How Are Diabetes and Prediabetes Diagnosed?
The following tests are used to diagnose diabetes:
- A fasting plasma glucose test measures your blood glucose after you have gone at least 8 hours without eating. This test is used to detect diabetes or prediabetes.
- An oral glucose tolerance test measures your blood sugar after you have gone at least 8 hours without eating and 2 hours after you drink a glucose-containing beverage. This test can be used to diagnose diabetes or prediabetes.
- In a random plasma glucose test, your doctor checks your blood sugar without regard to when you ate your last meal. This test, along with an assessment of symptoms, is used to diagnose diabetes but not prediabetes.
A hemoglobin A1c (HbA1c) test can be done without fasting and can be used to diagnose or confirm either prediabetes or diabetes.
If you get a positive test result, meaning you have diabetes or prediabetes, your doctor will confirm this by repeating the fasting plasma glucose test or the oral glucose tolerance test on a different day.
Another test, the zinc transporter 8 autoantibody test, can help determine if a person has type 1 diabetes and not another type.
Fasting Plasma Glucose (FPG) Test
This test is most reliable when done in the morning. The results and their meaning are shown in Table 1. If your fasting glucose level is 100 to 125 mg/dL, you have a form of prediabetes called impaired fasting glucose (IFG), meaning that you're more likely to develop type 2 diabetes but don't have it yet. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means you have diabetes.
Table 1. Fasting Plasma Glucose Test
Plasma Glucose Result (mg/dL) | Diagnosis |
99 and below | Normal |
100 to 125 | Prediabetes (impaired fasting glucose) |
126 and above | Diabetes* |
*Confirmed by repeating the test on a different day.
Oral Glucose Tolerance Test (OGTT)
Research has shown that the OGTT is more sensitive than the FPG test for diagnosing prediabetes, but it is less convenient to give. The OGTT requires you to fast for at least 8 hours before the test. Your plasma glucose is measured immediately before and 2 hours after you drink a liquid containing 75 grams of glucose dissolved in water. Results and what they mean are shown in Table 2. If your blood sugar level is between 140 and 199 mg/dL 2 hours after drinking the liquid, you have a form of prediabetes called impaired glucose tolerance or IGT, meaning that you're more likely to develop type 2 diabetes but don't have it yet. A 2-hour glucose level of 200 mg/dL or above, confirmed by repeating the test on another day, means that you have diabetes.
Table 2. Oral Glucose Tolerance Test
2-Hour Plasma Glucose Result (mg/dL) | Diagnosis |
139 and below | Normal |
140 to 199 | Prediabetes (impaired glucose tolerance) |
200 and above | Diabetes* |
*Confirmed by repeating the test on a different day.
Gestational diabetes is also diagnosed with the OGTT. Blood sugar levels are checked four times during the test. If your blood sugar levels are above normal at least twice during the test, you have gestational diabetes. Table 3 shows the above-normal results for the OGTT for gestational diabetes.
Table 3. Gestational Diabetes: Above-Normal Results for the Oral Glucose Tolerance Test
When | Plasma Glucose Result (mg/dL) |
Fasting | 95 or higher |
At 1 hour | 180 or higher |
At 2 hours | 155 or higher |
At 3 hours | 140 or higher |
Note: Some laboratories use other numbers for this test.
Glucose challenge test
This test, also called the 1-hour glucose tolerance test, is a shorter version of the OGTT. This shorter test can better predict type 2 diabetes than other types of tests.
If your blood glucose reading 1 hour after drinking the 75 grams of glucose dissolved in water is at or above 155 mg/dL, you’re considered to have intermediate hyperglycemia. If your blood glucose levels are greater than or equal to 209 mg/dL, you have type 2 diabetes.
Random Plasma Glucose Test
A random blood glucose level of 200 mg/dL or more, plus presence of the following symptoms, can mean that you have diabetes:
- Increased urination
- Increased thirst
- Unexplained weight loss
Other symptoms include fatigue, blurred vision, increased hunger, and sores that don't heal. Your doctor will check your blood glucose level on another day using the FPG or the OGTT to confirm the diagnosis of diabetes.
