The fasting plasma glucose (FPG) test and the oral glucose tolerance test (OGTT) are two of the most widely used blood tests for diagnosing diabetes and prediabetes. Together with the hemoglobin A1c, they form the standard set of tests used to detect and monitor problems with blood sugar control.
This article explains how each test is performed, what the results mean, and when one test is preferred over another.
The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.
Why are these tests done?
The body needs to keep blood glucose within a narrow range. After a meal, glucose rises and the pancreas releases insulin, which signals cells to take up glucose for energy or storage. Between meals, blood glucose falls back to baseline. When this regulation breaks down — most commonly because of type 2 diabetes — glucose levels rise above the normal range.
Glucose testing is used to:
- Diagnose diabetes. Fasting glucose, OGTT, and A1c are the three standard tests used to confirm a diagnosis of diabetes or prediabetes.
- Screen people without symptoms. Adults with risk factors — including overweight, family history, high blood pressure, abnormal lipids, gestational diabetes history, polycystic ovary syndrome, or certain ethnic backgrounds — are routinely screened, often beginning at age 35–45.
- Diagnose gestational diabetes. The OGTT is the standard test for gestational diabetes, performed during pregnancy.
- Investigate symptoms. Excessive thirst, frequent urination, unexplained weight loss, fatigue, blurred vision, slow-healing wounds, and recurrent infections may all prompt glucose testing.
- Monitor diabetes treatment. While A1c is the main long-term monitoring test, fasting and post-meal glucose levels are used to fine-tune day-to-day treatment.
Fasting plasma glucose (FPG)
The fasting plasma glucose test measures your blood sugar after you have gone without eating or drinking anything other than water for at least eight hours. Because food temporarily raises blood glucose, fasting allows the test to reflect your body’s baseline glucose regulation.
How the FPG is performed
The test is performed on a small blood sample drawn from a vein in the arm. Most people fast overnight and have the blood drawn first thing in the morning, before eating or drinking anything (water is allowed). Medications are usually continued as normal unless your doctor has specifically told you otherwise.
If you have known diabetes and are taking insulin or other diabetes medications, follow your doctor’s instructions carefully — fasting while continuing diabetes medications can sometimes cause low blood sugar.
What does the FPG result mean?
The American Diabetes Association uses the following thresholds:
- Less than 100 mg/dL (5.6 mmol/L) — normal fasting glucose.
- 100–125 mg/dL (5.6–6.9 mmol/L) — impaired fasting glucose, also called prediabetes. Indicates that fasting glucose is higher than normal but does not yet meet the threshold for diabetes. The risk of progressing to diabetes is significantly higher than in someone with normal results.
- 126 mg/dL (7.0 mmol/L) or higher — meets the threshold for diabetes when confirmed by a repeat test on a separate day, or by another diagnostic test such as the A1c or OGTT.
A single elevated fasting glucose is not enough to diagnose diabetes — it should be repeated or confirmed with another test before the diagnosis is made. The exception is when fasting glucose is very high (often above about 200 mg/dL) in someone with clear symptoms of diabetes, in which case the diagnosis is usually treated as established.
What can affect the FPG result?
- Recent food intake. Eating or drinking within 8 hours of the test invalidates the result. Even a small amount of food, beverage, or chewing gum can affect the reading.
- Stress and illness. Acute illness, infection, surgery, and emotional stress can temporarily raise blood glucose, sometimes significantly. A fasting glucose drawn during acute illness may not reflect your usual glucose control and is generally not used to diagnose diabetes in that situation.
- Medications. Corticosteroids such as prednisone, some diuretics, certain antipsychotic medications, and some HIV medications can raise glucose levels.
- Physical activity. Strenuous exercise within 24 hours can temporarily lower or raise blood glucose, depending on timing.
Oral glucose tolerance test (OGTT)
The oral glucose tolerance test measures how the body handles a defined amount of glucose given by mouth. Unlike the fasting glucose, which captures only the baseline level, the OGTT reveals how efficiently the body processes a sugar load — providing a more sensitive picture of glucose regulation, particularly in early diabetes and gestational diabetes.
How the OGTT is performed
The OGTT requires several hours and follows a specific protocol:
- You fast for at least 8 hours before the test. Most people do this overnight and arrive at the laboratory in the morning.
- A baseline blood sample is drawn to measure your fasting glucose.
- You drink a sweet beverage containing a measured amount of glucose — typically 75 grams in adults for routine OGTT testing, or 50 grams for an initial gestational diabetes screening test.
- One or more additional blood samples are drawn at specific times after drinking the glucose, usually at 1 hour and 2 hours, but sometimes at other intervals depending on why the test is being done.
You will need to remain at the laboratory or clinic for the duration of the test. Bring water; you cannot eat during the test, but you may drink. Many people experience some nausea from the sweet beverage, particularly the larger 75-gram dose.
OGTT results in non-pregnant adults
For diagnosing diabetes in non-pregnant adults, the standard 75-gram OGTT uses the 2-hour glucose result:
- Less than 140 mg/dL (7.8 mmol/L) — normal.
- 140–199 mg/dL (7.8–11.0 mmol/L) — impaired glucose tolerance, a form of prediabetes. Indicates the body is not handling glucose loads efficiently and the risk of progressing to diabetes is significantly increased.
- 200 mg/dL (11.1 mmol/L) or higher — meets the threshold for diabetes when confirmed by a repeat test or another diagnostic test.
