Leukemia Diagnosis on Peripheral Blood Smear: 3-Question Method

Conventional cytogenetics identifies t(9;22) and detects additional cytogenetic abnormalities but requires bone marrow metaphase cells and cannot detect atypical transcripts. FISH uses dual probes for BCR and ABL genes; fusion appears as yellow signals and detects atypical transcripts that PCR misses. RT-PCR detects specific BCR-ABL transcripts (P210, P190, P230) but cannot detect atypical variants. Additional cytogenetic abnormalities (ACAs) are present in 3% at diagnosis but increase to 30-40% during progression, contributing to TKI resistance. Major root ACAs (isochromosome 17q, trisomy 8, trisomy 19, additional Ph chromosomes) carry worse prognosis than minor root ACAs (deletion 11q23, deletion 7q, trisomy 21, chromosome 3q changes, complex karyotype).

CML diagnostic testing strategy: (1) Always perform cytogenetics to detect additional abnormalities; (2) Molecular testing (PCR BCR-ABL) is most sensitive (MR4/MR5); (3) FISH is useful when molecular testing is negative but only detects MR2; (4) Patients with cryptic translocations (negative PCR) are not candidates for TFR. Post-transplant TKI considerations: (1) Ponatinib at 45mg carries significant cardiovascular risk; (2) For BCR-ABL <1%, dose can be reduced to 30mg then 15mg; (3) TKI should be continued for at least 2 years post-transplant, with some patients requiring indefinite therapy.

Cytogenetic analysis (karyotyping) is the primary method for demonstrating the Philadelphia chromosome, identified as the shortened chromosome 22 with translocation 9-22. This is found in the majority of patients. For negative cytogenetics, fluorescence in situ hybridization (FISH) uses fluorescent probes for BCR and ABL genes that fuse in the translocation, showing characteristic fusion signals. RT-PCR (reverse transcription PCR) detects BCR-ABL transcripts and is highly sensitive, useful for detecting cases negative on cytogenetics. RTQ-PCR (quantitative RT-PCR) enables molecular monitoring and detection of molecular remission, which is essential for treatment decisions and assessing response to targeted therapy.

CML results from a balanced reciprocal translocation between chromosomes 9 and 22, fusing the ABL proto-oncogene with BCR to create the Philadelphia chromosome. This fusion gene produces a constitutively active tyrosine kinase protein (210 kDa) that promotes uncontrolled cell proliferation and inhibits apoptosis. The Philadelphia chromosome was first identified in Philadelphia, Pennsylvania in 1960. CML progresses through three phases: chronic phase (asymptomatic, responsive to tyrosine kinase inhibitors), accelerated phase (increased blasts and basophilia), and blast crisis (transformation to acute leukemia, refractory and fatal). Definitive diagnosis requires detecting the Philadelphia chromosome via cytogenetics (FISH or PCR) and confirming with bone marrow biopsy showing hypercellular marrow with myeloid hyperplasia. The leukocyte alkaline phosphatase score helps differentiate CML (low LAP from dysfunctional cells) from leukemoid reactions (high LAP from functional cells).

Chronic myeloid leukemia (CML) is characterized by elevated white blood cell count with left shift (myelocytes, bands, and blasts), basophilia, and low platelet count. The most specific and sensitive investigation for CML diagnosis is FISH (fluorescence in situ hybridization) with PCR for BCR-ABL gene rearrangement. While peripheral blood smear shows characteristic findings, FISH with PCR is the gold standard for confirming the diagnosis and detecting the Philadelphia chromosome.

Under microscopy, bone marrow aspirate shows both mature and immature blood cells, unlike peripheral blood smears. The presence of immature cells in peripheral blood indicates leukemia (malignant blood disease). In bone marrow sections, the bone trabeculae are visible, covered by endosteum. The marrow contains stromal elements, adipose tissue (appearing empty as fat dissolves during preparation), blood vessels (sinusoids and capillaries), and hematopoietic cells.

Bone marrow biopsy and aspirate are complementary procedures that must be interpreted together. Examination of bone marrow aspirate alone is insufficient because while cytomorphological features are well identified, full interpretation cannot be provided without architectural context. Conversely, bone marrow biopsy alone cannot provide complete evaluation because proper assessment of hematopoietic abnormalities requires correlation with aspirate findings and clinical context. Without clinical correlation and aspirate findings, there is always a risk of overinterpreting or failing to adequately interpret bone marrow biopsy findings. For optimal reporting, bone marrow biopsies should be interpreted by a hematopathologist who can evaluate both aspirate and biopsy, or alternatively by a histopathologist and hematologist working together to interpret findings in totality.

Bone marrow aspirate and biopsy are complementary—neither alone provides complete interpretation. Bone marrow biopsy interpretation requires systematic examination with working knowledge of normal microanatomy. Components include stroma, vessels, reticular fibers, fibroblasts, adipose tissue, erythroid series, myeloid series, lymphocytes, plasma cells, mast cells, and macrophages. Hematopoietic cells have consistent spatial relationships: early myeloid precursors near endosteal surface, mature cells in central intertrabecular regions. Cellularity assessment uses 100-age formula (normal 25-75% for adults 20-70 years), with subcortical regions always hypocellular and unsuitable for assessment.

