Assay-aware flow-cytometric interpretation of relapsed B-cell precursor acute lymphoblastic leukemia after sequential CD19- and CD22-directed therapy: A case report

Rationale: Sequential CD19- and CD22-directed therapies can make relapse immunophenotyping assay-dependent. Surface and cytoplasmic antigen results address distinct clinical questions and should not be interpreted interchangeably. This case uniquely integrates sequential target exposure, discordant surface and cytoplasmic findings, exclusion of mixed-phenotype acute leukemia, and interpretation of a post-inotuzumab ozogamicin (INO) population between the assay limits of detection and quantification. Patient concerns: A 73-year-old woman with B-cell precursor acute lymphoblastic leukemia received a 14-day course of blinatumomab and later developed marrow relapse after a prolonged gap in specialist follow-up. The available records did not permit reconstruction of the complete treatment-selection history. Diagnoses: At relapse, the blasts were positive for cytoplasmic CD19 and CD79a, lacked surface CD19, expressed weak surface CD22 and CD13/CD33, and were negative for cytoplasmic MPO, CD14, and CD64. These findings supported relapsed B-cell precursor acute lymphoblastic leukemia with aberrant myeloid-associated antigen expression rather than mixed-phenotype acute leukemia. Interventions: The records documented blinatumomab exposure followed by INO-based salvage on three occasions. The reason for the abbreviated blinatumomab course and body-surface-area-based INO dosing could not be reconstructed from the reviewed records. Outcomes: Posttreatment marrow contained 0.5% blasts and a low-level abnormal population of 0.007% (approximately 30 events), which was above the assay limit of detection but below the limit of quantification. Surface CD22 was not detected on this low-event population with the S-HCL-1 assay. At later assessment, no previously identified leukemia-associated abnormal population was detected under the reported assay conditions. Lessons: After sequential antigen-directed therapy, lineage assignment and therapeutic-target assessment require repeated, assay-aware interpretation. Cytoplasmic antigen positivity may support lineage assignment but should not be equated with surface therapeutic-target availability. A low-event multiparameter flow cytometry population below the limit of quantification should be described qualitatively. A single posttreatment sample cannot establish lineage switch, antigen loss, treatment causality, or durable disease control.