July 15, 2008
A1c (Glycohemoglobin, HgbA1c)
Question from Israel:
I have two questions regarding the relationship between A1cs and average blood glucose.
According to the Children with Diabetes web site, an A1C of 7% is equal to an average blood glucose of 170 mg/dl [9.4 mmol/L] (or 172 mg/dl [9.6 mmol/L]). However, according to other sources (including the ADAG — A1c Derived Daily Glucose, a study sponsored by the American Diabetes Association (ADA), European Association for the Study of Diabetes (EASD), and International Diabetes Federation (IDF)), an A1c of 7% is equal to a blood glucose of 150 mg/dl [8.3 mmol/L] (or 154 mg/dl [8.6 mmol/L]). Who is correct? See Translating the A1C Assay Into Estimated Average Glucose Values.
Secondly, my doctor told me that my daughter’s A1c reflects her average blood sugar over the last 90 days. However, on the Internet, I found that it reflects only the last 30 days, with most of the impact from the last two weeks before the A1c test. Which is correct? See Who determined the HbA1c reaction rates and the consequences?
Answer:
The average blood glucose in relation to A1c changes a bit depending upon which studies are quoted. There actually is not much difference between an average blood glucose of 154 mg/dl [8.6 mmol/L] or 172 mg/dl [9.6 mmol/L] for long term health consequences and all of these figures are estimates.
The A1c represents glycosylation of the protein hemoglobin. So, it also reflects what happens inside the red blood cell where the hemoglobin resides. Since there is also more glycosylation of the newer formed hemoglobin, there is some extra weight to more recent blood glucose variability. So, it best reflects past 30 days but can reflect reasonable association for the previous two to three months with average red blood cell life about three to four months. We usually tell our patients that the A1c mostly reflects the most recent one to two months. All are correct but some more accurately stated.
SB
