How is diabetes and prediabetes diagnosed?
The following tests are used to diagnose diabetes.
- Glucose is measured by a fasting glucose test when you have not eaten for at least 8 hours. This test is used to detect if you have diabetes or prediabetes.
- Glucose is measured by an oral glucose tolerance test after not eating for at least 8 hours and taking a glucose-containing drink for 2 hours. This test may be used to diagnose diabetes or prediabetes.
- In a randomized glucose test, the physician does not consider when the subject recently ate for the glucose test. This test and symptom assessment is used to diagnose diabetes, but not for the diagnosis of prediabetes.
A positive test result was confirmed by repeating the fasting glucose test or oral glucose tolerance test on a different date. When diabetes is first diagnosed, your doctor may recommend testing for zinc transporter 8 autoantibodies (ZnT 8 Ab). The results of this blood test, along with other information and test results, can help determine if you have type 1 diabetes or another type of diabetes. The purpose of the ZnT 8 Ab test is to obtain a timely and accurate diagnosis so that treatment can be started promptly.
Fast Fasting Glucose (FPG) test
The most reliable fasting glucose test is performed in the morning. The results and their meaning are shown in Table 1. A fasting blood glucose level of 100 to 125 milligrams per deciliter (mg/dl) suggests prediabetes called impaired fasting glucose (IFG), which means that type 2 diabetes is more likely, but not currently present. For those with a blood glucose level of 126 mg/dl or more, a repeat test on another day that confirms this result is indicative of diabetes.
| Glucose results (mg/dl) | Diagnosis |
| normal | |
| 100~125 | Pre-diabetes (impaired fasting glucose) |
| 126 and above | Diabetes* |
*A repeat test was performed on another day and confirmed this result.
Oral Glucose Tolerance Test (OGTT)
Studies have shown that oral glucose tolerance tests are more sensitive than fasting glucose tests for the diagnosis of prediabetes, but the test is less convenient. The oral glucose tolerance test requires fasting for at least 8 hours before the test and checking blood glucose 2 hours before and after drinking an aqueous solution containing 75 grams of glucose. The results and implications are shown in Table 2. A blood glucose level between 140 and 199 mg/dl at 2 hours after drinking the liquid indicates prediabetes called hypoglycemia (IGT), which means that type 2 diabetes is more likely to develop, but not currently. A 2-hour blood glucose level of 200 mg/dl or more, confirmed by a repeat test on another day, means diabetes is present.
| 2-hour glucose results (mg/dl) | Diagnosis |
| normal | |
| 140~199 | Pre-diabetes (reduced glucose tolerance) |
| 200 and above | Diabetes* |
*A repeat test was performed on another day and confirmed this result.
Gestational diabetes is also diagnosed based on the results of an oral glucose tolerance test. Check your blood glucose 4 times during the test. If blood glucose levels are higher than normal at least 2 times during the test, gestational diabetes is present. Table 3 shows the thresholds for higher-than-normal blood glucose when an oral glucose tolerance test is used to diagnose gestational diabetes.
| Time Point | Plasma glucose value (mg/dl) |
| Fasting | 95 or higher |
| 180 or higher | |
| 2 hours | 155 or higher |
| 3 hours | 140 or higher |
Note: Some labs have different thresholds.
Random blood glucose test
A random blood glucose level ≥200 mg/dl and the following symptoms may indicate diabetes.
- Increased urination.
- Increased thirst.
- Increased thirst.
- Unexplained weight loss.
Other symptoms include fatigue, blurred vision, increased hunger, and the development of ulcers that do not heal easily. Your doctor will check your blood glucose levels on another day with a fasting glucose test or an oral glucose tolerance test to confirm the diagnosis of diabetes.
Newer guidelines use glycated hemoglobin (HbA1c) as a screening tool for prediabetes or diabetes (this test is usually used to assess glycemic control over several months in people with diabetes). an HbA1c between 5.7% and 6.4% is consistent with a diagnosis of prediabetes, during which time the condition can be reversed by lifestyle changes to reverse the condition. A glycosylated hemoglobin of no less than 6.5% is consistent with a diagnosis of diabetes mellitus.