Understanding Leukaemia — Types, Diagnosis, and the Critical Role of the Laboratory
Decoding Leukaemia: How the Laboratory Unlocks the Diagnosis
Few diagnoses carry as much weight as leukaemia. The word itself can strike fear — a cancer of the blood, a malignancy that begins in the very marrow that sustains life. For generations, a diagnosis of leukaemia was, for many patients, a death sentence. Parents heard the word and knew the worst.
But modern medicine — driven in large part by sophisticated laboratory diagnostics — has transformed many forms of leukaemia from inevitably fatal diseases into manageable, and often curable, conditions. A child with acute lymphoblastic leukaemia today has a cure rate exceeding 85% in high-income settings, and improving outcomes across Africa as treatment protocols expand. Chronic myeloid leukaemia, once a progressive disease with a median survival of 3–5 years, is now managed as a chronic condition with oral tyrosine kinase inhibitors — patients live normal lifespans.
At the centre of this revolution is the medical laboratory. From the initial blood film that raises suspicion, to the flow cytometry and molecular tests that confirm the diagnosis and guide targeted therapy, the laboratory is the engine that drives leukaemia care. Every step matters. Every result shapes treatment.
In this exploration, we'll walk through the major types of leukaemia, their pathophysiology, clinical presentations, and — most importantly — the laboratory investigations that define them. Because understanding leukaemia begins with understanding the tests that diagnose it.
1. What Is Leukaemia?
Leukaemia is a malignancy of the blood-forming cells of the bone marrow. It begins when a single progenitor cell — a cell that would normally develop into a red cell, white cell, or platelet — undergoes malignant transformation. Something goes wrong in its DNA. It begins to proliferate uncontrollably, producing a clone of abnormal cells (leukaemic blasts or abnormal mature cells) that gradually crowd out normal haematopoiesis.
The consequences are predictable and devastating:
Anaemia — the bone marrow cannot produce enough red blood cells. The patient becomes fatigued, pale, short of breath.
Neutropenia — the marrow cannot produce enough neutrophils. The patient becomes vulnerable to life-threatening infections.
Thrombocytopenia — the marrow cannot produce enough platelets. The patient bruises easily, bleeds spontaneously.
Accumulation — leukaemic cells infiltrate the blood, bone marrow, lymph nodes, spleen, liver, and central nervous system, causing organ enlargement and dysfunction.
Leukaemia is not a single disease. It is a family of diseases, classified along two axes: lineage (which type of blood cell is affected) and pace (how quickly it progresses).
| Myeloid Lineage | Lymphoid Lineage | |
|---|---|---|
| Acute (rapid) | Acute Myeloid Leukaemia (AML) | Acute Lymphoblastic Leukaemia (ALL) |
| Chronic (slow) | Chronic Myeloid Leukaemia (CML) | Chronic Lymphocytic Leukaemia (CLL) |
2. The Acute Leukaemias: Rapid, Aggressive, Urgent
Acute leukaemias are characterised by the rapid accumulation of immature blast cells in the bone marrow — by WHO 2022 criteria, ≥20% blasts on bone marrow aspirate. They progress rapidly, often over weeks. Without treatment, they are fatal within months.
2.1 Acute Myeloid Leukaemia (AML): The Adult Acute Leukaemia
AML arises from myeloid progenitors — the cells that would normally become neutrophils, monocytes, eosinophils, basophils, or red cells. It is the most common acute leukaemia in adults, with peak incidence in older age (median age at diagnosis ~68 years).
Clinical Presentation:
Pancytopenia — fatigue (anaemia), fever and infections (neutropenia), bleeding/bruising (thrombocytopenia)
Bone pain — from marrow expansion
Gingival hypertrophy, skin infiltrates (leukaemia cutis) — in certain subtypes (monocytic differentiation)
Coagulopathy — particularly in acute promyelocytic leukaemia (APL) , a subtype of AML
Laboratory Findings:
FBC: Variable white count — can be very high (hyperleucocytosis), normal, or low. Anaemia and thrombocytopenia are common.
Blood film: The defining finding is leukaemic blasts — large cells with high nuclear-to-cytoplasmic ratio, fine nuclear chromatin, and prominent nucleoli. The presence of Auer rods — rod-like, red/purple cytoplasmic inclusions — is pathognomonic for myeloid lineage. Auer rods are never seen in ALL.
Bone marrow: Hypercellular with ≥20% blasts. Myeloperoxidase (MPO) and Sudan Black B stains are positive in myeloid blasts.
Immunophenotype (flow cytometry): Myeloid markers: CD13, CD33, CD117, MPO. Stem cell markers: CD34, TdT (often positive in immature AML).
