How do you diagnose leukemia?

How to Diagnose Leukemia

The diagnosis of leukemia requires a complete blood count with differential and peripheral blood smear as the essential first step, followed by bone marrow aspiration, immunophenotyping by flow cytometry, and cytogenetic analysis to establish the definitive diagnosis and classify the specific subtype. 1, 2

Initial Evaluation and Laboratory Testing

Complete Blood Count and Peripheral Blood Smear

• Obtain a CBC with differential as the critical first diagnostic test when leukemia is suspected 1
• Review the peripheral blood smear using May-Grünwald-Giemsa or Wright-Giemsa stain, counting at least 200 leukocytes 1
• Document relevant clinical data including age, sex, ethnicity, history of hematologic disorders, prior malignancies, exposure to cytotoxic therapy, radiation, or toxic substances 1
• Perform physical examination documenting neurologic findings, organomegaly, and lymphadenopathy 1

Key Blood Count Patterns by Leukemia Type

Acute Myeloid Leukemia (AML):

• Presence of myeloblasts, monoblasts, or megakaryoblasts in peripheral blood 1
• Often accompanied by anemia and thrombocytopenia 1
• Blood blast count of 20% or more is required for diagnosis in most cases 1

Chronic Myeloid Leukemia (CML):

• Leukocytosis with basophilia and immature granulocytes (metamyelocytes, myelocytes, promyelocytes) with few myeloblasts 1
• Thrombocytosis may be present 1

Chronic Lymphocytic Leukemia (CLL):

• Sustained peripheral blood lymphocyte count ≥5,000 monoclonal B lymphocytes/µL 3
• Small, mature-appearing lymphocytes with narrow cytoplasm border and dense nucleus lacking discernible nucleoli 3
• Larger atypical lymphocytes or prolymphocytes must not exceed 55% 3
• Monocytopenia is relatively sensitive and specific 3

Hairy Cell Leukemia:

• Pancytopenia with characteristic "hairy" lymphocytes on peripheral blood smear 1

Confirmatory Diagnostic Testing

Bone Marrow Examination

• Bone marrow aspiration is essential for definitive diagnosis when leukemia is suspected 1, 2
• Trephine bone marrow biopsy assesses extent of marrow infiltration 3
• Note that dry tap may occur in hairy cell leukemia due to extensive fibrosis 3
• For AML, diagnosis requires ≥20% myeloblasts in bone marrow or blood 1

Immunophenotyping by Flow Cytometry

• Flow cytometry is crucial to determine lineage involvement and confirm clonality 1, 2
• For CLL: Cells must co-express CD5 antigen with B-cell surface antigens CD19, CD20 (low), CD23, with characteristically low levels of surface immunoglobulin, CD20, and CD79b 3
• Each clone is restricted to expression of either kappa or lambda immunoglobulin light chains 3
• For AML: Markers include CD34, CD38, CD117, CD133, HLA-DR (precursor markers) and CD13, CD15, CD16, CD33, CD65, cytoplasmic myeloperoxidase (granulocytic markers) 1

Cytogenetic and Molecular Testing

• Cytogenetic analysis is mandatory for proper classification and risk stratification 1, 2
• For CML: Diagnosis must be confirmed by cytogenetics showing t(9;22)(q3.4;q1.1) and RT-PCR showing BCR-ABL transcripts 1
• For CLL: FISH for del(17p) and molecular genetics for TP53 mutation should be performed before treatment 3
• Molecular genetic testing identifies specific mutations and gene rearrangements 1, 2

Additional Pretreatment Evaluation

Laboratory Studies

• Serum chemistry including serum immunoglobulin and direct antiglobulin test (Coombs test) 3
• LDH, bilirubin, serum protein electrophoresis 3
• Hepatitis B and C, CMV, and HIV serology 3

Imaging

• Chest X-ray 3

Immunohistochemical Staining (for Hairy Cell Leukemia)

• CD20, annexin-1, BRAFV600E stain, and VE1 help establish diagnosis and assess degree of bone marrow infiltration 3
• BRAFV600E mutation is present in the overwhelming majority of HCL classic cases 3

Common Pitfalls and Caveats

• Recent growth factor therapy, transfusions, or certain medications can obscure or mimic features of acute leukemia 1
• Bone marrow biopsy is not required for CLL diagnosis but is recommended before initiating therapy to evaluate unclear cytopenia 3
• For differential diagnosis of CLL, make every effort to distinguish from mantle zone lymphoma using morphology, immunophenotyping, and FISH/molecular biology for t(11;14) translocation and cyclin D1 staining 3
• Approximately 10% of hairy cell leukemia patients will have a hypocellular bone marrow biopsy at diagnosis, reflecting decreased normal hematopoiesis 3

Referral Timing

Patients with suspected leukemia should be referred promptly to a hematologist-oncologist 4, 5, with urgent referral indicated for suspected acute leukemia due to potential life-threatening complications 6