The authors studied 5 diabetic patients and first realized that A1c was an ideal index to reveal the overall control of a patient’s blood
sugar over the past several months before a doctor’s visit. They were the first to state that “Hemoglobin A1c concentration appears to reflect the mean blood sugar concentration best over the previous weeks to months," …. And “the periodic monitoring of hemoglobin A1c levels provides a useful way of documenting the degree of control of glucose metabolism in diabetic patients….”

Order of authorship in the original publication: Koenig, Peterson, et al, Cerami.

(Thanks to Juan Weiss for this reference and its interpretation.)