HbA1c and Time in Range Explained

Contents

• What is HbA1c?
• What does the HbA1c test tell us?
• How often should my HbA1c be measured?
• HbA1c results and target
• What is Time in Range (TIR)?
• The difference between HbA1c and TIR

What is HbA1c?

The red blood cells in the blood contain a protein called haemoglobin. Glucose in the blood can stick to the haemoglobin, making a substance called ‘glycated haemoglobin’, also known as haemoglobin A1c or HbA1c for short.

The more glucose in your blood, the more glycated haemoglobin there will be, so the HbA1c level reported will be higher.

Red blood cells live for about 8–12 weeks before they are replaced by new ones, so the HbA1c test tells you how well your blood glucose has been controlled over the previous 8–12 weeks.

What does the HbA1c test tell us?

The better your blood glucose control the less chance there is of you developing diabetes complications such as eye, kidney or nerve damage, heart disease or stroke. The HbA1c test may change if you have changed your diet, physical activity, weight or medication and is a way of measuring whether you are on target to keep your risk of complications as low as possible.

How often should my HbA1c be measured?

Blood glucose levels vary throughout the day and from day to day, but HbA1c gives an average of what the glucose is over a 2-3 month period. HbA1c should be measured regularly but not too frequently. You will normally be tested every three to six months.

HbA1c results and target

It is important that you agree your own personal target with your diabetes care team, as different people will have different targets, depending on their individual circumstances.

For example, if you have had a lot of problems with low blood glucose levels and have had several hypoglycaemic episodes (‘hypos’), a higher target might be appropriate.

The HbA1c results will be expressed either in millimoles per mol (mmol/mol) or as a percentage (%). 

Table 1: Comparison of HbA1c results

What is Time in Range (TIR)?

Time in range (TIR) refers to the amount of time a person’s blood glucose levels remain within a specific target range. TIR is most accurately measured using a continuous glucose monitor (CGM),

Your CGM can tell you what your time in range is on a daily basis, however, it is generally more useful to look at the time in range over a 2 week period.

For most people with type 1 or type 2 diabetes, it is recommended to aim for:

• At least 70% of the day in 3.9-10 mmol/L (in-range)
• Less than 4% of the day below 3.9 mmol/L (below-range)
• Minimize time each day above 10 mmol/L (above-range)

Being in range 70% of the time equates to an HbA1c of about 53 mmol/mol. However, for some people, the target is greater than 50%, equating to an HbA1c of about 63 mmol/mol. Your diabetes team can discuss personal targets with you.

What is the difference between TIR and HbA1c?

Time in range is useful as it captures the daily variation in glucose levels that living with diabetes brings. HbA1c measures the average blood glucose over a two to three month period; it doesn’t capture the time spent in various blood glucose ranges throughout the day.

The graphic below from DiaTribe represents the advantages of looking at time in range. Each graph represents a different person but they all share the same HbA1c of 53 mmol/mol (average blood glucose of 8.6 mmol/L).

The first graph shows the person has a rollercoaster of dangerous highs and lows, the second still has highs and lows but much less pronounced and more time in range, the third has little variability and spends all the time in range.

Using time in range together with HbA1c is a good way for you and your healthcare team to monitor your diabetes and help reduce the risk of long-term health problems.