How far back can a glucose test detect?

A HbA1c (glycosylated or glycated haemoglobin) test offers an 'average' of the previous 3 months blood glucose level.

Edit:

Sorry, I meant to add that this won't tell your doctor what your blood sugar level was on a particular day 3 months ago, or what it was 3 days ago. It averages out the results of all your blood sugar levels over that period. (The average lifespan of red blood cells is approximately 90 to 120 days. It's these red blood cells ... the haemoglobin ... that the glucose gets attached to, so some cells will have higher levels of glucose attached and others will have loer levels of glucose attached. Obviously, not all of your blood cells die at the same time. This is why the result offers an 'average' for that period of time.)

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HbA1c is a lab test that shows the average level of blood sugar (glucose) over the previous 3 months. An HbA1c of 5.6% or less is normal.

The International Diabetes Federation and American College of Endocrinology recommend HbA1c values below 48 mmol/mol (6.5%), while American Diabetes Association recommends that the HbA1c be below 53 mmol/mol (7.0%) for most patients.

usually 3 months