India is estimated to have 10.13 crore people with diabetes, and another 13.6 crore people who are pre-diabetic, according to a nationwide study published in 2023. This apart, over 35% of Indians suffer from hypertension and nearly 40% from abdominal obesity, both of which are risk factors for diabetes. India accounts for 17% of all diabetes patients in the world.

Prevention and early detection are key to helping combat this non-communicable disease burden, experts say. One of the most commonly-used tests to diagnose pre-diabetes and diabetes (both type 1 and type 2) and to help manage diabetes, is the haemoglobin A1C (HbA1C) test, also known as the glycated haemoglobin or glycosylated haemoglobin test.

How does the test work?

Sugar enters your bloodstream from the food you eat. The sugar, or glucose, attaches to the haemoglobin in your red blood cells. Haemoglobin is a protein that transports oxygen to all the cells of your body.

Everybody has some sugar attached to their haemoglobin. Those with pre-diabetes and diabetes, however, have more. The HbA1C test measures the percentage of your red blood cells that have sugar-coated, or glycated, haemoglobin.

Why is the test used to check for diabetes?

A paper was published in the Cleveland Clinic Journal of Medicine in 2016 entitled ‘The role of haemoglobin A1c in the assessment of diabetes and cardiovascular risk’. It stated: “HbA1c was first discovered in 1955, but elevated HbA1c levels in diabetes patients were not noted until 1968. Another eight years passed before HbA1c was correlated with blood glucose values in hospitalised patients with diabetes and was proposed for monitoring glycemia.”

During the first few years of clinical use, the paper said, HbA1c measures were inconsistent. But as the importance of precise HbA1c measurements became apparent through studies that revealed better patient outcomes and mortality associated with lower average HbA1c, the need to reduce error margins in measurement became apparent.

Following programmes to regulate HBA1c measurements and calibrate them to reference standards, standardisation and accuracy greatly improved from 1993 to 2012, the paper noted.

The American Diabetes Association approved HbA1c as a diagnostic tool in 2009. In 2011, after an expert consultation with the World Health Organisation (WHO), it said HbA1c could be used as a diagnostic test for diabetes “provided … stringent quality assurance tests are in place and assays are standardised to criteria aligned to the international reference values, and there are no conditions present which preclude its accurate measurement.”

What do HbA1C test results look like?

The HbA1C levels are provided as either a percentage or in mmol/mol (which stands for millimoles per mole). A mole is a unit of measurement often used for chemical substances.

The higher the percentage, the higher your blood glucose levels are. An Hb1A1C below 5.7% is considered normal; between 5.7 and 6.4% may indicate you are pre-diabetic; and 6.5% or higher can indicate diabetes. In mmol/mol: below 42 corresponds to below 6.0%; 42-47 mmol/mol to 6.0 to 6.4%; and 48 mmol/mol to 6.5% or over.

However, the test’s results may change under certain conditions, including if a patient has kidney or liver failure, severe anaemia or a blood disorder such as thalassemia; if they have a less common type of haemoglobin found in some populations; or are under certain medications including steroids, opiates or dapsone (a drug used to treat leprosy). They may even change if a person is in early or late pregnancy.

Generally, for those whose results indicate pre-diabetes or diabetes, doctors specify a goal to achieve specific HbA1C levels. But these vary from person to person and also depend on their age, health conditions, medications being taken, and other factors.

Who needs to take the test and when?

According to the Indian Council of Medical Research’s Guidelines for Management of Type 2 Diabetes (2018), all individuals older than 30 years should be screened for diabetes. Those with one or more risk factors including, among others, obesity, an increased waist circumference, a history of or being treated for hypertension, a history of heart disease, and a history of polycystic ovarian syndrome should be screened earlier.

Retests should be conducted after three years in case of normal glucose tolerance. If a person is pre-diabetic, retests should be annual. Your doctor may also ask you to be tested more frequently if, for instance, you’re planning to have a baby.

If you have diabetes, your doctor may ask you to take it every three to six months, to keep an eye on your blood sugar levels and to check if your treatment plan is working.

How does the test differ from others?

While fasting and post-prandial or post-meal blood sugar tests give you blood sugar levels within a specific time frame, the HbA1C test reflects your average blood glucose levels over the last two to three months.

Also, while the traditional blood sugar tests may fluctuate depending on items in the person’s latest meal and when they last consumed it, the HbA1C test is independent of these variables, making it more reliable. It can be taken irrespective of when the latest meal was consumed.

What are the test’s limitations?

It is important to note the HbA1C test does not replace other tests and may be carried out alongside others, such as the traditional blood sugar tests to test for diabetes and pre-diabetes. It also does not replace regular blood-sugar testing at home, which a doctor may have recommended, as the blood sugar levels may spike and dip through the day or night, and the HbA1C test may not capture this.

This apart, while the HbA1C test remains one of the best to assess long-term control of diabetes in people known to have diabetes, it is not uniformly accepted as a diagnostic test by all global medical bodies because of its relatively low sensitivity arising from difficulties in assay standardisation. In other words, a doctor may recommend a glucose test alongside an HbA1C test to obtain a clearer picture when diagnosing a person.

The test may also have limitations particular in India. A 2013 paper published in the journal Diabetes Technology and Therapeutics noted that in some clinical situations, accurate measurements are harder to make. These include having conditions like thalassaemia, structural haemoglobin variants in the population, iron-deficiency anaemia (which is relatively high in India), and the use of certain drugs.