Bone marrow examination requires systematic assessment of both aspirate and trephine biopsy together, with the aspirate providing cytological details and cell counts while the biopsy preserves architectural framework for evaluating cellularity, fibrosis, and focal lesions; adequate interpretation demands examining multiple smears (at least 3-4), counting 200-500 nucleated cells for differential counts, assessing myeloid-erythroid ratio, recognizing cell maturation stages, evaluating iron stores using Prussian blue stain, and correlating findings with peripheral blood smears to avoid diagnostic errors.

Bone marrow biopsy provides architectural information about marrow composition and cellularity. Hypercellular biopsy corresponds to increased cellularity on aspirate. Erythroid hypoplasia indicates decreased red cell production. Accelerated erythropoiesis shows increased numbers of erythroid precursors. Myeloid suppression may indicate stress on the marrow. The biopsy complements aspirate morphology by showing tissue architecture and distribution of cell populations.

The WHO 2008 classification of myeloid neoplasms includes: (1) Acute myeloid leukemia and related precursor neoplasms (myelodysplastic syndromes, myeloid sarcoma); (2) Classical myeloproliferative neoplasms (chronic myeloid leukemia, polycythemia vera, primary myelofibrosis, essential thrombocythemia); (3) Non-classical myeloproliferative neoplasms (chronic neutrophilic leukemia, chronic eosinophilic leukemia, mastocytosis, unclassifiable myeloproliferative neoplasm); (4) Myelodysplastic syndromes (refractory cytopenia with dysplasia, refractory anemia with excess blasts, refractory anemia with ring sideroblasts, refractory anemia with multilineage dysplasia, myelodysplastic syndromes associated with isolated del(5q), unclassifiable myelodysplastic syndromes); (5) Myeloid and lymphoid neoplasms with eosinophilia and genetic alterations affecting PDGFRA, PDGFRB, or FGFR1 receptors.

According to the latest WHO classification, myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase fusion genes include PDGF receptor alpha/beta, FIP1L1-PDGFRA, JAK2, FLT3, and ETV6-ABL1 rearrangements. These neoplasms originate from mutated pluripotent stem cells capable of differentiating into myeloid and lymphoid lineages, explaining their clinical heterogeneity. The common mechanism involves constitutive tyrosine kinase signaling from gene fusions, though peripheral eosinophilia is not always present. Different rearrangements show varying responses to targeted therapies, with PDGF receptor alpha forms showing excellent imatinib sensitivity.

The WHO classification of myeloid neoplasms has undergone fundamental changes in the fifth edition. AML is now divided into two main categories: AML with defining genetic abnormalities and AML defined by differentiation. The term 'AML NOS' is no longer used. The 20% blast requirement has been eliminated for most AML types with defining genetic abnormalities, except AML with BCR-ABL fusion and AML with SEPAR mutation. AML with myelodysplasia-related changes (AML-MRC) is now included under AML with defining genetic abnormalities, requiring specific cytogenetic abnormalities and somatic mutations from eight defined genes. Secondary myeloid neoplasms encompass diseases arising from predisposing factors including cytotoxic therapy exposure and germline predisposition. Myeloid neoplasms post cytotoxic therapy include AML, MDS, and MDS/MPN arising after chemotherapy for unrelated conditions, with majority associated with TP53 mutation. Myeloid neoplasms with germline predisposition include conditions like Fanconi anemia, Down syndrome, and RASopathies, classified using a framework coupling disease phenotype with predisposing genotype.

Myeloid neoplasms arise from transformed hematopoietic progenitor/stem cells along the myeloid differentiation pathway. Normal maturation follows: myeloblast → pro-myelocyte → myelocyte → metamyelocyte → band → segmented neutrophil. In AML, maturation arrest occurs at various stages, causing blast accumulation. WHO defines AML by ≥20% blasts in bone marrow. Three main categories exist: acute myeloid leukemia, myelodysplastic syndromes (defective maturation), and myeloproliferative neoplasms (increased production). Both MDS and MPN can transform to AML over time, with CML potentially transforming to ALL due to shared hematopoietic stem cell origin. The FAB classification categorizes AML into seven subtypes based on morphology: M0 (undifferentiated), M1 (<10% differentiated), M2 (10-80% differentiated), M3 (APL with abnormal promyelocytes and Auer rods), M4 (myelomonocytic, 20-80% monocytic), M5 (>80% monocytic), M6 (erythroid), M7 (megakaryocytic). The WHO classification groups AML into four categories: recurrent genetic aberrations (t(8;21), inv(16), t(15;17)), AML with myelodysplastic features, therapy-related AML, and AML NOS. This evolution reflects advances in understanding molecular mechanisms underlying leukemogenesis.

The WHO classification of hematological malignancies divides tumors into myeloid neoplasms and lymphoid neoplasms. Myeloid neoplasms include MPN, MDS, and AML. The classification is based on biological behavior, clinicopathological features, immunophenotype, and genetics. This classification enables accurate diagnosis and guides treatment decisions. The WHO 2016-2017 classification represents the current standard, incorporating molecular findings that were not available in earlier classifications.