Diabetes causes complications and collateral diseases, reducing quality of life and increasing medical costs. The Japanese government has promoted measures for the prevention of diabetes aggravation. Although glycemic control is reported to prevent the development of complications, assessment of the effects on overall medical cost is insufficient. We examined the medical cost by the analysis of hemoglobin A1c (HbA1c) level.

A Japanese employee-based health insurance claims database with annual medical check-up data was analyzed. Excess medical cost was calculated as the difference between medical cost and standard medical cost (defined as the average medical cost for individuals of same age and sex). Percentage of excess medical cost was calculated by dividing excess medical cost by standard cost, and compared between individuals with or without treatment for diabetes.

Of 4,307,184 individuals with HbA1c data, four percent of them received treatment for diabetes. For treatment of 6.5 percent of HbA1c, excess medical cost increased to 124 percent. The medical cost increased by an additional 20.4 percent (95% CI: 17.1–23.8) when the HbA1c level increased one percent. Treatment for less than six percent of HbA1c caused an increase consistent with the HbA1c level. The relative risk of iron deficiency anemia, unspecified of those with less than six percent of HbA1c against those with seven to eight percent was the highest, 2.15.

An increase of medical cost for individuals with treatment for high HbA1c is likely associated with diabetic complication. The raise for individuals with lower HbA1c level may be related to anemia. Despite the younger age and healthier life of the analyzed individuals, since they were insured by employee-based health insurance who took regular medical check-ups, more expensive medical cost was observed for those having higher HbA1c level.