Hemoglobin A1c (HbA1c) Test
This test tells you what percentage of your hemoglobin (the protein in your blood that carries oxygen) is coated with glucose. Your doctor gives you this test by using a needle to collect a sample of blood from your arm. The blood sample goes to a lab for results. Your HbA1c reading gives your doctor a snapshot of your blood glucose control over several months.
An HbA1c of 5.7% to 6.4% points to prediabetes and marks a time when it can be reversed by lifestyle changes. An HbA1c of 6.5% or higher is considered diabetes.
Table 4. HbA1c Test
HbA1c | Diagnosis |
Below 5.7% | Normal |
Between 5.7% and 6.4% | Prediabetes |
6.5% or higher | Diabetes |
HbA1c tests don’t diagnose type 1 or gestational diabetes, only type 2 diabetes.
Diabetes Diagnostic Criteria
As your doctor collects data to confirm a diabetes diagnosis, these are the values they follow as a guideline:
Table 5. Diagnostic Criteria for Prediabetes
Test | Result |
HbA1c | 5.7-6.4% |
Fasting blood glucose | 100-125 mg/dL |
2-hour OGTT blood glucose | 140-199 mg/dL |
Table 6. Diagnostic Criteria for Diabetes
Test | Result |
HbA1c | 6.5% or higher |
Fasting blood glucose | 126 mg/dL or higher |
2-hour OGTT blood glucose | 200 mg/dL or higher |
Type 1 Diabetes Diagnosis
If your doctor confirms a diabetes diagnosis, they may want to do further tests to decide if you have type 1 diabetes. This is especially important for children and people who don’t have other risk factors for diabetes who get a diabetes diagnosis.
Autoantibody test. Your doctor may suggest a zinc transporter 8 autoantibody (ZnT8Ab) test. The goal of having this blood test is to get a prompt and accurate diagnosis that can lead to timely treatment.
C-peptide test (Insulin in blood test). This test helps measure how much insulin your pancreas is making. Your pancreas releases equal amounts of insulin and C-peptide into your bloodstream at the same time. C-peptide doesn't affect your blood glucose levels, but it stays in your blood longer than insulin. Your doctor will take a small sample of blood and send it to a lab for this test.
Ketones. Your doctor can test your urine using a paper test strip to look for ketones. Ketones are acids your body makes when it’s using fat instead of glucose for energy. This can be a sign of type 1 diabetes.
Takeaways
There are many tests that confirm a diabetes diagnosis, including a fasting plasma glucose test, oral glucose tolerance test, random plasma glucose test, and hemoglobin A1c test. These tests tell your doctor if you have diabetes or are at risk of developing it (prediabetes).
Diabetes Diagnosis FAQs
How do you diagnose diabetes? Your doctor can diagnose diabetes with blood tests. They may want to do more than one type to be sure of the diagnosis. If your doctor thinks you may have type 1 diabetes, they may do further tests on your blood to look for autoantibodies or look at your urine for ketones.
How do I accept a diabetes diagnosis? Learning you have diabetes can make you feel a range of emotions. Know that over 38 million Americans live with diabetes. There are many organizations and support groups to help with emotional health, diet management, and lifestyle tips to help you live well with diabetes.
What should you do when first diagnosed with diabetes? Educate yourself on your condition so you feel empowered to take good care of yourself. Write down any questions you have and bring them with you to your doctor so you continue to understand more. Reach out to other people already living with diabetes for advice and support. Follow your treatment plan as your doctor directs.
What do you say to a newly diagnosed diabetic person? If someone in your life gets a diabetes diagnosis, ask them how you can help them process the news. Be a good listener. Learn about diabetes yourself so you can be a helpful partner in conversation or in day-to-day life as they learn to manage it. Support from friends and family can help people with diabetes manage their condition better.
How does a doctor diagnose diabetes type 1? After a diabetes diagnosis, there are further tests your doctor can use to see if you have type 1 diabetes. These include tests on your blood to look for autoantibodies, tests to see how much insulin your pancreas is making, or urine tests that look for ketones.