Prediabetes is diagnosed with a simple blood test, and there are three standard options your doctor can use. Any one of them can confirm a prediabetes diagnosis based on where your blood sugar falls within a specific range. The most common is the A1C test, which requires no fasting and gives a snapshot of your average blood sugar over the past two to three months.
The Three Blood Tests Used
Each test measures blood sugar differently, but all three are equally accepted for diagnosing prediabetes. Your doctor will typically choose based on convenience, your medical history, and what’s available at the lab.
A1C test: This measures the percentage of your red blood cells that have sugar attached to them. Because red blood cells live about three months, the result reflects your average blood sugar over that period rather than a single moment. A result between 5.7% and 6.4% indicates prediabetes. Below 5.7% is normal, and 6.5% or higher points to diabetes. The big advantage of this test is that you don’t need to fast beforehand, so it can be done at any time of day.
Fasting blood sugar test: You fast for at least 8 hours (usually overnight), then have blood drawn. A result of 100 to 125 mg/dL falls in the prediabetes range. Normal is 99 mg/dL or below, and 126 mg/dL or above indicates diabetes.
Oral glucose tolerance test (OGTT): This one takes more time. You fast overnight, have your blood drawn, then drink a sugary solution containing 75 grams of glucose. Two hours later, your blood is drawn again. A two-hour result between 140 and 199 mg/dL means prediabetes. This test shows how efficiently your body processes sugar in real time, which makes it particularly useful for catching early problems that the other tests might miss.
What to Expect During Each Test
The A1C test is the simplest experience. It’s a standard blood draw with no preparation needed. You can eat and drink normally beforehand, and results typically come back within a day or two.
For the fasting blood sugar test, you’ll need to stop eating and drinking anything other than water for at least 8 hours before the blood draw. Most people schedule it first thing in the morning so the fasting period overlaps with sleep. The draw itself takes just a few minutes.
The oral glucose tolerance test requires the most commitment. After fasting overnight, you’ll arrive at the lab, have an initial blood draw, and then drink the glucose solution, which tastes like a very sweet flat soda. You’ll wait at the lab or clinic for two hours before the second blood draw. Plan to bring something to read or do, because you’ll need to stay put. Some people feel a bit nauseous from the sugary drink, but this usually passes quickly.
Do You Need More Than One Test?
A single test result in the prediabetes range is generally enough for your doctor to make the diagnosis, especially if you have risk factors. However, for a diabetes diagnosis (results above the prediabetes range), a second test on a separate day is typically required to confirm. This distinction matters because a diabetes diagnosis carries more significant treatment implications.
Once you’ve been identified as having prediabetes, your doctor will usually recheck your blood sugar at least once a year to track whether your levels are improving, holding steady, or moving toward diabetes. For children with prediabetes, annual testing is recommended, or more frequently if they gain weight or develop symptoms like increased thirst, frequent urination, or fatigue.
When A1C Results May Be Unreliable
The A1C test works by measuring sugar on red blood cells, which means anything that affects those cells can throw off the result. Conditions that change how long your red blood cells survive, such as sickle cell trait, iron-deficiency anemia, or recent blood loss, can produce misleadingly high or low A1C readings. Pregnancy, kidney disease, and recent blood transfusions can also skew results.
If you have any of these conditions, your doctor will likely rely on the fasting blood sugar test or the oral glucose tolerance test instead. Both measure your blood sugar directly rather than through a red blood cell proxy, so they aren’t affected by the same variables.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends screening for adults aged 35 to 70 who are overweight or obese, defined as a BMI of 25 or higher. The American Diabetes Association casts a wider net, recommending that all adults 45 and older get tested regardless of weight, and that younger adults get tested if they’re overweight and have at least one additional risk factor.
Those risk factors include a family history of type 2 diabetes, a history of gestational diabetes, polycystic ovary syndrome, physical inactivity, and belonging to a racial or ethnic group with higher diabetes rates. American Indian, Alaska Native, Black, Hispanic/Latino, and Native Hawaiian/Pacific Islander populations all face disproportionately higher risk.
For Asian Americans, the BMI threshold is lower. Both the USPSTF and ADA recommend screening at a BMI of 23 or higher rather than 25, because Asian adults tend to develop type 2 diabetes at lower body weights. Research on Chinese, Filipino, and South Asian adults has shown that prediabetes and diabetes risk can be elevated even in what would traditionally be considered a healthy weight range.
For children and adolescents, the American Diabetes Association recommends risk-based screening starting at age 10 or the onset of puberty, whichever comes first, if the child is overweight (BMI at or above the 85th percentile) and has at least one additional risk factor.
At-Home A1C Test Kits
Over-the-counter A1C test kits are available at most pharmacies and online. These use a finger prick rather than a full blood draw. In a study published in the Journal of Diabetes Science and Technology, 93.2% of home test results fell within the acceptable accuracy range compared to lab values, and the correlation between home and lab results was strong. Most users found the kits easy to use.
That said, a home kit is best used as a screening tool rather than a final diagnosis. If your result comes back in the prediabetes range (5.7% to 6.4%), it’s worth getting a lab-confirmed test. A home result can’t account for the conditions that affect A1C accuracy, and your doctor will want a verified number before making any recommendations. In the study, over 93% of people who used the home kit said they were very or extremely likely to discuss their results with their doctor, which is exactly how these kits work best: as a starting point for a conversation, not an endpoint.