From the Guidelines
Diabetes Mellitus (DM) should be diagnosed based on specific blood glucose criteria, including a hemoglobin A1C level of 6.5% or higher, a fasting plasma glucose level of 126 mg/dL (7.0 mmol/L) or higher, a 2-hour plasma glucose level of 200 mg/dL (11.1 mmol/L) 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.
Diagnostic Criteria
The diagnostic criteria for DM include:
- A1C 6.5% or higher, as measured in a laboratory using a method that is NGSP certified and standardized to the DCCT assay 1
- Fasting plasma glucose (FPG) 126 mg/dL (7.0 mmol/L) or higher, with fasting defined as no caloric intake for at least 8 hours 1
- 2-hour plasma glucose 200 mg/dL (11.1 mmol/L) or higher during an oral glucose tolerance test (OGTT), using a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water 1
- Random plasma glucose 200 mg/dL (11.1 mmol/L) or higher with symptoms of hyperglycemia or hyperglycemic crisis 1
Confirmation of Diagnosis
Abnormal results should be confirmed by repeat testing on a different day, unless clinical symptoms are clear 1. If two different tests are available in an individual and the results are discordant, the test whose result is above the diagnostic cut point should be repeated, and the diagnosis is made on the basis of the confirmed test 1.
Screening Recommendations
Testing for DM should be considered in adults who are overweight or obese with additional risk factors, and all adults should be screened beginning at age 45 regardless of weight 1. The 2019 ESC guidelines recommend investigating DM using fasting plasma glucose (FPG) or haemoglobin A1c (HbA1c), and using an oral glucose tolerance test (OGTT) to diagnose impaired glucose tolerance (IGT) 1.
From the Research
Diagnostic Guidelines for Diabetes Mellitus (DM)
The diagnosis of Diabetes Mellitus (DM) can be made using various tests, including Hemoglobin A1c (HbA1c), Fasting Plasma Glucose (FPG), and Oral Glucose Tolerance Test (OGTT) 2, 3, 4, 5, 6.
- 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.
- FPG and OGTT are recommended for the diagnosis of diabetes only if HbA1c testing is not possible due to unavailability of the assay, patient factors that preclude its interpretation, and during pregnancy 2.
- HbA1c testing has the advantages of greater clinical convenience, preanalytic stability, and assay standardization, but when used as the sole diagnostic criterion for diabetes, it has the potential for systematic error 2.
- A diagnostic cut-off point of HbA1c greater than or equal to 6.5% misses a substantial number of people with type 2 diabetes, including some with fasting hyperglycemia, and misses most people with impaired glucose tolerance 2.
Comparison of Diagnostic Tests
- A systematic review and network meta-analysis compared the diagnostic accuracy of HbA1c ≥ 6.5%, FPG ≥ 126 mg/dl, and the combination of HbA1c ≥ 6.5% or FPG ≥ 126 mg/dl (HbA1c| FPG) with OGTT ≥ 200 mg/dl for diagnosing diabetes 3.
- The study found that HbA1c| FPG is superior based on sensitivity, whereas FPG is ranked best based on specificity and positive likelihood ratio 3.
- Another study found that the diagnostic agreement in the clinical setting revealed the current HbA1c ≥ 6.5% is less likely to detect diabetes than those defined by FPG and 2hPG 5.
Combined Use of Diagnostic Tests
- The combined use of FPG and HbA1c may be a more sensitive and specific screening tool for identifying high-risk individuals with diabetes and impaired glucose tolerance at an early stage 6.
- A study found that the screening model using FPG > or = 6.1 mmol/l or HbA1c > or = 6.1% had a sensitivity of 96.5% for detecting undiagnosed diabetes, and the screening model using FPG > or = 6.1 mmol/l and HbA1c > or = 6.1% had a specificity of 96.3% for detecting undiagnosed diabetes 6.