From the Guidelines
Diabetes can be diagnosed through several blood tests, including the fasting plasma glucose (FPG) test, the random plasma glucose test, the hemoglobin A1c (HbA1c) test, and the oral glucose tolerance test, with the most recent and highest quality study recommending these methods for diagnosis 1. The most common methods for diagnosing diabetes include:
- Fasting plasma glucose (FPG) test: a blood sample is taken after an 8-hour fast, with a result of 126 mg/dL or higher indicating diabetes
- Random plasma glucose test: can be done at any time, with a result of 200 mg/dL or higher, along with symptoms of diabetes, being diagnostic
- Hemoglobin A1c (HbA1c) test: measures average blood sugar levels over the past 2-3 months, with a result of 6.5% or higher indicating diabetes
- Oral glucose tolerance test: measures blood sugar before and two hours after drinking a glucose solution, with a two-hour result of 200 mg/dL or higher being diagnostic These tests are effective because they capture blood sugar levels at different points in time, allowing healthcare providers to assess how well the body is processing glucose, as noted in the 2018 study 1. The diagnosis of diabetes is crucial for identifying the condition early and enabling prompt treatment to prevent complications associated with chronic high blood sugar levels, with the American Diabetes Association (ADA) adopting the use of the A1C test to diagnose diabetes in 2010 1. In some cases, a combination of these tests may be used to confirm a diagnosis, with the 2014 study recommending the use of plasma glucose criteria, either the fasting plasma glucose (FPG) or the 2-h plasma glucose (2-h PG) value after a 75-g oral glucose tolerance test (OGTT) 1. The 2004 study also notes that the diagnosis of diabetes can be made based on symptoms of diabetes plus casual plasma glucose concentration ≥200 mg/dl (11.1 mmol/l), or fasting plasma glucose ≥126 mg/dl (7.0 mmol/l), or 2-h postload glucose ≥200 mg/dl (11.1 mmol/l) during an OGTT 1. The 2010 study recommends that the diagnosis of diabetes be based on one of three methods of blood glucose measurement, including fasting blood glucose, random blood glucose, and oral glucose tolerance test 1.
From the Research
Diagnostic Methods for Diabetes Mellitus (DM)
The diagnosis of Diabetes Mellitus (DM) can be made using several methods, including:
- Hemoglobin A1c (HbA1c) 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 2
- Fasting plasma glucose (FPG) testing: The American Diabetes Association recommends a diagnostic cutoff for FPG of 126 mg/dL (7.0 mmol/L) 3
- 2-hour postglucose-load plasma glucose testing: A 2-hour plasma glucose level of 200 mg/dL (11.1 mmol/L) or more during an oral glucose tolerance test (OGTT) is diagnostic of diabetes 3
- Oral glucose tolerance test (OGTT): The 100-g, 3-hour OGTT remains the "gold standard" for gestational diabetes mellitus (GDM) 3
- Random plasma glucose testing: A random plasma glucose level of 200 mg/dL (11.1 mmol/L) or more is diagnostic of diabetes 3, 4
Comparison of Diagnostic Methods
Studies have compared the effectiveness of different diagnostic methods for DM, including:
- HbA1c vs. FPG: The sensitivity of HbA1c ≥ 6.5% was only 43.3% compared to the FPG criterion 5
- HbA1c vs. 2-hour postglucose-load plasma glucose: The sensitivity of HbA1c ≥ 6.5% was only 28.1% compared to the 2-hour postglucose-load plasma glucose criterion 5
- FPG vs. 2-hour post-load glucose: The degree of agreement (kappa) in establishing the diagnosis of diabetes between the FPG and 2-hour post-load glucose cut-offs was 0.48 6
Recommendations for Diagnostic Testing
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 4 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 4