"First, a negative DAT does not exclude immune-mediated hemolysis, particularly in ICI-treated patients."
BUFFALO, NY — September 10, 2026 — A new case report was published in Volume 13 of Oncoscience on August 12, 2026, titled " Pembrolizumab-associated Coombs-negative autoimmune hemolytic anemia in metastatic lung adenocarcinoma with concurrent thromboembolism and bacteremia: A case report with literature review ."
The report was led by first and corresponding author Swapnil Surpur from The Wright Center for Graduate Medical Education in Scranton, Pennsylvania .
Immune checkpoint inhibitors (ICIs) such as pembrolizumab have become an important part of treatment for metastatic non-small cell lung cancer (NSCLC), but they can also trigger immune-related adverse events. Hematologic complications are among the rarest, with an estimated incidence below 1%, yet they can carry substantial morbidity and mortality. Autoimmune hemolytic anemia (AIHA), in which red blood cells are prematurely destroyed, represents one particularly serious hematologic immune-related adverse event.
The case involved a 68-year-old woman with stage IV lung adenocarcinoma and brain metastases. She underwent resection of a metastatic brain lesion followed by stereotactic radiation. With no actionable molecular alterations identified, systemic treatment with carboplatin, pemetrexed and pembrolizumab was initiated. Approximately four weeks after the first cycle, she experienced repeated hospitalizations involving fever and major thromboembolic complications, including pulmonary embolism and dural venous sinus thrombosis.
Over approximately six weeks, her hemoglobin declined from 11–12 g/dL to 6.4 g/dL. Laboratory testing showed reticulocytosis, markedly elevated lactate dehydrogenase, undetectable haptoglobin and indirect hyperbilirubinemia—findings consistent with active hemolysis. Imaging showed no active gastrointestinal or retroperitoneal bleeding, while a peripheral blood smear demonstrated polychromasia without schistocytosis.
The diagnostic challenge was that direct antiglobulin testing (DAT), also known as the Coombs test, was negative for both IgG and complement. The patient also had several competing potential explanations for anemia, including advanced cancer, chemotherapy, infection, anticoagulation and extensive thromboembolic disease. She additionally developed transient Streptococcus bacteremia, which was treated with intravenous ceftriaxone.
Despite the negative DAT, the temporal association with pembrolizumab, biochemical evidence of hemolysis and exclusion of alternative causes led the treating team to diagnose pembrolizumab-associated Coombs-negative autoimmune hemolytic anemia. The event was classified as grade 3 immune-related hematologic toxicity because of the severity of the anemia and need for transfusion.
Pembrolizumab was discontinued, and the patient was treated with high-dose prednisone at 1 mg/kg/day, along with folic acid and vitamin B12 supplementation. Transfusion was reserved for symptomatic anemia or hemoglobin below 7 g/dL. Following corticosteroid treatment, her hemoglobin stabilized and laboratory markers of hemolysis gradually improved.
"Up to 20% of AIHA cases may be DAT-negative, reflecting low-affinity antibodies, non-IgG immune mechanisms, or assay limitations."
The case highlights why a negative Coombs test should not automatically exclude immune-mediated hemolysis. The authors note that immune checkpoint inhibitor-associated AIHA has predominantly been reported with PD-1 and PD-L1 inhibitors and can develop within weeks to months after treatment begins. Diagnosing the condition can become particularly difficult in patients with advanced cancer because chemotherapy, malignancy-associated inflammation, infection, anticoagulation and other complications may produce overlapping clinical findings.
The proposed mechanism of ICI-associated AIHA involves loss of peripheral immune tolerance, expansion of autoreactive T cells and subsequent autoantibody production against red blood cell antigens. Current management discussed in the report includes prompt high-dose corticosteroids for clinically significant hemolysis, with intravenous immunoglobulin or rituximab considered in refractory cases. The authors also note that permanent discontinuation of immune checkpoint therapy is generally advised for grade 3 or higher hematologic toxicity.
Importantly, this report describes a single patient and cannot establish how frequently pembrolizumab causes Coombs-negative AIHA or prove that pembrolizumab alone was responsible for the hemolysis. The patient's concurrent metastatic cancer, chemotherapy, thromboembolic disease, bacteremia and anticoagulation complicated the clinical picture. Instead, the case illustrates the diagnostic reasoning required when severe hemolysis develops during immune checkpoint inhibitor treatment despite negative standard serologic testing.
Overall, the report emphasizes that pembrolizumab-associated AIHA is rare but potentially serious and that negative DAT results do not necessarily rule out immune-mediated hemolysis. In patients receiving immune checkpoint inhibitors who develop otherwise unexplained anemia accompanied by laboratory evidence of red blood cell destruction, early recognition and appropriate evaluation may be important for preventing severe complications.
DOI: https://doi.org/10.18632/oncoscience.670
