A macrocytic blood picture was present in 62 per cent of 222 patients with various types of hepatic disease.

Three different types of macrocytes were present in the blood films: a thin macrocyte, a target macrocyte and a thick macrocyte. Just as leukocytosis might be described as neutrophilic, basophilic or eosinophilic according to the predominate leukocyte present, so macrocytosis has been defined according to the predominate macrocyte present: thin macrocytosis, target macrocytosis and thick macrocytosis. This report deals with the first type, thin macrocytosis.

1. Thin macrocytosis is, by definition, the type of macrocytosis in which all the macrocytes are thin macrocytes. It is the commonest type of macrocytosis (59 per cent of all types of macrocytosis).

2. The thin macrocyte is a flattened erythrocyte. It has the same volume as the normal erythrocyte but is broader and thinner. As the diameter of the cell increases its thickness decreases.

3. Thin macrocytosis is caused by an alteration in erythropoiesis in the bone marrow and not by a flattening or swelling of normal erythrocytes in the peripheral circulation, as suggested by various workers, because:

(a) Normal size erythrocytes transfused into the circulation of patients with thin macrocytosis do not become macrocytic.

(b) Anisocytosis is increased.

(c) The nucleated red cells in the marrow are macrocytic.

4.The thin macrocyte is produced by a macronormoblastic (or rarely atypical megaloblastic) type of maturation in the bone marrow.

5. This altered erythrocyte maturation which gives rise to thin macrocytosis is a specific response to nonspecific hepatic parenchymal cell damage. It occurs in a significant percentage of patients suffering from all types of parenchymal and obstructive hepatic disease. It does not occur in patients with simple fatty liver or obstructive jaundice of short duration where hepatic cells are not damaged. A deficiency of any known hematopoietic factor plays no part in the etiology of this disorder.

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