It is possible to have a normal fasting glucose and an abnormal OGTT, or vice versa. The OGTT is generally more sensitive than the fasting glucose for detecting early or mild glucose dysregulation, but it is less convenient and is no longer required for routine diabetes diagnosis — the A1c and fasting glucose are sufficient in most adults.
OGTT in pregnancy (gestational diabetes screening)
Pregnancy is the most common situation in which the OGTT is used. The procedure varies between countries and clinical practices, but two common approaches are:
Two-step approach (commonly used in North America):
- Step 1 — 50-gram glucose challenge test. Performed at 24–28 weeks of pregnancy. Fasting is not required. A blood sample is drawn 1 hour after drinking the glucose. If the result is below the screening threshold (commonly 130, 135, or 140 mg/dL depending on the lab), no further testing is needed.
- Step 2 — 100-gram OGTT. If the screening test is abnormal, a fasting 3-hour OGTT is performed using a 100-gram glucose load, with samples at fasting, 1 hour, 2 hours, and 3 hours. Two or more elevated values diagnose gestational diabetes.
One-step approach (commonly used in Europe and increasingly in North America):
- A single 75-gram fasting OGTT is performed at 24–28 weeks. Samples are drawn at fasting, 1 hour, and 2 hours. Any one elevated value diagnoses gestational diabetes.
The exact cutoff values used in pregnancy differ from those used outside pregnancy because gestational diabetes is diagnosed at lower glucose levels — the goal is to detect glucose abnormalities that could affect the developing baby.
What can affect the OGTT?
- Recent dietary changes. A very low-carbohydrate diet for several days before the test can produce a falsely abnormal result. Patients are generally advised to eat a normal mixed diet for at least three days before the test.
- Acute illness. As with fasting glucose, acute illness can raise OGTT results. The test should generally be postponed during acute illness.
- Medications. Corticosteroids and some other medications can affect OGTT results.
- Vomiting during the test. If you vomit shortly after drinking the glucose solution, the test cannot be completed accurately and may need to be rescheduled.
Random plasma glucose
A random plasma glucose is a blood sugar measurement taken at any time, regardless of when you last ate. It is not used for routine diabetes screening because results are too variable, but a very high random glucose in someone with classic symptoms of diabetes — frequent urination, excessive thirst, and unexplained weight loss — is one of the standard ways to diagnose diabetes:
- Random plasma glucose of 200 mg/dL (11.1 mmol/L) or higher with classic symptoms of diabetes meets the diagnostic threshold.
Which test should be used?
All three main tests — fasting glucose, OGTT, and A1c — are acceptable for diagnosing diabetes in most adults. They sometimes give discordant results because they measure different things:
- Fasting glucose is the simplest, fastest, and least expensive. It requires only one blood draw after an overnight fast. It is widely available and is the most common first test in most clinical situations.
- A1c is convenient because it does not require fasting and reflects average glucose over two to three months. It is less affected by short-term factors such as recent illness, stress, or what you ate the day before. However, it can be inaccurate in certain conditions (described in our A1c article) and is generally not used in pregnancy.
- OGTT is more sensitive than fasting glucose for detecting early or mild glucose abnormalities. It is the standard test for gestational diabetes. Outside of pregnancy, it is mostly used when other tests are inconclusive or unreliable.
For most adults being screened for diabetes, either fasting glucose or A1c is used as the first test, often as part of a routine annual blood draw. If results are borderline or inconsistent, additional testing may be added.
What happens after the test?
The next steps depend on the result:
- Normal results: No immediate action is needed. Periodic repeat testing is recommended depending on age and risk factors.
- Prediabetes: Lifestyle changes are the primary recommendation, including weight loss (5–10% of body weight), increased physical activity, and dietary changes. These can significantly reduce the risk of progressing to type 2 diabetes. Some patients benefit from metformin, particularly those at higher risk. Repeat testing is typically done in 6–12 months.
- First-time results suggesting diabetes: Confirmation is needed before the diagnosis is made. This typically means repeating the same test on a separate day, or using a different test (e.g., confirming an abnormal fasting glucose with an A1c). The exception is severely elevated glucose in someone with clear symptoms, where diagnosis is usually made on the first abnormal test.
- Confirmed diabetes: Treatment planning begins, including dietary changes, exercise, weight management, and often medication. Type 2 diabetes treatment usually starts with metformin and lifestyle changes, with additional medications or insulin added as needed. Type 1 diabetes always requires insulin from the time of diagnosis.
- Gestational diabetes: Treatment focuses on dietary management and home blood glucose monitoring, sometimes with insulin or other diabetes medications. After delivery, glucose typically returns to normal, but women who have had gestational diabetes are at significantly higher risk of developing type 2 diabetes later in life and need ongoing screening.
- Follow-up testing: Once diabetes is diagnosed, the A1c is the main test used for long-term monitoring, repeated every 3–6 months. Additional testing for complications — including kidney function, urine albumin, lipid panel, and eye exams — is also done at regular intervals.
Questions to ask your doctor
- Was my fasting glucose or OGTT result normal, in the prediabetes range, or consistent with diabetes?
- If the result is abnormal, does it need to be confirmed with another test?
- Could any of my medications, recent illness, or fasting practices have affected my result?
- If I have prediabetes, what should I do to reduce my risk of progressing to diabetes?
- If I have gestational diabetes, what does this mean for my pregnancy and for my long-term health?
- How often should glucose testing be repeated?
- Should I also have an A1c test, and how do my results compare?
- If I have diabetes, what type do I have, and what treatment do you recommend?
- Should I be referred to an endocrinologist, a diabetes educator, or a registered dietitian?
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