Cytogenetics and molecular: Critical for risk stratification. Favorable-risk AML includes t(8;21) , inv(16) , and t(15;17) (APL). Acute promyelocytic leukaemia (APL) — t(15;17); PML-RARA fusion — is a haematological emergency. APL presents with severe coagulopathy (DIC) and requires immediate initiation of ATRA (all-trans retinoic acid) on clinical suspicion, before molecular confirmation.
The APL Emergency:
APL is a distinct subtype of AML (M3 in the French-American-British classification) where the leukaemic cells release procoagulant and fibrinolytic substances, causing disseminated intravascular coagulation (DIC) . Patients present with bleeding — gum bleeding, epistaxis, menorrhagia, intracranial haemorrhage. The coagulation screen (PT, aPTT, fibrinogen, D-dimer) must be checked urgently. Treatment with ATRA (and arsenic trioxide in many protocols) rapidly differentiates the blasts and corrects the coagulopathy. Delay is fatal.
2.2 Acute Lymphoblastic Leukaemia (ALL): The Paediatric Acute Leukaemia
ALL arises from lymphoid progenitors — B-cell precursors (most common) or T-cell precursors. It is the most common leukaemia in children, accounting for approximately 75% of childhood leukaemias. Peak incidence is between 2 and 5 years of age. In adults, ALL is less common but carries a poorer prognosis.
Clinical Presentation:
Similar to AML: fatigue, fever, bleeding
Prominent lymphadenopathy, hepatosplenomegaly — more common than in AML
Bone pain — often out of proportion to findings
CNS involvement — headache, vomiting, cranial nerve palsies (more common in T-ALL)
Laboratory Findings:
FBC: Anaemia, thrombocytopenia. White count may be extremely high (hyperleucocytosis) or low. Circulating blasts are present.
Blood film: Blasts with high nuclear-to-cytoplasmic ratio (scant cytoplasm), coarse chromatin, and no Auer rods. In T-ALL, mediastinal mass may be present on imaging.
Bone marrow: Hypercellular with ≥20% blasts. Periodic acid-Schiff (PAS) stain often shows coarse granular positivity in lymphoblasts (block positivity). MPO and Sudan Black B are negative.
Immunophenotype (flow cytometry): B-ALL: CD19, CD20, CD22, CD79a, TdT. T-ALL: CD2, CD3, CD5, CD7, TdT. CD10 (CALLA) positivity is common in precursor B-ALL.
Cytogenetics and molecular: Risk stratification based on cytogenetics. High-risk ALL includes t(9;22) (BCR-ABL1) — Philadelphia chromosome — historically a poor prognostic marker but now targeted with tyrosine kinase inhibitors (imatinib) added to chemotherapy. Other risk factors: KMT2A rearrangements (11q23) , hypodiploidy, iAMP21.
CSF examination: CNS involvement is common in ALL, particularly T-ALL. CSF cytocentrifuge preparations are examined for leukaemic blasts.
3. The Chronic Leukaemias: Slower, Insidious, Incidental
Chronic leukaemias involve more mature, functional cells and progress slowly over months to years. They are often diagnosed incidentally on a routine blood test — a patient presenting for an unrelated issue, a lab result that flags an unexpected elevation.
3.1 Chronic Myeloid Leukaemia (CML): The Targeted Therapy Success Story
CML is characterised by the Philadelphia chromosome — a reciprocal translocation between chromosomes 9 and 22: t(9;22)(q34;q11) . This creates the BCR-ABL1 fusion gene, which encodes a constitutively active tyrosine kinase that drives uncontrolled proliferation of mature and maturing myeloid cells.
Clinical Presentation:
Often asymptomatic — detected on routine FBC
Splenomegaly — massive splenomegaly is a hallmark; patients may complain of early satiety or left upper quadrant discomfort
Fatigue, weight loss, night sweats — in more advanced disease
Three phases: Chronic phase (managed with TKI) → accelerated phase → blast phase (acute leukaemia, myeloid ~70% or lymphoid ~30%)
Laboratory Findings:
FBC: Markedly elevated WBC — often >50 × 10⁹/L, sometimes >200–500 × 10⁹/L. Basophilia (absolute basophil count >200/μL) is characteristic. Eosinophilia may be present. Anaemia and thrombocytosis (or thrombocytopenia) occur.
Blood film: Left shift — presence of immature myeloid cells: myelocytes, metamyelocytes, promyelocytes, occasional blasts. The smear shows the full spectrum of myeloid maturation. Basophils are prominent.
Bone marrow: Hypercellular with myeloid hyperplasia. Blasts <10% in chronic phase.
