From the Guidelines
Diabetes mellitus (DM) should be diagnosed based on a hemoglobin A1C level of 6.5% or higher, a fasting plasma glucose level of 126 mg/dL or higher, a 2-hour plasma glucose level of 200 mg/dL or higher during an oral glucose tolerance test, or a random plasma glucose level of 200 mg/dL or higher with symptoms of hyperglycemia, as recommended by the most recent guidelines 1. The diagnostic criteria for DM have been established based on the association between glucose levels and the presence of retinopathy, as well as other microvascular and macrovascular complications 1.
- The primary diagnostic tests include:
- Hemoglobin A1C (HbA1C) testing, which reflects average blood glucose levels over the past 2-3 months
- Fasting plasma glucose (FPG) testing, which requires at least 8 hours of fasting
- Oral glucose tolerance test (OGTT), which involves drinking a solution containing 75 grams of glucose after fasting overnight
- Random plasma glucose testing, which can be used in patients with symptoms of hyperglycemia For accurate diagnosis, abnormal results should be confirmed with repeat testing unless clinical symptoms are clear, as stated in the guidelines 1. It is essential to note that the choice of diagnostic test may depend on the individual patient's circumstances, and the healthcare provider should consider factors such as the patient's risk factors, symptoms, and medical history when selecting a diagnostic test, as recommended by the guidelines 1. Additionally, the guidelines emphasize the importance of using certified and standardized methods for diagnostic testing, such as NGSP-certified assays for HbA1C testing, to ensure accurate and reliable results 1.
From the Research
Diagnosis of Diabetes Mellitus
To diagnose diabetes mellitus (DM), several tests can be used, including:
- Fasting plasma glucose (FPG) test
- Oral glucose tolerance test (OGTT)
- Glycated hemoglobin (HbA1c) test
Diagnostic Criteria
The diagnostic criteria for DM are as follows:
- FPG: 126 mg/dL (7.0 mmol/L) or higher 2
- OGTT: 2-hour plasma glucose level of 200 mg/dL (11.1 mmol/L) or higher 2
- HbA1c: 6.5% or higher 3
Combination of Tests
Using a combination of tests can improve the accuracy of diagnosis:
- FPG and HbA1c: sensitivity of 96.5% for detecting undiagnosed diabetes 4
- FPG and OGTT: 100% of diabetic patients were identified 5
- HbA1c and OGTT: 91.7% of diabetic patients were identified 5
Advantages and Limitations of Each Test
Each test has its advantages and limitations:
- HbA1c: advantages include greater clinical convenience, preanalytic stability, and assay standardization, but may have systematic error 3
- FPG: advantages include simplicity and low cost, but may not detect all cases of diabetes 2
- OGTT: advantages include high sensitivity, but may be time-consuming and inconvenient for patients 2
Clinical Recommendations
Based on the evidence, the following clinical recommendations can be made:
- HbA1c can be used as a screening test for diabetes, but should be confirmed with a repeat test 3
- FPG and OGTT can be used to diagnose diabetes, especially in patients with equivocal HbA1c results 2, 5
- A combination of tests can improve the accuracy of diagnosis and reduce the risk of systematic bias 4, 5