Preventive Health · 2026-08-27 · 4 min read
Diabetes Screening: A1C, Fasting Glucose, and Who Should Be Tested
The two tests, the cutoffs for prediabetes and diabetes, when to start screening, and the situations where A1C misleads.
About one in three American adults has prediabetes. Most do not know it. The number who progress to type 2 diabetes each year is large and mostly preventable. The screening is one blood test. This guide covers which test, what the numbers mean, and who should be getting it.
Who should be screened
The US Preventive Services Task Force recommends screening adults 35 to 70 who are overweight or obese, repeated every three years if normal. The American Diabetes Association goes further: all adults at 35, and any adult at any age who is overweight and has one more risk factor.
The risk factors that move screening earlier:
- A parent or sibling with type 2 diabetes
- High blood pressure or high cholesterol
- A history of gestational diabetes
- Polycystic ovary syndrome
- Physical inactivity
- Belonging to a group with higher rates, including Black, Hispanic, Native American, Asian American, and Pacific Islander populations
- A prior A1C in the prediabetes range, which moves the interval to yearly
If you are unsure, the CDC has a one minute risk test online. A high score is reason enough to ask for the blood test.
The two tests
Hemoglobin A1C. Measures the share of hemoglobin in red blood cells that has glucose attached. Because red cells live about three months, A1C reflects average blood sugar over that window. No fasting needed. One tube of blood, or a fingerstick on some point of care devices.
Fasting plasma glucose. Measures blood sugar after at least eight hours without food or drink other than water. A single snapshot. Cheap and widely available.
A third option, the oral glucose tolerance test, involves drinking a sugar solution and testing two hours later. It is more sensitive but less convenient, and it is mostly used in pregnancy.
The cutoffs
| Result | A1C | Fasting glucose |
|---|---|---|
| Normal | Below 5.7% | Below 100 mg/dL |
| Prediabetes | 5.7% to 6.4% | 100 to 125 mg/dL |
| Diabetes | 6.5% or higher | 126 mg/dL or higher |
A diabetes diagnosis normally requires two abnormal results, either the same test repeated or two different tests on the same day, unless blood sugar is very high with symptoms.
When A1C misleads
A1C depends on red blood cells behaving normally. When they do not, the number is wrong in a predictable direction.
- Iron deficiency anemia raises A1C. Cells live longer and pick up more glucose.
- Blood loss, hemolysis, or recent transfusion lowers it. Cells are younger.
- Hemoglobin variants such as sickle trait can interfere with some lab methods. Most modern methods correct for this, but not all.
- Pregnancy lowers A1C in the second and third trimesters. Glucose tests are used instead.
- Advanced kidney disease and some HIV medications can shift it.
If any of these apply, fasting glucose or the tolerance test is the better screen. A clinician who knows your history will pick the right one.
Home testing
Home A1C kits exist. Most are fingerstick cartridges read by a small meter or mailed to a lab. The good ones are accurate to within a few tenths of a percent, which is fine for screening and rough tracking. They are not accurate enough to diagnose on their own, and they do not replace the lab test that a diagnosis requires.
Home glucose meters, the kind used by people with diabetes, can also give a fasting reading. Meter results are allowed to vary by up to 15 percent from a lab value, so a fasting reading of 105 on a meter could be 95 or 118 in the lab. Use a meter to decide whether to get a lab test, not to skip one.
What a prediabetes result means
It is the most useful result on the list. It means the process has started and can still be reversed. The evidence for structured lifestyle programs is strong: the Diabetes Prevention Program trial cut progression to diabetes by 58 percent over three years with modest weight loss and regular activity. Many insurers and the CDC fund these programs.
It also means a yearly recheck instead of every three years, a look at blood pressure and cholesterol, and a conversation about kidney screening. See kidney health urine tests.
What a diabetes result means
A confirmed result starts a standard workup: repeat testing, a kidney check, an eye exam, a foot exam, and a treatment plan. It is not the emergency it once was. It is a long project that goes better when it starts early. Which is the whole argument for screening.
By the Ribbon Health Press editorial team. Reviewed against current clinical guidelines at publication. Report an error: editors@ribboncheckup.org. See our editorial policy.