Association between HbA1c and breast cancer risk in women with type 2 diabetes mellitus: A case-control study

Type 2 diabetes mellitus (T2DM) has been linked to several types of cancer, including breast cancer. Glycated hemoglobin (HbA1c) serves as an indicator of long-term blood sugar control, and elevated levels may contribute to the development of breast cancer. This study aims to evaluate the association between elevated HbA1c levels and breast cancer risk in women with T2DM attending a tertiary care hospital in South India, and to compare HbA1c levels between diabetic women with and without breast cancer. A retrospective case-control study was conducted at Kovai Medical Center from 2014 to 2016 among women with T2DM. Breast cancer was histopathologically confirmed. Data on HbA1c (<7.0% vs ≥7.0%), body mass index, diabetes duration, menopausal status, and family history were collected. Associations with breast cancer risk were analyzed using Chi-square tests and logistic regression adjusted for confounders, with significance set at P < .05. The study included 182 women with T2DM and breast cancer and 197 age-matched controls without breast cancer. Poor glycemic control (HbA1c ≥ 7.0%) was more prevalent among cases and significantly associated with breast cancer in multivariable logistic regression adjusting for age, body mass index, menopausal status, diabetes duration, and insulin use (adjusted OR = 2.98, 95% CI: 1.08-8.21, P < .0001). Additional risk factors included postmenopausal status (OR = 4.52, 95% CI: 1.36-15.00), age 50 to 59 years (OR = 8.30, 95% CI: 2.80-19.32), and insulin use (OR = 2.57, 95% CI: 1.04-3.21). Most cases were estrogen receptor and progesterone receptor negative, with 63% diagnosed at stage I. Advanced stages (II/III), triple-negative breast cancer (OR = 2.78, 95% CI: 1.05-3.65), and higher Ki-67 index (intermediate: OR = 1.91, high: OR = 3.00) were linked to more aggressive disease. In this retrospective case-control study, higher HbA1c levels (≥7.0%) were associated with breast cancer presence and more advanced tumor stages among women with T2DM, although the odds ratio for HbA1c should be interpreted cautiously. Postmenopausal status and insulin use were more common among cases, while metformin use showed a nonsignificant inverse trend. triple-negative breast cancer status and higher Ki-67 index were also more frequent in advanced disease.