--- title: "Diabetes tests: A1C, fasting, glucose tolerance and random" description: "The four blood tests used to diagnose diabetes and prediabetes, every cutoff in one table, how to prepare, and why a home meter can't give a diagnosis." canonical: https://glucosebench.com/diabetes-tests alternate_html: https://glucosebench.com/diabetes-tests language: en-US last_updated: 2026-09-27 type: guia --- # Which blood test diagnoses diabetes? All four, compared 1. [Home](https://glucosebench.com/) 2. [Blood sugar basics](https://glucosebench.com/blood-sugar) 3. Which blood test diagnoses diabetes? All four, compared There isn't one diabetes test. There are four, each measuring something a little different, and a doctor often wants two results before calling it diabetes. By **GlucoseBench editorial team**Updated September 27, 2026 ![Which blood test diagnoses diabetes? All four, compared](https://glucosebench.com/img/guia-diabetes-tests-640.webp) Short answer Type 2 diabetes and prediabetes are diagnosed with blood tests: the A1C, the fasting plasma glucose test, the oral glucose tolerance test (OGTT) and, when there are symptoms, the random plasma glucose test. The diabetes cutoffs are an A1C of 6.5% or above, a fasting result of 126 mg/dL or above, and 200 mg/dL or above on the 2-hour OGTT or on a random test, per the NIDDK. Usually a second test confirms the diagnosis. Testing equipment sold over the counter, such as a blood glucose meter, can't diagnose diabetes. ## Diagnosis cutoffs for each test Result A1C Fasting plasma glucose Oral glucose tolerance test (2 hours) Random plasma glucose Normal Below 5.7% 99 mg/dL or below 139 mg/dL or below Not applicable Prediabetes 5.7% to 6.4% 100 to 125 mg/dL 140 to 199 mg/dL Not applicable Diabetes 6.5% or above 126 mg/dL or above 200 mg/dL or above 200 mg/dL or above, used when there are symptoms Source: NIDDK, "Diabetes Tests & Diagnosis" (last reviewed July 2022), read on September 27, 2026. For people who aren't pregnant. The CDC's "Diabetes Testing" page (May 15, 2024), read the same day, has the same cutoffs with one quirk: for the 2-hour test it prints normal as "140 mg/dL or below" and prediabetes as "140–199 mg/dL", so 140 sits in both rows. NIDDK puts normal at 139 or below. ## What each test measures, and what it asks of you ### A1C Your average blood glucose over the last 3 months, reported as a percentage. No fasting. NIDDK says it might not be accurate in the second or third trimester of pregnancy, with certain types of anemia, or with some hemoglobin variants, though "most A1C tests used in the United States are not affected by the most common variants." ### Fasting plasma glucose Your glucose at a single point in time, after at least 8 hours with nothing but sips of water. For the most reliable result, it's done in the morning. ### Oral glucose tolerance test (OGTT) A fasting blood draw, a drink high in sugar, then another draw 2 hours later. NIDDK notes it costs more than the fasting test and "is not as easy to give." ### Random plasma glucose Drawn at any time, no fasting. Doctors use it when you have symptoms of diabetes and they don't want to wait for you to fast. ## Why a doctor tests twice NIDDK puts it simply: "Usually, your doctor will use a second test to confirm you have diabetes." Blood glucose rises and falls with meals, exercise, illness and stress, and different tests don't always agree. In people at an early stage, glucose may not have risen high enough to show up on every test, so a doctor may watch closely and repeat the test in several months. A "high glucose" flag on routine bloodwork deserves the same patience. Glucose is part of standard panels such as the BMP, and MedlinePlus says what a result means "will depend on which test you had, the results of other tests, and your medical history." Prediabetes and diabetes are the most common causes of a high level, not the only ones; our fasting blood sugar page lists the others. A number from a health fair or a pharmacy counts as a first look: the CDC's advice is to bring it to your doctor, since a retest may be needed "to make sure the results are accurate." A reading from a meter at home falls in the same category. NIDDK's advice: "Don't try to diagnose yourself if you think you might have diabetes." ## Who should be tested ### Anyone with symptoms "Anyone who has symptoms of diabetes should be tested for the disease," says NIDDK. Symptoms of high blood glucose listed by MedlinePlus include increased thirst, frequent urination, blurred vision, fatigue, losing weight without trying, and numbness or tingling in the feet or hands. ### Adults with risk factors Experts recommend routine type 2 testing if you have risk factors, which include being 35 or older; being American Indian, Black or African American, Asian American, Hispanic/Latino or Pacific Islander; having overweight or obesity plus at least one other risk factor; or having had gestational diabetes. ### Children and teens NIDDK says experts recommend testing ages 10 to 18 who have overweight or obesity and at least one more risk factor. ### How often Every 3 years after normal results, and every year after a prediabetes diagnosis. Medicare may cover the tests if you have certain risk factors; with other insurance, ask your insurer. ## What a supplement label can't do to a test result Blood sugar supplements borrow lab language without showing lab results. One GlucoTrust testimonial reads: "My blood sugar has stabilized at around 125 and if this keeps up I should be able to get off my prescription meds." A number without its test can't be read. On a fasting test, 125 is the top of the prediabetes range; two hours into a glucose tolerance test, it's normal. And coming off a prescription is your doctor's call, never a label's. A label can't move a diagnosis, and the products that hint otherwise are the ones to worry about. NCCIH, the NIH's complementary health center, passes on an FDA warning about diabetes products that claim to "replace your diabetes medicine": "These products are marketed illegally." If a test puts you in the prediabetes or diabetes range, the next step is your doctor, and if you already take diabetes medication, any supplement goes past them first. ## Frequently asked questions **What blood test is used to diagnose diabetes?** Any of four: the A1C, the fasting plasma glucose test, the oral glucose tolerance test, or the random plasma glucose test when you have symptoms. Doctors usually confirm with a second test. **What is a normal random blood sugar?** Neither NIDDK nor the CDC gives a normal or prediabetes range for the random test. It's used to diagnose diabetes when there are symptoms, at 200 mg/dL or above. **How does a glucose tolerance test work?** It starts with a blood draw after at least 8 hours without food. Then you drink a liquid high in sugar and give blood again 2 hours later. At the 2-hour mark, NIDDK's cutoffs are 139 mg/dL or below for normal, 140 to 199 for prediabetes, and 200 or above for diabetes. **Can a home glucose meter diagnose diabetes?** No. NIDDK says over-the-counter equipment such as a blood glucose meter cannot diagnose diabetes. If a reading worries you, ask your doctor for a lab test. **My blood test shows high glucose. Do I have diabetes?** Not from one result. Prediabetes and diabetes are the most common causes, but MedlinePlus lists others, and doctors confirm with a second test. Ask which test it was and what comes next. **Do you need to fast for a diabetes blood test?** For the fasting plasma glucose test and the OGTT, yes, at least 8 hours. For the A1C and the random test, no. ## Related pages Blood sugar basics ### [Your A1C result, read against the NIH's cutoffs and the CDC's chart](https://glucosebench.com/a1c-levels) A normal A1C is below 5.7%. Read more Blood sugar basics ### [Fasting blood sugar: three cutoffs and one blood draw](https://glucosebench.com/fasting-blood-sugar) The CDC's cutoffs for fasting blood sugar: 99 mg/dL or below is normal, 100 to 125 mg/dL is prediabetes, and 126 mg/dL or above is diabetes. Read more Blood sugar basics ### [Prediabetes: a number worth acting on, usually without a symptom](https://glucosebench.com/prediabetes) Prediabetes means your blood sugar is higher than normal but not high enough for a type 2 diabetes diagnosis. Read more Blood sugar basics ### [Insulin in your blood: what the test measures and what it misses](https://glucosebench.com/insulin-test) An insulin in blood test, often called a fasting insulin test, measures how much insulin is in a sample of your blood. Read more Updated September 27, 2026. 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