SN5 SHORT NOTE 5M ★ High-yield

Define leukaemoid reaction. Differentiate it from chronic myeloid leukaemia and add a note on the leucoerythroblastic reaction.

Pathology · 2nd Professional — Para-Clinical

Definitions

Term	Definition
Leukaemoid reaction	A reactive, benign leucocytosis above 50,000/mm3 with immature cells in the blood, resembling leukaemia but with an identifiable cause
Leucoerythroblastic reaction	Nucleated red cells and immature white cells together in the blood, indicating marrow infiltration or severe stress

Causes of a Leukaemoid Reaction

Severe infection — pneumonia, septicaemia, empyema, tuberculosis (may be lymphoid), whooping cough
Severe haemorrhage or haemolysis
Metastatic carcinoma involving the marrow
Severe burns, tissue necrosis, diabetic ketoacidosis, eclampsia
Drugs — corticosteroids, granulocyte colony stimulating factor
Poisoning — mercury, carbon monoxide

Distinguishing It from Chronic Myeloid Leukaemia

Feature	Leukaemoid reaction	Chronic myeloid leukaemia
Underlying cause	Identifiable — infection, malignancy, haemorrhage	None
White cell count	Usually 50,000–100,000	Often above 100,000, may exceed 300,000
Leucocyte alkaline phosphatase	High or normal	Low — the classical discriminator
Basophilia	Absent	Present — almost invariable
Eosinophilia	Absent	Present
Toxic granulation, Dohle bodies, cytoplasmic vacuolation	Present	Absent
Splenomegaly	Absent or mild	Massive
Philadelphia chromosome / BCR-ABL	Absent	Present in over 95%
Course	Resolves when the cause is treated	Progressive

CLINICAL PEARL

BCR-ABL settles it, and everything else is supporting evidence. The lap score was the classical answer and is still examined, but molecular testing is definitive and now widely available. If the cause of a very high white count is not obvious, test for BCR-ABL rather than agonising over the smear.

Leucoerythroblastic Reaction

Defined by nucleated red cells and immature white cells appearing together in the peripheral blood
Causes — marrow infiltration by carcinoma, lymphoma, myeloma or granuloma; myelofibrosis; severe haemolysis; severe sepsis; massive haemorrhage; miliary tuberculosis
It always warrants a bone marrow trephine biopsy, since it usually indicates that the marrow architecture is disturbed
Tear-drop red cells alongside it point specifically to myelofibrosis

Applied Aspects

A very high white count is not by itself a diagnosis of leukaemia, and in Indian practice severe infection and tuberculosis are far commoner explanations
Always look for a cause before ordering expensive tests — a chest radiograph, blood cultures and a careful examination often explain it
The blood film is what distinguishes them — toxic granulation and Dohle bodies indicate a reactive process, basophilia a neoplastic one
Leukostasis — counts above 100,000 with blasts cause sludging in cerebral and pulmonary vessels, and require urgent leucapheresis; this is a feature of leukaemia, not of a leukaemoid reaction
A leukaemoid reaction resolves with the underlying illness, and repeating the count after treatment is a cheap and decisive test

All questions in Haematology II: Leukaemias, Lymphomas & Haemostasis

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