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Hb = hemoglobin, the compound in the red blood cells that transports oxygen. Hemoglobin occurs in several variants; the one which composes about 90% of the total is known as hemoglobin A. A1c is a specific subtype of hemoglobin A. The 1 is actually a subscript to the A, and the c is a subscript to the 1. "Hemoglobin" is also spelled "haemoglobin", depending on your geographic allegiance. Glucose binds slowly to hemoglobin A, forming the A1c subtype. The reverse reaction, or decomposition, proceeds relatively slowly, so any buildup persists for roughly 4 weeks. Because of the reverse reaction, the actual HbA1c level is strongly weighted toward the present. Some of the HbA1c is also removed when erythrocytes (red blood cells) are recycled after their normal lifetime of about 90-120 days. These factors combine so that the HbA1c level represents the average bG level of approximately the past 4 weeks, strongly weighted toward the most recent 2 weeks. It is almost entirely insensitive to bG levels more than 4 weeks previous. In non-diabetic persons, the formation, decomposition and destruction of HbA1c reach a steady state with about 3.0% to 6.5% of the hemoglobin being the A1c subtype. Most diabetic individuals have a higher average bG level than non-diabetics, resulting in a higher HbA1c level. The actual HbA1c level can be used as an indicator of the average recent bG level. This in turn indicates the possible level of glycation damage to tissues, and thus of diabetic complications, if continued for years. Interpreting HbA1c values can be tricky for several reasons. See the following section for more details.