Leukaemia Classification
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Direct answer
Leukaemias are neoplastic proliferations of haematopoietic cells that replace the marrow and spill into the blood. They are classified along two axes — lineage (lymphoid or myeloid) and clinical behaviour (acute or chronic): acute leukaemias present abruptly with marrow failure and at least twenty per cent blasts, while chronic leukaemias accumulate more mature cells and run an indolent course. The WHO classification (updated 2022) defines entities by lineage plus recurring genetic abnormalities rather than morphology alone.
What you must remember
- Four classical groups: acute lymphoblastic leukaemia (ALL), acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML) and chronic lymphocytic leukaemia (CLL).
- ALL is the commonest cancer of children, peaking at two to five years, with bone pain, lymphadenopathy, hepatosplenomegaly and a propensity for meningeal and testicular relapse; childhood ALL is also the paradigm of curable cancer with combination chemotherapy.
- AML predominates in adults; the FAB M0 to M7 morphological subtypes remain in Indian teaching, while WHO (5th edition, 2022) pivots to defining genetics — t(8;21), inv(16) and t(15;17) — with APL (M3, PML-RARA) diagnosable at any blast count.
- Acute promyelocytic leukaemia (M3) is the exam favourite: Auer rods, t(15;17), disseminated intravascular coagulation from granule release, and cure with all-trans retinoic acid plus arsenic trioxide.
- CML is a myeloproliferative neoplasm defined by the Philadelphia chromosome, t(9;22), creating the BCR-ABL1 fusion; it presents with neutrophilia with basophilia, striking splenomegaly, low leucocyte alkaline phosphatase, and progresses through chronic, accelerated and blast phases; imatinib is first-line therapy.
- CLL is the commonest leukaemia of older adults in Western populations: mature CD5-positive, CD23-positive B cells, smudge cells on the smear, and complications of warm autoimmune haemolytic anaemia, infection from hypogammaglobulinaemia and Richter transformation; Rai and Binet stages guide care.
- Marrow failure — anaemia, infection and bleeding, with gum hypertrophy or chloromas in monocytic AML — is the usual presentation of acute leukaemia.
Common confusion
Acute versus chronic is confused when the white cell count alone is considered. The deciding criterion is blast percentage and cell maturity: acute leukaemias show at least twenty per cent blasts in blood or marrow, while chronic leukaemias show mature, differentiated cells. A second classic discrimination is CML from a leukaemoid reaction — the latter has toxic granulation and a high leucocyte alkaline phosphatase score, opposite to CML.
Exam-focused takeaway
Theory answers should present the four-way classification, then one paragraph each on presentation, typical age and hallmark laboratory features. Viva examiners ask about the Philadelphia chromosome, Auer rods and blast thresholds. MCQs match disease to marker — ALL with childhood, M3 with DIC and ATRA, CML with basophilia and imatinib, CLL with smudge cells and CD5 positivity.
Frequently asked questions
What blast percentage defines acute leukaemia?
Twenty per cent or more blasts in blood or marrow, although WHO 2022 permits diagnosis of genetically-defined AML such as PML-RARA at lower counts.
What is the Philadelphia chromosome?
The t(9;22) translocation creating the BCR-ABL1 fusion tyrosine kinase — the hallmark of CML and the target of imatinib.
Which leukaemia is associated with disseminated intravascular coagulation?
Acute promyelocytic leukaemia (FAB M3), through release of procoagulant granules; all-trans retinoic acid differentiates the blasts and resolves it.
Which leukaemia is the commonest in children?
Acute lymphoblastic leukaemia — also among the most curable of childhood cancers with modern protocols.
What are smudge cells?
Fragile CLL lymphocytes that rupture during blood smear preparation — a characteristic laboratory clue to CLL.