Haemoglobin A1c (HbA1c), also known as glycated haemoglobin, provides a measure of average glucose levels in the blood over a longer period than a simple blood glucose test. When glucose attaches to the haemoglobin in red blood cells, it forms glycated haemoglobin, which can be used to estimate the average level of sugar in the blood over the preceding 2-3 months, as red blood cells have a lifespan of around 12-16 weeks.