Blast Percentage Defining Remission in AML
In acute myeloid leukemia, complete remission requires less than 5% blasts in the bone marrow and 0% blasts in peripheral blood. 1
Bone Marrow Criteria for Complete Remission
The bone marrow must contain <5% myeloblasts with normal maturation of all cell lines and no evidence of dysplasia. 1
Critical Technical Considerations:
When erythroid precursors constitute <50% of bone marrow nucleated cells, blast percentage is calculated based on all nucleated cells. 1
When erythroid precursors constitute ≥50% of bone marrow cells, blast percentage must be calculated based on nonerythroid cells only. 1
The bone marrow aspirate must contain spicules for adequate evaluation; if spicules are absent, a bone marrow biopsy is mandatory. 1
A minimum 200-cell differential count should be performed on the bone marrow aspirate, though a 500-cell count provides superior diagnostic precision. 1, 2
Peripheral Blood Criteria for Complete Remission
Complete remission mandates 0% blasts in peripheral blood—any detectable blasts constitute failure to achieve CR. 1, 3, 4, 2
Additional Hematologic Requirements:
Absolute neutrophil count must be >1,000/mcL (some guidelines specify ≥1,500/mm³). 1
Platelet count must be >100,000/mcL. 1
Hemoglobin must be >11 g/dL without transfusion support. 1
The patient must be transfusion-independent. 1
No extramedullary disease can be present. 1
Complete Remission with Incomplete Count Recovery (CRi)
Some patients achieve <5% bone marrow blasts but fail to recover adequate peripheral blood counts—this is designated CRi, not CR, and carries inferior prognosis. 1
CRi is defined as meeting all morphologic CR criteria except persistent neutropenia (<1,000/μL) or thrombocytopenia (<100,000/μL). 1
CRi should not be grouped with CR when reporting outcomes, as survival differs significantly between these categories. 1
Partial Remission (Not a Treatment Goal)
Partial remission requires 5-25% bone marrow blasts (representing ≥50% reduction from pretreatment) with normalization of blood counts—this designation is only relevant for phase I/II trials evaluating new agents, not standard therapy. 1
Critical Pitfall: Distinguishing Regeneration from Residual Disease
A bone marrow showing ≥5% blasts shortly after chemotherapy may represent regenerating marrow rather than treatment failure—repeat examination in 5-7 days is essential. 1
Flow cytometry can help distinguish regenerating normal marrow from persistent leukemia when morphology is ambiguous. 1, 5
Research demonstrates that patients with ≥5% morphologic blasts but negative flow cytometry for leukemia-associated immunophenotypes typically show <5% blasts on repeat marrow within 1-3 weeks without additional therapy. 5
However, negative flow cytometry should never provide false reassurance when clinical suspicion for relapse is high based on peripheral blood findings. 4
Minimal Residual Disease Considerations
Patients in morphologic CR (<5% blasts) but with detectable MRD by flow cytometry or molecular testing have outcomes comparable to patients with active disease (≥5% blasts), not to MRD-negative CR patients. 6
Three-year relapse rates are approximately 67% for MRD-positive CR versus 22% for MRD-negative CR. 6
Three-year overall survival is 26% for MRD-positive CR versus 73% for MRD-negative CR. 6
This evidence supports that true remission should ultimately be defined by MRD negativity, not just morphologic criteria, particularly when considering allogeneic transplantation. 6
Relapse Definition
Relapse is defined as reappearance of blasts in peripheral blood at any percentage, or ≥5% blasts in bone marrow not attributable to regeneration, or development of extramedullary disease. 1, 3, 4