1.
Can You Test at Home?
Home A1C kits are available over the counter, and two of them (Home Access and A1cNow+) are FDA-cleared for home use. But accuracy varies widely. A University of Florida study of 219 people with diabetes compared home kits to standard lab results. Home Access had 82% of its samples within the accepted accuracy range. A1cNow+ hit that benchmark only 46% of the time, and a third kit, CoreMedica, managed just 29%.
A home kit can give you a rough idea of where you stand, but it’s not reliable enough to diagnose or rule out diabetes on its own. If a home test shows an elevated result, follow up with a lab test. And if you have symptoms, skip the home kit and go straight to your doctor.
What the Numbers Mean in Practice
Understanding where your results fall helps you know what comes next. Here’s a quick breakdown of the three categories across the most common tests:
- Normal: A1C below 5.7%, fasting blood sugar 99 mg/dL or below, two-hour glucose tolerance 140 mg/dL or below
- Prediabetes: A1C 5.7% to 6.4%, fasting blood sugar 100 to 125 mg/dL, two-hour glucose tolerance 140 to 199 mg/dL
- Diabetes: A1C 6.5% or higher, fasting blood sugar 126 mg/dL or higher, two-hour glucose tolerance 200 mg/dL or higher
A prediabetes result isn’t a diabetes sentence. It means your blood sugar is higher than normal but hasn’t crossed the diabetes threshold. Losing 5% to 7% of your body weight and getting regular physical activity can significantly lower the risk of progressing to type 2 diabetes.
How to Prepare for Your Test
If your test requires fasting, stop eating and drinking everything except water at least eight hours beforehand. Most people schedule these tests for early morning so the fasting period overlaps with sleep. For the A1C and random blood sugar tests, no preparation is needed.
Some medications and supplements can affect results. If you take anything that influences blood sugar, such as steroids or certain psychiatric medications, let your doctor know before the test. Conditions that affect red blood cells, like sickle cell trait or recent blood loss, can also make A1C results less reliable, in which case your doctor will likely use a fasting or glucose tolerance test instead.