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PA18.1-2 | Acute Leukaemias — AML & ALL — Part 3

Distinguishing AML from ALL: Cytochemistry and Immunophenotyping

A color-coded medical diagram compares AML and ALL using cytochemistry stain results and immunophenotyping markers. — **Panel A:** Diagnostic overview showing acute leukaemia blasts branching into AML and ALL through cytochemistry and immunophenotyping.. **Panel B:** Cytochemistry comparison table showing MPO, Sudan Black B, NSE, PAS, and TdT results in AML versus ALL.. **Panel C:** AML immunophenotype showing myeloid blast, possible Auer rod, MPO positivity, and CD13, CD33, CD117 markers.. **Panel D:** ALL immunophenotype showing lymphoblast, TdT positivity, B-ALL markers CD10, CD19, CD20, and T-ALL markers CD3, CD7..

The AML vs ALL distinction is the single most important diagnostic decision in acute leukaemia workup. It requires a multi-modal approach.

Cytochemistry (bench tests, rapid):

Stain	AML	ALL
Myeloperoxidase (MPO)	Positive (≥3% blasts)	Negative
Sudan Black B (SBB)	Positive (parallels MPO)	Negative
Non-specific esterase (NSE)	Positive in M4/M5 (monocytic)	Negative
PAS (periodic acid-Schiff)	Weak/diffuse or negative	Strongly positive (coarse block pattern) in B-ALL
TdT (terminal deoxynucleotidyl transferase)	Negative (except M0 occasionally)	Positive in ALL and T-ALL

Side-by-side cytochemistry panels showing AML blasts with MPO-positive brown granular staining, ALL lymphoblasts with PAS-positive magenta blocks, and TdT-positive nuclear staining in ALL. — **Panel A:** AML blasts with MPO-positive brown granular cytoplasmic staining, myeloid lineage marker. **Panel B:** ALL lymphoblasts with coarse PAS-positive magenta block cytoplasmic staining. **Panel C:** ALL lymphoblasts with TdT-positive nuclear staining, lymphoid lineage marker.

Immunophenotyping by flow cytometry (definitive):

B-ALL markers: CD19, CD10 (CALLA), CD22, TdT, HLA-DR, cIgM (pre-B).
T-ALL markers: CD3 (cytoplasmic → surface), CD7, CD2, CD5, TdT.
AML markers: CD13, CD33, CD117 (c-KIT), CD34, MPO (flow). Monocytic: CD14, CD64.

CD34 is a stem cell marker expressed in both AML and ALL; TdT is the key lymphoid marker.

Diagnostic flowchart for differentiating AML from ALL with clinical features comparison and marrow failure manifestations. — **Panel A:** Diagnostic decision tree showing blast count threshold, morphological features (Auer rods), cytochemical stains (MPO/SBB vs PAS/TdT), immunophenotyping markers, and cytogenetic analysis leading to AML vs ALL diagnosis. **Panel B:** Clinical feature comparison table highlighting organ infiltration differences - gum hypertrophy in AML M4/M5, lymphadenopathy more prominent in ALL, hepatosplenomegaly in both, mediastinal mass specific to T-ALL. **Panel C:** Marrow failure triad showing anaemia (fatigue, pallor, dyspnoea), neutropenia (infections, fever), and thrombocytopenia (bleeding, petechiae, bruising) as interconnected consequences of blast displacement.

Clinical Features: Marrow Failure and Organ Infiltration

Diagram showing leukaemic marrow replacement causing anaemia, neutropaenia, and thrombocytopaenia, with AML and ALL organ infiltration features compared. — **Panel A:** Bone marrow cavity, leukaemic blasts, displaced normal haematopoiesis, reduced red blood cells, reduced neutrophils, reduced platelets, anaemia, neutropaenia, thrombocytopaenia. **Panel B:** Anaemia: fatigue, pallor, dyspnoea on exertion, tachycardia; Neutropaenia: recurrent bacterial/fungal infections, fever without obvious source; Thrombocytopaenia: petechiae, easy bruising, gum bleeding, epistaxis, haemorrhagic diathesis. **Panel C:** AML M4/M5: gum hypertrophy and gingival infiltration; ALL: lymphadenopathy, hepatosplenomegaly, T-ALL anterior mediastinal thymic mass, bone pain, sternal tenderness, periosteal infiltration.

Clinical features arise from two processes: marrow failure (displacement of normal haematopoiesis) and organ infiltration by leukaemic blasts.

Marrow failure triad:
• Anaemia (normocytic normochromic): fatigue, pallor, dyspnoea on exertion, tachycardia.
• Neutropaenia: recurrent bacterial/fungal infections, fever without an obvious source.
• Thrombocytopaenia: petechiae, easy bruising, mucosal bleeding (gum bleed, epistaxis), haemorrhagic diathesis.

Organ infiltration — AML vs ALL differences:

Sign	More prominent in
Gum hypertrophy (gingival infiltration)	AML M4/M5 (monocytic)
Lymphadenopathy	ALL (especially in children)
Hepatosplenomegaly	ALL (more marked), both
Mediastinal mass	T-ALL (anterior mediastinum — thymic infiltration)
Bone pain / sternal tenderness	ALL (periosteal infiltration; classic in children)
CNS involvement (headache, cranial nerve palsies, papilloedema)	ALL (sanctuary site)
Testicular enlargement	ALL (sanctuary site in boys)
Skin infiltration (leukaemia cutis)	AML M4/M5, infant ALL
DIC / bleeding	APL (M3) predominantly

Gum hypertrophy: monocytic blasts express adhesion molecules (CD11b, CD18) that promote tissue homing to the periodontium. This is pathognomonic of monocytic differentiation when seen in the context of acute leukaemia.

