From the Guidelines
Diabetes mellitus can be diagnosed using HbA1c (glycated hemoglobin) with a threshold value of 6.5% or higher, as recommended by the most recent guidelines 1. To properly diagnose diabetes using HbA1c, a blood sample should be collected and analyzed in a laboratory using a method that is certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) reference assay. The test should be performed in the absence of conditions that affect red blood cell turnover, such as hemolytic anemia, recent blood transfusion, or pregnancy, as these can lead to falsely low or high values. If the initial HbA1c result is 6.5% or higher, a repeat test is recommended for confirmation unless clinical symptoms and a random plasma glucose level above 200 mg/dL (11.1 mmol/L) are present. Some key points to consider when diagnosing diabetes using HbA1c include:
- HbA1c values between 5.7% and 6.4% indicate prediabetes, suggesting an increased risk for developing diabetes in the future 1.
- HbA1c testing is advantageous because it reflects average blood glucose levels over the previous 2-3 months, doesn't require fasting, has less day-to-day variability than glucose measurements, and is less affected by acute stress or illness 1.
- The frequency of HbA1c testing for diagnosis has not been established, but guidelines similar to those for glucose-based testing seem appropriate 1.
- HbA1c assays are not recommended for screening for or diagnosis of gestational diabetes mellitus 1. It is essential to note that while HbA1c is a valuable tool for diagnosing diabetes, it should be used in conjunction with clinical judgment and other diagnostic criteria to ensure accurate diagnosis and effective management of the disease.
From the Research
Diagnosis of Diabetes Mellitus using HbA1c
- HbA1c is a widely used test to evaluate glycemic control in patients with diabetes, but its effectiveness in diagnosing diabetes mellitus (DM) is still controversial 2, 3, 4, 5, 6.
- The American Diabetes Association recommends a diagnostic cut-off point of HbA1c ≥ 6.5% for DM diagnosis 4, 5, 6.
- However, studies have shown that this cut-off point may not be sensitive enough to detect all cases of diabetes, particularly in certain populations such as older adults, females, and those with impaired glucose tolerance 2, 3, 5.
- The sensitivity and specificity of HbA1c in diagnosing DM vary among different ethnic groups and populations 3, 6.
- HbA1c testing has several advantages, including convenience, preanalytic stability, and assay standardization, but it also has limitations, such as the potential for systematic error and the impact of factors that may not be clinically evident on test results 4, 5.
Comparison with other Diagnostic Methods
- HbA1c has been compared to other diagnostic methods, such as fasting plasma glucose (FPG) and 2-hour plasma glucose (2hPG), and has been found to have lower sensitivity and specificity 2, 3.
- The use of HbA1c as the sole diagnostic criterion for DM may lead to systematic bias and miss a substantial number of people with type 2 diabetes 5.
- Combining HbA1c with plasma glucose measurements may offer the benefits of each test and reduce the risk of systematic bias inherent in HbA1c testing alone 5.
Practical Considerations
- The cost and availability of HbA1c testing, as well as the lack of standardization in some countries, are major arguments against its use as a diagnostic criterion for DM 6.
- Further guidelines are needed to clarify the appropriate use of HbA1c screening in certain populations, such as children, women with gestational diabetes, and those with prediabetes 4.