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Leucocythaemia was the historical description of a persistent excess of white corpuscles in the blood, commonly accompanied by enlargement of blood-forming or lymphoid organs. The corresponding modern disease group is leukaemia, but a raised leukocyte count alone is not a sufficient modern diagnosis.

A period investigation would compare the proportions and appearances of the white and red corpuscles in a fresh blood preparation and examine the spleen, lymph nodes and liver. Splenic enlargement could be striking, and enlarged lymphatic glands supplied another important pattern. The white material was not simply pus circulating from an unnoticed abscess; microscopic resemblance between a leukocyte and a pus corpuscle does not establish suppuration. A persistent excess must also be distinguished from a temporary reactive rise during infection or inflammation.

In present terms, leukaemia comprises malignant disorders of blood-forming cells. Acute and chronic forms, and myeloid and lymphoid lineages, differ in the maturity and identity of their abnormal cells and in their course. Bone marrow disease can interfere with normal production of red blood cells, platelets and functioning leukocytes. Thus pallor, fatigue and breathlessness may reflect anaemia; bruising or bleeding may reflect inadequate platelets; recurrent infection can occur despite a large total white count. Infiltration or enlargement of the spleen, lymph nodes and liver can produce swelling, early satiety or abdominal discomfort.^[ref]

The useful modern examination includes a complete blood count and differential, a stained blood film, and investigation of bone marrow and cellular markers when indicated. The morphology and proportions of mature and immature cells matter. A count can be normal or low in some leukaemias, while a high count can have a non-malignant cause.^[ref] The historical appearance suggests a disorder of blood formation, but classification requires more than counting white cells.

“Leukhæmia” denotes leukaemia: historically a persistent excess of colourless corpuscles in the blood, now understood as a group of malignant disorders of blood-forming biological cells. It is distinct from a temporary rise of ordinary leukocytes during inflammation or infection.

The historical laboratory finding was an unusually large proportion of colourless blood corpuscles on microscopy, often accompanied by enlargement of the spleen or lymph nodes. Such a blood appearance did not mean that ordinary pus had simply entered the circulation. The present classification separates acute and chronic disorders and myeloid and lymphoid lineages. The abnormal population can interfere with production of normal red blood cells, platelets and effective leukocytes in the bone marrow. A high total white-cell count is common, but some acute leukaemias present with a normal or low count; the count alone is neither a complete definition nor a modern subtype diagnosis.^[ref]

The symptoms follow several mechanisms:

These are possible manifestations, not a required syndrome in every affected person.^[ref][ref]

For an 1866 explanation, the important connection is between the persistent abnormal blood picture, enlargement of blood-forming or lymphatic organs, progressive debility and bleeding. Modern diagnosis goes further: a complete blood count with differential and a stained blood film establish which cellular populations are abnormal; selected bone marrow studies, immunophenotyping and genetic tests identify the disease. A microscope image of numerous white corpuscles is the beginning of that distinction, not proof of every later molecular category. The dominant clinical problem can be failure of normal blood formation despite an apparent abundance of white cells.