John B. Buse, MD, PhD


September 09, 2003


How long do you need to wait to see a change in hemoglobin A1C after making a modification in a diabetes medication regimen?

Response from John B. Buse, MD, PhD

The glycosylated hemoglobin (A1C) concentration reflects the time-weighted average of mean daily plasma glucose over 2-3 months.[1,2] Thus, it is generally appropriate to wait 3 months to see the full effect of a change in treatment on A1C. However, if the change in treatment involves the addition or change in dose of a thiazolidinedione, it may take 2-3 months more to see the full effect on fasting glucose. Thus, one might expect that it would take 4-6 months or more to see the full effect on A1C.

A practical issue is that the Medicare National Coverage Determination (MNCD)[3] suggests that the 3-month interval may be necessary to monitor patients with diabetes whose control has been, on average, within the target range. If substantial changes in mean glucose have occurred, one can repeat the measurement more often if there is a need to estimate the proportion and magnitude of the change in average glycemia. Because A1C seems to have a half-life of about 28 days, of the eventual change in A1C that would occur over 3 months after a change in treatment, perhaps 50% will occur within 1 month and 75% in 2 months. The MNCD does make specific mention that testing as frequently as monthly may be appropriate in pregnant women, and that testing every 1-2 months (with appropriate documentation) may be appropriate in patients with uncontrolled diabetes undergoing treatment changes. Please consult your local Medicare benefits administrator, since the guidelines are interpreted locally.