Is there an approximate conversion formula for A1c to the average blood glucose for the previous one to two months? For example, how does one interpret an A1c of 7% to estimate the previous month’s blood glucose average?

Hemoglobin A1c values are slightly different in different labs, but usually average about 4-6% if adjusted for what is called the DCCT. In the February 2002 issue of Diabetes Care, there is an article by David Goldstein and colleagues that equates A1c to average blood flucose as follows:

   A1c   
   Average Blood glucose   

4%
65 mg/dl [3.6 mmol/L]

5%
100 mg/dl [5.6 mmol/L]

6%
135 mg/dl [7.5 mmol/L]

7%
170 mg/dl [9.4 mmol/L]

8%
205 mg/dl [11.4 mmol/L]

9%
240 mg/dl [13.3 mmol/L]

10%
275 mg/dl [15.3 mmol/L]

11%
310 mg/dl [17.2 mmol/L]

12%
345 mg/dl [19.2 mmol/L]

These data all comes from analysis of DCCT with 26,056 samples and 1439 patients having complete blood glucose profiles as well as A1c analyses every three months during the DCCT study period. the exact reference is Rohlfing CL, Wiedmeyer HM, Little RR, England JD, Tennil A and Goldstein DE. Defining the relationship between plasma glucose and HbA1c. Analysis of glucose profiles and HbA1c in the DCCT. Diabetes Care 2002:25:275-8.

You should discuss this with your own laboratory since they may have standards already set up for their assay.

SB