Glycated hemoglobin (HbA1c, also known as glycosylated hemoglobin) is a laboratory test that reflects average blood glucose levels over the past 2-3 months. It is one of the key indicators for diagnosing and managing diabetes. It allows for the early detection of carbohydrate metabolism disorders, even in the absence of symptoms. Elevated HbA1c levels may indicate long-term elevated blood sugar and indicate the risk of complications (particularly those affecting the cardiovascular system, kidneys, vision, and nervous system).
What is glycated hemoglobin?
This is a key biochemical indicator reflecting the average blood glucose level over 2-3 months (depending on the lifespan of red blood cells). It occurs when glucose in the blood binds to hemoglobin in red blood cells. Since red blood cells live for approximately 3 months, this indicator allows one to assess blood sugar levels over a long period. The higher the blood sugar level, the more glycated hemoglobin is formed. Unlike a single glucose measurement, the HbA1c test is not affected by food intake, stress, recent illness, or physical activity, so it requires no special preparation before testing. The test should be performed in a laboratory using a method certified by the NGSP** and standardized for the DCCT test.*
Glycated hemoglobin: Normal values
HbA1c testing is used to diagnose carbohydrate metabolism disorders and manage diabetes. Reference values are provided on the result form, and the results should be evaluated in conjunction with symptoms, health status, and other laboratory parameters. Reference ranges are used when interpreting HbA1c levels. These are the ranges of test results considered normal and listed on the result form, and ranges that may indicate prediabetes or diabetes. Results are interpreted differently depending on whether the test is being used to diagnose diabetes or to manage established diabetes. To identify diabetes and prediabetes, many expert organizations, including the American Diabetes Association (ADA) and the U.S. Centers for Disease Control and Prevention (CDC), use the following reference ranges for HbA1c:
- Normal carbohydrate metabolism: <5.7%
- Prediabetes: 5.7-6.4%
- Diabetes: ≥6.5%
To diagnose diabetes, your doctor may order a repeat glycated hemoglobin test or additional diagnostic tests, such as a blood glucose level.
Causes and consequences of decreased values
A decreased HbA1c level is less common than an increased level and is usually associated not with sugar levels, but with changes in red blood cells or their faster turnover, and the influence of other factors.
The main causes of a decreased value are:
- hemolytic anemia – a condition in which red blood cells are destroyed faster than usual, which reduces the time it takes for hemoglobin to interact with glucose;
- acute or chronic blood loss, leading to red blood cell turnover;
- blood transfusion, especially if the donor red blood cells have different characteristics;
- pregnancy (due to an increase in the volume of the liquid portion of the blood and rapid red blood cell turnover);
- hemoglobinopathy – genetically determined changes in the structure of hemoglobin that can affect the results.
Why is glycated hemoglobin elevated and what are the effects?
When glycated hemoglobin is elevated, it is most often a marker of long-term elevated blood glucose.
The main causes of elevated HbA1c include:
- diabetes mellitus, both newly diagnosed and poorly controlled;
- prediabetes;
- iron or vitamin B12 deficiency, which can affect the lifespan of red blood cells and consequently increase the level;
- chronic renal failure and other metabolic disorders that alter metabolism and blood composition.
Possible consequences of long-term elevation:
Persistently elevated HbA1c levels are associated with gradual damage to blood vessels and tissues, significantly increasing the risk of chronic complications:
- cardiovascular diseases (coronary heart disease, heart attack, stroke);
- chronic kidney disease, which over time, if kidney function deteriorates, may lead to the need for dialysis (blood purification) or a transplant;
- nerve damage and circulatory problems in the extremities, which can increase the risk of amputation, as well as damage to internal organs (heart, bladder, stomach, intestines, genitals, eyes);
- Retinal damage: Long-term elevated blood glucose levels damage blood vessels, which can lead to decreased vision or blindness.
Results should be assessed comprehensively, taking into account symptoms, medical history, and other laboratory test results.
*DCCT – Diabetes Control and Complications Study;
**NGSP – National Glycated Hemoglobin Standardization Program.