Neutrophil alkaline phosphatase (NAP) score: Low in CML (in contrast to reactive neutrophilia, where NAP is elevated).
Cytogenetics and molecular: Philadelphia chromosome on karyotyping or FISH. BCR-ABL1 detected by RT-PCR — the gold standard for diagnosis and monitoring. BCR-ABL1 levels (International Scale) are used to assess treatment response: MR4.5 (0.0032% IS) indicates deep molecular response; loss of response signals treatment failure.
Treatment: Tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib) — a landmark in targeted cancer therapy. Most patients achieve long-term remission and near-normal life expectancy.
3.2 Chronic Lymphocytic Leukaemia (CLL): The Adult Lymphoid Leukaemia
CLL is a clonal proliferation of mature B lymphocytes that accumulate in the blood, bone marrow, and lymphoid organs. It is the most common leukaemia in adults in Western countries — less common in Africa, though incidence is rising with increased life expectancy and improved diagnostics.
Clinical Presentation:
Often asymptomatic — detected on routine FBC
Lymphadenopathy — generalized, non-tender
Fatigue, recurrent infections (hypogammaglobulinaemia), autoimmune haemolytic anaemia (Coombs-positive), thrombocytopenia
Rai and Binet staging systems — based on extent of disease
Laboratory Findings:
FBC: Lymphocytosis — absolute lymphocyte count >5 × 10⁹/L (often >20–50 × 10⁹/L). Anaemia and thrombocytopenia in advanced disease.
Blood film: Smear cells (Gumprecht shadows) — fragile CLL cells ruptured during film preparation, leaving smudged remnants. This is a characteristic finding. Lymphocytes are small, with condensed chromatin and scant cytoplasm.
Immunophenotype (flow cytometry): The CLL immunophenotype is distinctive: CD5+, CD19+, CD23+, CD20dim, surface Igdim. This distinguishes CLL from other B-cell lymphoproliferative disorders.
Bone marrow: Interstitial, nodular, or diffuse infiltration by small lymphocytes. Trephine biopsy shows characteristic patterns.
Cytogenetics: FISH detects high-risk abnormalities: del(17p) (TP53 deletion) — associated with poor response to chemoimmunotherapy; del(11q) ; trisomy 12; del(13q) (better prognosis).
Treatment: Not all patients require treatment. Indications include progressive cytopenias, massive lymphadenopathy/splenomegaly, or symptomatic disease. Targeted therapies (BTK inhibitors: ibrutinib; BCL2 inhibitors: venetoclax) have transformed CLL management.
4. Laboratory Investigations: The Diagnostic Toolkit
4.1 Full Blood Count and Blood Film
The FBC is often the first alert. Common findings include:
Anaemia
Thrombocytopenia
Abnormal white cell count — very high, normal, or low (leukaemic blasts can present as apparent leukopaenia if they fail to be counted accurately by the analyser)
The blood film is indispensable:
| Finding | Suggests |
|---|---|
| Auer rods | AML (myeloid lineage) — pathognomonic |
| Smear cells (Gumprecht shadows) | CLL |
| Blasts with high N:C ratio, scant cytoplasm, no Auer rods | ALL |
| Left shift with basophilia, eosinophilia, full myeloid spectrum | CML |
| Occasional blasts with prominent nucleoli | Suspect acute leukaemia |
4.2 Bone Marrow Aspirate and Trephine Biopsy
Definitive diagnosis requires bone marrow examination.
Aspirate: Provides cells for morphology, cytogenetics, flow cytometry, and molecular studies. Blast percentage confirms diagnosis (≥20% in acute leukaemias).
Trephine biopsy: Provides bone marrow architecture, cellularity, reticulin fibrosis assessment, and is essential when aspirate is "dry" (fibrotic marrow, as in some leukaemias).
4.3 Immunophenotyping (Flow Cytometry)
Flow cytometry uses fluorescently labelled antibodies to identify cell surface and intracellular antigens (CD markers). This is the gold standard for lineage assignment and classification of leukaemias.
| Lineage | Key Markers |
|---|---|
| Myeloid | CD13, CD33, CD117, MPO (myeloperoxidase) |
| B-lymphoid | CD19, CD20, CD22, CD79a |
| T-lymphoid | CD2, CD3, CD5, CD7, CD8 |
| Stem cell/immature | CD34, TdT (terminal deoxynucleotidyl transferase) |
| CLL | CD5+, CD19+, CD23+, CD20dim, surface Igdim |
4.4 Cytogenetics and Molecular Studies
Chromosomal and molecular abnormalities are central to leukaemia classification, risk stratification, and treatment decisions.