“Because of the relatively frequent occurrence of some of these conditions in some parts of India, it is important that they are looked for when evaluating an inappropriately high or low HbA1c level. Alternative indices may have to be used for assessing glycemic control in these cases,” the paper noted.

Textbooks have said for decades that haemoglobin is found in the red blood cells (RBCs), that it makes blood red, carries oxygen, and is essential for our survival.

A new and serendipitous discovery has revealed that haemoglobin isn’t used by RBCs alone. In a study published in Nature, scientists from China have reported that chondrocytes – cells that make cartilage, the connecting tissue between bones – also make haemoglobin and seem to depend on it for their survival.

‘Haemoglobin bodies’

Feng Zhang, a pathologist in the Fourth Military Medical University in China, had been working on bone development since 2010. In 2017, when he was studying growth plates – cartilaginous tissue at the end of certain long bones that allows the bones to become longer – he stumbled upon a few spherical blob-like structures. They seemed to bear an uncanny resemblance to RBCs, and they contained haemoglobin.

Dr. Zhang then teamed up with Quiang Sun at the Beijing Institute of Biotechnology and used advanced microscopy techniques to investigate further.

Picture what happens when oil is mixed into water: the oil separates out into little globules in a process called phase separation. That’s what seemed to be happening in the chondrocytes in the cartilage as well. Dr. Zhang ascertained that the chondrocytes within the growth plates of newborn mice were not only producing large amounts of haemoglobin, but also that it was coalescing and forming large blobs without a membrane.

The scientists called these blobs haemoglobin bodies, or Hedy.

The haemoglobin does something

Now that they knew chondrocytes were making haemoglobin bodies, the question was: were the Hedy functional? That is, did they actually do something? To test this, the scientists used genetically modified mice, in this case mice in which the gene making haemoglobin had been removed. These mice produced almost no haemoglobin molecules and they died as embryos. But it turned out that if one looked closely at the growth plate cartilage tissue from these mice, most of the chondrocytes were dying.

Removing the gene that made haemoglobin specifically in the cartilage tissue also resulted in the same outcome: cell death among the chondrocytes. It was clear that Hedy was essential for the chondrocytes to live.

In RBCs, haemoglobin carries oxygen and makes sure that different parts of the body receive the oxygen to function correctly. The scientists conducted a series of experiments to check whether haemoglobin also carries oxygen in chondrocytes. First, they checked if the cartilage cells showed signs of stress before dying when haemoglobin molecules were absent. They focused on a special type of stress called hypoxic stress, caused by low-oxygen conditions. And indeed they did: cartilage that didn’t contain haemoglobin showed signs of hypoxic stress.

An oxygen store

Now they knew that the absence of haemoglobin caused the chondrocytes to go through some sort of low-oxygen stress. They then wanted to see how normal and haemoglobin-free chondrocytes behaved when there is little oxygen in the cells’ environment. The researchers proceeded to test the cells in a low-oxygen, or hypoxic, environment. In the presence of haemoglobin, the cells seemed to release more oxygen. But in the absence of haemoglobin, the chondrocytes started dying.

This further confirmed their hunch that the haemoglobin in the chondrocytes was most likely storing oxygen and supplying it to the cells when required.

“What’s really interesting about this paper is that they picked up on this unusual finding and delved deep into its aspects,” said Noriaki Ono, a bone biologist at the University of Texas Health Science Center at Houston. He wasn’t involved in the study.

Haemoglobin in other places

In a developing growth plate, where oxygen is limited due to a lack of blood supply to the region, the chondrocytes still manage to thrive. Based on the scientists’ findings, it’s the haemoglobin molecules that manage to bring them the oxygen they need to survive.

“What surprised me the most was that cartilage tissue synthesised a large amount of hemoglobin to cope with hypoxic stress,” Dr. Zhang told this writer by email.

A 2003 study had shown that chondrocytes adapt to low oxygen by, among other things, using an alternate pathway to break down sugars to release energy – one that doesn’t require oxygen. This study shed light on a different mechanism with which chondrocytes dealt with reduced oxygen supply.

The scientists also found that cartilage in regions outside the growth plate, like the one in the ribs or the spine of mice, also contained haemoglobin. What we don’t know yet for sure is whether the haemoglobin in these regions plays a similar role, in storing and releasing oxygen.

‘Really exciting possibility’

“What is important in this paper is that it breaks down barriers between haematology and skeletal biology, and shows that, in fact, these fields are more connected than it seems,” said Gerard Karsenty, a professor studying skeletal biology at Columbia University, New York.

We also don’t know if the Hedy has more functions or other effects on cells in the growth plate. Dr. Ono, who discovered a bone-making stem cell population in the growth plate in 2018, is intrigued by the possibilities this discovery opens for stem cells and their fates in the growth plate. “One really exciting possibility is that the haemoglobin in the growth plate could be doing something about changing the … fate of stem cells in the growth plate,” he said.

The discovery of functional haemoglobin in cartilage also leads to the possibility that it plays a role in certain joint diseases. “There are many bone deformities that develop from defects in chondrocytes,” according to Dr. Ono. “Maybe there is more cell death in some conditions due to having something wrong with the chondrocyte haemoglobin.”

Dr. Zhang added that he “hopes this discovery can reinterpret the mechanisms underlying some joint diseases.”

Rohini Subrahmanyam is a freelance journalist.