From the Guidelines
Diabetes should be diagnosed based on A1C or plasma glucose criteria, including fasting plasma glucose, 2-h plasma glucose during a 75-g oral glucose tolerance test, or random glucose with classic hyperglycemic symptoms/crises, as outlined in the 2024 standards of care in diabetes 1. The diagnostic criteria for diabetes include a fasting plasma glucose (FPG) value, 2-h plasma glucose (2-h PG) value during a 75-g oral glucose tolerance test (OGTT), or random glucose value accompanied by classic hyperglycemic symptoms/crises criteria.
- Key diagnostic thresholds are:
- Fasting plasma glucose (FPG)
- 2-h plasma glucose (2-h PG) during a 75-g oral glucose tolerance test (OGTT)
- Random glucose with symptoms of hyperglycemia
- These criteria are used to identify individuals with diabetes, and confirmatory testing is required in the absence of unequivocal hyperglycemia, as stated in the 2024 standards of care in diabetes 1. The American Diabetes Association recommends using A1C, FPG, and 2-h PG during 75-g OGTT for diagnostic screening, as these tests reflect different aspects of glucose metabolism and can identify different groups of individuals with diabetes 1.
- The choice of test may depend on the individual patient and the clinical context, but A1C and plasma glucose criteria are the primary methods for diagnosing diabetes. In clinical practice, it is essential to use standardized methods for blood tests and confirm results with repeat testing, unless there are clear symptoms of hyperglycemia, to ensure accurate diagnosis and appropriate management of diabetes, as outlined in the 2024 standards of care in diabetes 1.
From the Research
Diagnostic Criteria for Diabetes
The diagnosis of diabetes can be made using various tests, including:
- Fasting plasma glucose (FPG) level of 126 mg per dL or greater 2
- Hemoglobin A1c (HbA1c) level of 6.5% or greater 3, 2
- Random plasma glucose level of 200 mg per dL or greater 2
- 75-g two-hour oral glucose tolerance test with a plasma glucose level of 200 mg per dL or greater 2
Recommendations for Diagnostic Testing
The International Expert Committee recommends that the diagnosis of diabetes be made if HbA1c level is greater than or equal to 6.5% and confirmed with a repeat HbA1c test 3. The American Diabetes Association recommends screening for type 2 diabetes annually in patients 45 years and older, or in patients younger than 45 years with major risk factors 2. The U.S. Preventive Services Task Force recommends screening for abnormal blood glucose and type 2 diabetes in adults 40 to 70 years of age who are overweight or obese, and repeating testing every three years if results are normal 2.
Limitations of HbA1c Testing
HbA1c testing has the potential for systematic error, and factors that may not be clinically evident can impact HbA1c test results 3. The diagnostic agreement in the clinical setting revealed that the current HbA1c ≥ 6.5% is less likely to detect diabetes than those defined by FPG and 2hPG 4. HbA1c ≥ 6.5% detects less than 50% of diabetic patients defined by FPG and less than 30% of diabetic patients defined by 2hPG 4.
Combined Use of Diagnostic Tests
Combining the use of HbA1c and plasma glucose measurements for the diagnosis of diabetes offers the benefits of each test and reduces the risk of systematic bias inherent in HbA1c testing alone 3. The simultaneous measurement of FPG and HbA1c (FPG and/or HbA1C) might be a more sensitive and specific screening tool for identifying high-risk individuals with diabetes and impaired glucose tolerance at an early stage 5.