Understand your tests / Guide
HbA1c: a view of glucose beyond today
The difference between a current glucose reading and HbA1c, preparation, and why context matters.
Key takeaway
HbA1c reflects average glucose over roughly three months. It does not show every rise or fall during the day.
An average over time
Glucose attaches to haemoglobin in red blood cells. HbA1c measures the percentage of haemoglobin with glucose attached and reflects the average level over approximately the last three months.
A glucose measurement shows the level at the moment the sample is taken. HbA1c adds a longer-term perspective.
Do you need to fast?
HbA1c itself does not require fasting. If you have other tests at the same visit, their preparation instructions may differ.
When an average may not tell the whole story
Blood loss, transfusions and some haemoglobin variants can affect the result. Severe iron deficiency may also distort it.
When HbA1c and glucose readings do not agree, the discrepancy needs clarification. A single number should not be used to change treatment on your own.
Sources
- The A1C Test & Diabetes ↗NIH · National Institute of Diabetes and Digestive and Kidney Diseases · Accessed 5 September 2026
- Factors that Interfere with HbA1c Test Results ↗NGSP · Accessed 5 September 2026
An editorial explanation referring to the listed sources. General information; individual interpretation and treatment require clinical assessment.
Updated 5 September 2026. How we prepare our content