CLINICAL PEARL

Sternal tenderness is an underappreciated clinical sign: pressing firmly on the sternum in a child with pancytopaenia and lymphadenopathy may elicit disproportionate pain from marrow expansion — a clue pointing toward ALL (or lymphoma). Do not skip this examination step in a child presenting with unexplained anaemia and lymphadenopathy.

Haematologic Indices and Bone Marrow Findings

A three-panel diagram summarizes acute leukaemia peripheral blood indices, marrow blast replacement with the ≥20% diagnostic threshold, and subtype-specific findings for AML, APL, and ALL. — **Panel A:** Peripheral blood smear showing variable TLC, blasts, nucleated RBCs, immature myeloid cells, normocytic normochromic anaemia, and severe thrombocytopaenia.. **Panel B:** Normal versus leukaemic bone marrow showing hypercellular marrow, replacement of normal architecture by blasts, reduced haematopoietic islands, and the WHO ≥20% blast threshold.. **Panel C:** Subtype clues and marrow investigations: AML with Auer rods and MPO-positive blasts, APL with hypergranular promyelocytes and faggot cells, ALL with TdT-positive lymphoblasts, plus flow cytometry, cytogenetics, FISH, and PCR testing..

Peripheral blood:
• TLC: highly variable — can be low (<5,000/µL, aleukemic leukaemia), normal, or dramatically elevated (>100,000/µL — hyperleukocytosis).
• Blasts visible on smear in most cases (can be absent in aleukemic variants).
• Leukoerythroblastic picture: blasts + nucleated RBCs + immature myeloid cells — seen when marrow is replaced.
• Anaemia: normocytic normochromic.
• Thrombocytopaenia: often severe (<50,000/µL).

Bone marrow aspirate and trephine biopsy:
• Hypercellular marrow with replacement of normal architecture by blasts.
• ≥20% blasts (of all nucleated cells) is the WHO diagnostic threshold — key criterion.
• Normal haematopoietic islands (erythroid, myeloid, megakaryocytic islands) markedly reduced.
• Additional staining (MPO, PAS, TdT, flow panel) performed on marrow aspirate.
• Cytogenetic analysis (conventional karyotype + FISH) and molecular testing (PCR for BCR-ABL, FLT3, NPM1) on marrow sample.

Characteristic findings by subtype:
• AML: Auer rods on smear; MPO-positive blasts; cytoplasmic granularity may be visible.
• APL: hypergranular promyelocytes + faggot cells; marrow often packed.
• ALL: lymphoblasts fill marrow; 'starry sky' pattern may be seen in L3/Burkitt-type; TdT+ by immunostain.

SELF-CHECK

Bone marrow biopsy of a 45-year-old shows 24% blasts. The blasts are MPO-negative, TdT-positive, CD19+, CD10+. Conventional karyotype shows t(9;22). What is the diagnosis and what therapy should be added to standard chemotherapy?

A. AML with t(9;22); add ATRA

B. Philadelphia-positive B-ALL; add a BCR-ABL tyrosine kinase inhibitor (e.g., imatinib/dasatinib)

C. T-ALL with favourable genetics; standard paediatric protocol

D. Blast crisis of CML; cytarabine monotherapy

Reveal Answer

Answer: B. Philadelphia-positive B-ALL; add a BCR-ABL tyrosine kinase inhibitor (e.g., imatinib/dasatinib)

The blast immunophenotype (TdT+, CD19+, CD10+) confirms B-lymphoblastic lineage. MPO negativity rules out AML. t(9;22) in this B-ALL context = Philadelphia-positive B-ALL — historically a dismal prognosis. The addition of BCR-ABL TKIs (imatinib, dasatinib) to chemotherapy has dramatically improved outcomes. ATRA is for APL (t(15;17)). Blast crisis of CML has a CML history with a prior chronic phase; de novo Ph+ B-ALL is treated as ALL.

Tumour Lysis Syndrome

Tumour lysis syndrome (TLS) is an oncological emergency caused by the massive, rapid destruction of leukaemic blasts — either spontaneous or triggered by chemotherapy. It is most common in ALL (especially Burkitt) and hyperleukocytic AML.

Pathophysiology: Blast lysis → release of intracellular contents:
• Potassium → hyperkalaemia (cardiac arrhythmia risk)
• Phosphate → hyperphosphataemia → reciprocal hypocalcaemia (tetany, seizures)
• Nucleic acids → xanthine → hyperuricaemia → urate crystals in renal tubules → acute kidney injury

Cairo-Bishop criteria (laboratory TLS): ≥2 of: uric acid ≥476 µmol/L, potassium ≥6 mmol/L, phosphate ≥1.45 mmol/L, calcium ≤1.75 mmol/L — within 3 days before to 7 days after starting treatment.

Prevention and management:
• Aggressive IV hydration (2–3 L/m²/day)
• Allopurinol (xanthine oxidase inhibitor — blocks uric acid production)
• Rasburicase (recombinant uricase — converts uric acid to allantoin; faster, more potent)
• Electrolyte correction; cardiac monitoring
• Avoid nephrotoxins; avoid potassium supplements