Diabetes is diagnosed through blood tests that measure how much sugar is in your blood. The most common tests are the A1C test, the fasting blood sugar test, and the oral glucose tolerance test, each with specific cutoff numbers that separate normal results from prediabetes and diabetes. In most cases, an abnormal result needs to be confirmed with a second test before a diagnosis is official.
The A1C Test
The A1C test measures your average blood sugar over the past two to three months. It works by looking at how much sugar has attached to your red blood cells, giving a broader picture than a single-moment snapshot. No fasting is required, which makes it one of the most convenient options.
The results come back as a percentage:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
One thing to know: certain conditions can make A1C results less reliable. If you have sickle cell trait, are pregnant, or have recently had significant blood loss, your doctor may rely on a different test instead.
Fasting Blood Sugar Test
This test measures your blood sugar after you haven’t eaten for a set period, typically 8 to 12 hours. You can drink plain water during the fast, but flavored water and other beverages should be avoided since they may contain sugars or sweeteners that affect the result. Most people schedule this as a morning blood draw after skipping breakfast.
The thresholds are straightforward:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or above
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) checks how well your body handles a large dose of sugar. You fast overnight, have your blood drawn, then drink a sugary liquid containing 75 grams of glucose. Two hours later, your blood is drawn again.
Your two-hour reading determines the result:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or above
This test is more time-consuming than the others, so it’s used less often for routine screening. It’s most commonly used during pregnancy to check for gestational diabetes.
Random Blood Sugar Test
A random blood sugar test can be done at any time, without fasting. It’s not used for routine screening. Instead, it comes into play when someone already has obvious symptoms of high blood sugar: frequent urination, excessive thirst, and unexplained weight loss. A random reading of 200 mg/dL or higher, combined with those symptoms, is enough to diagnose diabetes without a second confirmatory test.
Why You Usually Need Two Tests
A single abnormal result on an A1C, fasting glucose, or glucose tolerance test is not enough for a formal diagnosis. Confirming diabetes requires two abnormal results, either from the same blood sample using two different tests or from two separate samples taken at different times. If you get a high A1C but a normal fasting glucose (or vice versa), your doctor will repeat whichever test came back abnormal. The diagnosis is based on whichever test is consistently elevated.
This double-check exists because blood sugar levels can fluctuate day to day. Illness, stress, and even a poor night of sleep can temporarily push numbers higher. Requiring confirmation prevents a misdiagnosis based on a single off reading.
Telling Type 1 From Type 2
The blood sugar tests above confirm that diabetes is present, but they don’t distinguish between Type 1 and Type 2. In most cases, the distinction is clear from context: a child or teenager who loses weight rapidly and develops sudden, severe symptoms almost certainly has Type 1, while an overweight adult with gradually rising blood sugar typically has Type 2.
When the picture is less clear, a C-peptide test can help. C-peptide is a substance your pancreas releases alongside insulin, so measuring it reveals how much insulin your body is actually producing. A low C-peptide level points toward Type 1 diabetes, where the immune system has destroyed the insulin-producing cells. A normal or high level suggests Type 2, where the body still makes insulin but can’t use it efficiently. Autoantibody blood tests can also confirm Type 1 by detecting immune proteins that attack the pancreas.
Gestational Diabetes Screening
Pregnant women are typically screened for gestational diabetes between 24 and 28 weeks. The most common approach in the U.S. uses two steps. First, you drink a smaller sugar solution (50 grams) without any fasting required. Your blood is drawn one hour later. If your glucose is at or above the screening threshold (commonly 140 mg/dL, though some providers use 130 or 135 mg/dL), you move on to the second step.
The confirmatory test is a three-hour glucose tolerance test. You fast overnight, then drink a 100-gram glucose solution. Your blood is drawn at fasting, one hour, two hours, and three hours. The cutoffs for abnormal results at each point are:
- Fasting: 95 mg/dL or higher
- One hour: 180 mg/dL or higher
- Two hours: 155 mg/dL or higher
- Three hours: 140 mg/dL or higher
Two or more abnormal values out of those four readings establish a gestational diabetes diagnosis. A single abnormal value doesn’t meet the threshold, but your provider may still monitor you more closely.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends that adults aged 35 to 70 who are overweight or obese get screened for prediabetes and Type 2 diabetes, even if they feel perfectly fine. This age threshold was recently lowered from 40 to 35. “Overweight” in this context means a BMI of 25 or higher.
Certain groups face higher risk and should consider screening earlier or at a lower weight. Asian Americans should be screened starting at a BMI of 23. People who are American Indian, Alaska Native, Black, Hispanic or Latino, or Native Hawaiian or Pacific Islander have a disproportionately high prevalence of diabetes and may benefit from screening before age 35. A family history of diabetes, a history of gestational diabetes, or polycystic ovary syndrome also warrant earlier testing.
If your results come back normal, repeat screening every three years is a reasonable timeline. If you fall in the prediabetes range, your provider will likely recommend retesting more frequently, along with lifestyle changes that can delay or prevent progression to diabetes.