| Technique | Purpose |
|---|---|
| Karyotyping | Conventional analysis of all 46 chromosomes. Identifies translocations (t(9;22), t(15;17)), deletions, trisomies. |
| FISH (Fluorescence In Situ Hybridisation) | Detects specific chromosomal abnormalities with higher sensitivity than karyotyping. Used for BCR-ABL1, PML-RARA, and high-risk FISH panel in CLL. |
| RT-PCR | Detects fusion transcripts (BCR-ABL1 for CML; PML-RARA for APL). Used for diagnosis and minimal residual disease (MRD) monitoring. |
| Next-generation sequencing (NGS) | Detects somatic mutations (FLT3, NPM1, IDH1/2, DNMT3A, RUNX1) with prognostic and therapeutic implications in AML. |
4.5 Coagulation Screen in APL
In suspected APL, the coagulation screen is an emergency:
PT prolonged
aPTT prolonged
Fibrinogen low
D-dimer elevated
APL is a haematological emergency. ATRA must be started immediately on clinical suspicion — do not wait for molecular confirmation.
4.6 CSF Examination
In ALL (particularly children, T-ALL) and some other acute leukaemias, CSF examination is performed to detect CNS involvement. CSF cytocentrifuge preparations are examined for leukaemic blasts.
5. Monitoring and Minimal Residual Disease (MRD)
Modern leukaemia management requires ongoing monitoring of response to therapy.
Minimal Residual Disease (MRD) refers to the small number of leukaemic cells remaining after treatment — too few to detect by morphology (which can only detect 1 blast in 100 cells, or 1% sensitivity) but detectable by more sensitive techniques.
| Technique | Sensitivity | Use |
|---|---|---|
| Flow cytometry MRD | 0.01% (1 in 10,000) | ALL, AML (if aberrant phenotype identified) |
| PCR MRD (quantitative RT-PCR for fusion transcripts) | 0.001% (1 in 100,000) | BCR-ABL1 (CML, Ph+ ALL), PML-RARA (APL) |
| NGS MRD (clonality-based) | 0.0001% (1 in 1,000,000) | ALL, AML, CLL |
MRD negativity — the absence of detectable leukaemic cells by sensitive techniques — is increasingly used as a treatment endpoint and prognostic marker. In ALL, MRD negativity after induction is one of the strongest predictors of long-term remission. In CML, sustained deep molecular response (MR4.5) allows consideration of treatment discontinuation (treatment-free remission) in eligible patients.
6. The Laboratory Scientist's Role: Beyond the Bench
The medical laboratory scientist is not a passive observer in leukaemia management. Responsibilities include:
Recognising abnormal cells on peripheral blood film and escalating urgently — a single Auer rod, a suspicious blast, an unexpected smear cell
Accurate FBC reporting with appropriate flags and comments — leukaemia may present with a normal WBC; the analyser may flag blasts or atypical cells
Performing and interpreting bone marrow stains — MPO, PAS, Sudan Black B, non-specific esterase (NSE)
Quality assurance in flow cytometry and molecular testing — ensuring panels are appropriate, controls are valid, results are accurate
Ensuring accurate cytogenetic specimen handling — bone marrow for karyotyping must be collected in heparin, transported immediately, and kept at room temperature (not refrigerated)
Communicating critical results to clinical teams promptly — a new diagnosis of APL, a positive CSF, a patient with hyperleucocytosis
Conclusion: Precision Begins in the Laboratory
Leukaemia is complex — a family of diseases with distinct biology, prognosis, and treatment. But laboratory science has the tools to decode it. From the first glance at a blood film to sophisticated molecular analysis, the medical laboratory scientist is at the frontline of diagnosis, classification, and monitoring of these malignancies.
A child with ALL — the blood film shows blasts, the flow cytometry confirms B-ALL, the cytogenetics show standard-risk features, and MRD monitoring guides therapy. An adult with fatigue — the FBC shows leukocytosis with basophilia, the blood film shows a left shift, the FISH shows BCR-ABL1, and the patient starts imatinib. A patient with bleeding — the coagulation screen shows DIC, the blood film shows Auer rods with bundles of Auer rods (faggot cells), and ATRA is administered within hours.
In an era of targeted therapies, precision matters. And precision begins in the laboratory.
Your Results. Your Understanding. Your Health.
Whether you're a patient navigating a leukaemia diagnosis, a healthcare professional interpreting complex haematological results, or someone seeking to understand one of the most challenging areas of laboratory medicine, knowledge is essential.
Visit our free interpretation tool at:
https://VincentAkwas.github.io/lablens
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