Early-Onset Direct Antiglobulin Test-Negative Autoimmune Hemolytic Anemia Induced by Pembrolizumab in Perioperative Triple-Negative Breast Cancer: A Case Report.

Introduction: Hematological immune-related adverse events associated with immune checkpoint inhibitors (ICIs) are rare, and autoimmune hemolytic anemia (AIHA) is particularly uncommon. Case Presentation: A 71-year-old woman with stage IIA triple-negative breast cancer received perioperative pembrolizumab with carboplatin and paclitaxel. During the first treatment cycle, she developed acute severe anemia with laboratory findings consistent with hemolysis. Despite a negative direct antiglobulin test (DAT), major alternative causes of hemolysis were considered unlikely, leading to a clinical diagnosis of suspected ICI-associated AIHA. Oral prednisolone (1 mg/kg) promptly resolved hemolysis without recurrence. Pembrolizumab was discontinued; however, imaging demonstrated a clinical complete response, and subsequent surgery confirmed a pathological complete response (pCR). Conclusion: This case demonstrates that AIHA during ICI therapy may occur despite a negative DAT and underscores the importance of prompt recognition and corticosteroid therapy. Favorable oncologic outcomes, including pCR, may still be achieved despite early discontinuation of ICI therapy.