Over the last three decades, HbA1c testing has gained a prominent role in the diagnosis of Type 2 diabetes as well as monitoring of blood glucose control in Type 1 and Type 2 diabetes.
In this post, we describe HbA1c and provide an overview of its use in the diagnosis and monitoring of diabetes. We also introduce an emerging approach based on dried blood spots, which can simplify HbA1c testing and improve life quality by allowing for self-sampling at home.
What is HbA1c?
HbA1c is the abbreviated name for glycated haemoglobin or haemoglobin A1c. Haemoglobin is the iron-containing oxygen-transport protein present in red blood cells that gives blood its colour. In simple terms, glycated haemoglobin refers to the fraction of haemoglobin in the blood that is biochemically joined to glucose.
The rate at which haemoglobin is glycated is dependent upon blood glucose levels, so the more glucose in the blood, the higher the percentage of glycated red blood cells, and the higher the HbA1c result will be. HbA1c is formed slowly and non-enzymatically via the condensation of glucose with the N-terminal amino groups of the beta-chains of haemoglobin A.
HbA1c’s use as a biomarker for long-term glucose control originates from work by Samuel Rahbar and co-workers in the late 1960s, who demonstrated that glycated haemoglobin was two-fold more abundant in the blood of diabetics compared to healthy individuals (1).
Some years later, it was proposed by Ronald Koenig and co-workers that HbA1c could be used clinically to monitor the control of glucose metabolism in diabetic patients (2), and today it is well established that the concentration of glycated haemoglobin in the blood is directly related to the long-term blood glucose level.
How is HbA1c testing used today?
In contrast to a standard blood glucose test, which determines the blood sugar level at the time of testing, a HbA1c test result gives an indication of the blood sugar levels in the 8-12 weeks prior to testing.
So how does this work? Because red blood cells are freely permeable to glucose, HbA1c is formed throughout their entire lifespan, which is approximately 120 days. Since its formation is directly related to the blood glucose level as explained above, determining the HbA1c concentration provides a “glycemic history” of the previous 120 days. Because of the relatively long lifespan of a red blood cell, HbA1c levels do not change rapidly, making it a very suitable long-term marker for glucose control.
As mentioned above, HbA1c testing is the main approach to diagnosing Type 2 diabetes. It is not the primary test for diagnosing Type 1 diabetes but it may be used to supplement a range of blood glucose tests depending on whether or not the individual presents with symptoms.
HbA1c testing is also used for regular monitoring of patients with Type 1 and Type 2 diabetes. This allows early intervention when HbA1c levels are deemed too high, which is crucial because the higher the HbA1c levels, the greater risk of diabetes-associated complications, e.g., severe and irreversible nerve and kidney damage. Another common use for HbA1c is in the screening of at-risk individuals, e.g., those over 40 years of age or obese; in these groups a positive HbA1c result needs to be confirmed with a repeat test or another type of test.
When should HbA1c testing not be used?
HbA1c testing should not be used to diagnose Type 2 diabetes in:
- Children and young people below the age of 18 years, because of concerns that the threshold of ≥48 mmol/mol is not sensitive enough, and the risk of missing cases of Type 2 diabetes in this group.
- Pregnant women or women who are less than two months postpartum, because it has been shown to be less reliable than other diagnostic methods for gestational diabetes.
- Individuals who have had symptoms of diabetes for less than 2 months.
- Anyone with anaemia or another chronic disease that impacts haemoglobin, e.g., sickle cell disease or thalassemia.
How is HbA1c currently tested?
In clinical practice today, HbA1c levels are usually measured in venous or finger-prick blood samples collected in a doctor’s office, diabetes clinic or other relevant healthcare facility.
HbA1c testing is typically performed quarterly or half-yearly, and the testing frequency and HbA1c target levels (measured as mmol/mol or % of total haemoglobin) depend on the individual’s situation. In general, the acceptable target level is usually higher for a confirmed diabetic than a person at risk of developing diabetes.
In 2011, the World Health Organization recommended the use of HbA1c as an alternative diagnostic test suggesting an HbA1c level of ≥6.5% (≥48 mmol/mol) as a cut-off for diagnosing type 2 diabetes in non-pregnant adults (3).
The most widely used analytical methods to measure HbA1c include:
- Immunoassay using antibodies that detect HbA1c and the glycated forms of the most common genetic variants of haemoglobin.
- Ion-exchange high-performance liquid chromatography (HPLC), which separates haemoglobin species based on charge differences between HbA1c and other haemoglobins.
- Enzymatic assays that cleave HbA1c to release amino acids (usually valine) that serve as substrates for downstream detectable reactions.
- Boronate affinity HPLC, which allows the selective purification of glycosylated- from non-glycosylated haemoglobins based on highly specific biological interactions between glycosylated haemoglobin and boronic acid derivatives. This method is used in an FDA-approved point-of-care diagnostic test.
For patient convenience, at-home or remote HbA1c testing has been made possible using microsamples of fresh finger-prick blood; however, those tests are time-sensitive and require the patient to send their sample back to a lab in a short space of time for processing. It should also be noted that currently available home tests are not recommended as substitutes for testing performed by a medical professional and should not be used in place of an official diagnosis.
Current HbA1c testing approaches can lead to non-compliance
While minimally-invasive finger-prick sampling strategies have emerged for HbA1c testing, as mentioned above, the predominant approach in many countries still involves testing in venous blood samples, leaving much room for improvement when it comes to patient convenience and quality of life.
The treatment and management of diabetes has advanced so much in recent years that the majority of diabetes patients nowadays, assuming their disease is well-managed, live active and normal lives. The possibility for home sampling for an official HbA1c test would greatly reduce the need for regular medical visits, and positively impact life quality.
In addition, centralised testing for HbA1c can lead to access issues for people living in remote areas, affecting their attendance for regular check-ups and ultimately impacting their disease control and risk of developing diabetic complications. On a related note, a 77% reduction in monitoring of HbA1c was reported in the UK during the Coronavirus disease 2019 (COVID-19) pandemic (4); it is likely that this reduction in compliance could have been mitigated by access to home sampling for HbA1c testing.
Dried blood spot as an emerging approach to HbA1c sampling
Recent studies highlight the potential of dried blood spot (DBS) sampling in HbA1c testing, with robust and accurate HbA1c detection in DBS samples, and excellent correlation and agreement with whole blood values. For example, in one study published in 2000, a comparative analysis of home prepared DBS samples vs. standard venous blood analysis revealed a strong linear correlation between HbA1c levels determined from both sample types (5). Another study performed under ISO15189 guidelines at the French National Institute of Health and Medical Research (INSERM) in 2017 revealed highly correlated HbA1c levels from whole blood or DBS samples from a cohort of diabetic patients (6).
The studies mentioned above, as well as other similar studies carried out in recent years, support a role for DBS in future HbA1c testing, and the high analytical quality that comes with quantitative DBS has the power to transform the HbA1c testing procedure for millions of diabetics worldwide.
