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Issue 93 Item 7 Why Relating A1C (GHB) To Mean Blood Glucose (MBG) Is Important?

Know your numbers and what they mean is crucial for every patient We are frequently asked about the relationship between A1C (or GHB) and blood glucose levels. Many patients with diabetes mellitus now perform self-monitoring of BG (SMBG) in the home setting, and understanding the relationship between A1C and BG can be very meaningful in setting goals for day-to-day testing.

A1c is a “Weighted” Average and many studies have shown that A1C is an index of mean BG (MBG) over the preceding weeks-to-months. Erythrocyte (red blood cell) life-span averages about 120 days. The level of A1C at any point in time is contributed to by all circulating erythrocytes, from the oldest (120 days old) to the youngest. However, A1C is a "weighted" average of blood glucose levels during the preceding 120 days, meaning that BG levels in the preceding 30 days contribute substantially more to the level of A1C than do BG levels 90-120 days earlier. Blood Glucose levels 30 days immediately preceding blood sampling contributes approximately 50% to the GHB result, the 30 days prior to that contributes about 25%, the remaining 25% is determined by the glucose level during a 2 month period before these 2 months (Tahara & Shima, Diab Care 1993;16:1313-14). This explains why the level of A1C can increase or decrease relatively quickly with large changes in BG; it does not take 120 days to detect a clinically meaningful change in A1C following a clinically significant change in MBG.  According to the studies, the last 30 days can represent up to 50% of the A1C reading.

So, How does A1C relate to MBG?  The largest set of data relating BG to A1C comes from the Diabetes Control and Complications Trial or DCCT (New Engl J Med 1993;329:977-986). Although quarterly A1C determinations were the principal measure of glycemic control in the DCCT, study subjects also performed quarterly 24-hour, 7-point capillary-blood glucose profiles. Blood specimens were obtained by subjects in the home setting, pre-meal, 90 minutes post-meal, and at bed-time. Analysis of the DCCT BG profile data was recently performed (Diabetes 1997;46 (suppl 1):8A). Mean A1C and BG were calculated for each study subject (n= 1439). Results showed a linear relationship between A1C and BG (MBG = ( 31.7 x A1C ) – 66.1) with a Pearson correlation coefficient (r) of 0.81.

The table below depicts the relationship between A1C, MBG and mean plasma glucose (MPG).

In summary, each 1% change in A1C represents a change of approximately 30 mg/dl in MBG or 35 mg/dl in MPG. Please note that this relationship applies only to A1C methods certified as traceable to the DCCT reference, and that it is based on overall averages and may vary slightly in individual patients.  

 

* MPG=MBG x 1.11 (Clin Chem 1998;44:655-59)
#Diabetes Care 1999;22 (Suppl. 1):S32-S41

  Rohlfing CL, Wiedmeyer HM, Little RR, England JD, Tennill A, Goldstein DE. Defining the Relationship Between Plasma Glucose and Hemoglobin A1c (HbA1c): Analysis of Glucose Profiles and HbA1c in the Diabetes Control and Complications Trial. Diab Care 25:275-8, 2002